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Using this book
Most of the interactive features in this series
of GCSE Revision Guides are intuitive. The
other features described in this introduction
will help you make the most of these books.
Making the most of this interactive book
Syllabus coverage
This series of interactive science books covers the complete GCSE Science ‘A’
specifications for AQA, OCR (21st Century Science) and Edexcel.
Differences in the specifications are indicated by specific references. For example,
(Edexcel, OCR) means that the material covers the OCR and Edexcel specifications,
but is not relevant to AQA.
Where no indication is given, the material is relevant to all three awarding bodies.
In this example, ‘the halogens’ and ‘formulae of ionic compounds’ are relevant to the
AQA, OCR and Edexcel specifications, but ‘more halogen reactions’ is only relevant to
Edexcel and OCR:
In this section (all boards) • the halogens • more halogen reactions (Edexcel, OCR) • formulae
of ionic compounds
ii
Navigation and features
The Contents in portrait view is the
easiest way to navigate your book – by
chapter, section, and sub-section, as
shown here:
iii
You can also navigate in the landscape Contents view:
iv
Generally red texts are hyperlinks to
another part of the page or another
part of the book. Bookmark the current
page if you need to return to it.
All terms in the glossary are defined
every time they appear in the book, not
just when they appear in bold.
In vertical view, tapping the iPad’s
black status bar scrolls quickly back to
the start of the chapter.
v
Test yourself with study
cards from the glossary, or
make your own cards:
vi
All your study cards are organised by chapter. Your own highlights and notes appear
as a list under ‘My Notes’:
vii
Test yourself or a friend using your study cards. Use the gear wheel to decide whether
to test from the glossary, from your cards, or from both:
viii
Accessibility features such as
VoiceOver are available from the iPad’s
General menu:
ix
Chapter 1
How structure affects the
properties of a substance
Syllabus
AQA
Coverage
C2.2 How structure influences the properties and uses of substances
C4.3.7 How do chemists explain the properties of compounds of Group
1 and Group 7 elements?
OCR
21st Century
C5.1.4–5.1.8, 5.1.11 What types of chemicals make up the atmosphere?
C5.2.2–5.2.4 What reactions happen in the hydrosphere?
C5.3.2–5.3.4, 5.3.7–5.3.8 What types of chemicals make up the Earth’s
lithosphere?
C5.4.23–5.4.26 How can we extract useful metals from minerals?
1.3–1.8, 1.10-1.11 Atomic structure and the periodic table
Edexcel
2.7–2.8 Ionic compounds and analysis
3.6–3.9 Covalent compounds and separation techniques
4.2–4.5 Groups in the periodic table
1.1
Structure and properties of ionic compounds
In this section (all boards) • structure of ionic
compounds • properties of ionic compounds
Structure of ionic compounds
Ionic compounds like sodium chloride form a
tightly-packed structure called a giant lattice.
The position of each ion in the lattice is regular
(predictable) and the pattern repeats millions of
times over.
The overall formula of sodium chloride is NaCl,
but at the atomic level it is not meaningful to try
and say which sodium ion is bonded to which
chloride ion — the forces of attraction work in all
directions.
What we can say is that there is one Na+ ion for
every one Cl– ion overall, in this regular crystal
structure.
Figure 1.1
The strong electrostatic forces of attraction
between positive sodium ions (purple) and
negative chloride ions (green) in NaCl work in
all directions, holding the oppositely-charged
ions together. There are billions of ions even
in a single grain of salt
11
Properties of ionic compounds
The properties of ionic compounds relate to their
giant lattice structure. Ionic compounds:
have strong forces of attraction between the
oppositely-charged ions; this holds the lattice
structure tightly together
do not conduct electricity in their solid state —
they have no free electrons that can move
around (contrast with metals: see Section 2.2)
have high melting points and boiling points.
Ionic compound
melting point (ºC)
LiCl
605
NaCl
801
Ionic compound
boiling point (ºC)
MgO
3600
CaO
2850
Tip: It requires plenty of energy to raise the
temperature of ionic compounds high enough to
break the bonds holding their lattices together
However, when ionic substances are in a liquid
state — molten or
in aqueous
solution, they do
conduct electricity
— see Section 5.3.
The ionic
compounds
described in the
tables have a
similar structure:
one positive metal
ion for each
negative ion. The
difference is in the
sizes of the ions.
Figure 1.2
Lithium ions (purple, left) are
smaller than potassium ions
(purple, right) in their respective
chlorides, but the giant crystal
structure is exactly the same in
both salts
12
1.2
Structure and properties of metals
In this section (all boards) • structure of metals •
properties of metals
Structure of metals
Most metals are solids at room temperature and
have the following giant crystal structure:
positively-charged ions of the metal, e.g. Li+,
Na+, Mg2+, Fe3+ …
… are layered and
packed tightly
together …
… and held together
by a ‘sea’ of their
own freely-moving
(delocalised)
electrons.
Figure 1.4
Figure 1.3
A 3D representation of
the tightly-layered
structure of sodium
Metal atoms often only have 1, 2 or 3 electrons in
their outer shell. These electrons are not strongly
tied to the atom’s nucleus — the electrons are free
to move around between the closely-packed
atoms in the crystal structure
13
Properties of metals
The properties of metals relate to their structures
described above. Metals are:
strong and malleable (easily shaped), because
when a force is applied to them, metal atoms
slide over each other. The sea of electrons
helps give metals flexibility, holding the
positive ions together while also allowing them
to move over each other.
have high melting points, because of the tight
packing, and the strong electrostatic forces of
attraction between the positive metal ions and
their sea of electrons.
AQA — Pure metals can be strengthened by
mixing them with other elements to make alloys.
This makes them less malleable.
good conductors of electricity (and heat),
because the delocalised electrons can move in
one direction, e.g. if a potential difference
(voltage) is applied to them.
Figure 1.6
A pure metal such as iron can be mixed with a small
percentage of carbon to make steel: a strong alloy
of iron
Figure 1.5
Used zinc is malleable and can be cut,
bent and compacted for recycling
14
AQA — Shape-memory alloys are a recent
innovation. For example the shape of nitinol, a
nickel-titanium alloy can be altered when cooled;
its original shape returns when it is warmed up.
Tiny nitinol meshed tubes called stents can be
used to unblock arteries: the tube is cooled,
squashed and inserted into the artery. Once the
stent is at the blocked position, the body’s heat
helps it to expand.
Most metals are transition metals. These are in
purple in the Periodic Table, Appendix 1.
Transition metals tend to:
form coloured compounds.
have high melting points.
Stent mesh
Figure 1.8
Plaque
Aqueous solutions of transition metal compounds,
from left to right: cobalt(II) nitrate, potassium
dichromate, potassium chromate, nickel(II) chloride,
copper(II) sulfate, potassium permangenate
Figure 1.7
Tiny nitinol meshes can be used to unblock arteries
15
1.3
Structure and properties of covalent
substances
In this section (all boards) • structure and
properties of covalent substances • separating
mixtures of liquids (Edexcel)
Structure and properties of giant
covalent substances
Diamond and graphite are both minerals and
both made entirely from carbon atoms. Yet their
unique and different structures give them unique
and different properties.
Diamonds are extremely hard and cut through
anything, including metals. This is their most
useful practical property.
Diamonds do not conduct electricity — all of the
electrons in each carbon atom’s outer shell are
used to form covalent bonds with 4 other carbon
atoms in the structure.
Figure 1.9
Diamonds have many industrial uses: this grinding
wheel is being returned to its original shape by the
diamond ‘dressing’ tool touching it at the bottom
Graphite is another type of carbon; it is what
pencil ‘lead’ is made from. Like diamond, it has a
high melting point and is insoluble in water.
16
However, unlike diamond:
graphite’s layers can slide over
each other. Using a pencil, layers
of carbon atoms slide easily onto
paper. This characteristic also
makes graphite a good lubricant.
graphite conducts electricity and
heat — each carbon atom is only
bonded to 3 others rather than 4;
so each atom has a free
(delocalised) electron in its outer
shell. The same is true of
nanotubes (see Section 2.5).
Figure 1.10
Graphite is unusual in being a giant molecular structure that nevertheless
includes weak intermolecular forces between the layers of stronglybonded hexagons of carbon atoms
17
Silica is silicon dioxide, SiO2, the main
component of sand and quartz. Sand grains are
very difficult to melt, as is quartz. They both have
a giant covalent structure similar to diamond.
Figure 1.11
Rose quartz is made from silicon dioxide, as is sand.
The rose colour comes from trace metals mixed in
with the quartz
Comparing the melting points of giant covalent
substances to simple covalent substances, we
can see a general trend in melting points that is
explained by their structures.
Covalent molecular
substance
Melting point (ºC)
Oxygen, O2
–219
Hydrogen chloride, HCl
–114
Ammonia, NH3
–77
Buckminsterfullerene,
C60
sublimates at 600
Silica, SiO2
1600
Diamond, C
3500
The boiling points of the substances in the table
follow a similar pattern.
18
Tip: Always remember that both ionically-bonded
substances and giant covalently-bonded
substances have strong bonds throughout.
Generally it is only simple covalently-bonded
substances — e.g. gases like Cl2 and liquids like
water — that have some weak forces of attraction
between each molecule: and low melting/boiling
points as a result.
Separating mixtures of liquids (Edexcel)
Figure 1.12
A simple covalent molecule like water has very
strong bonds between each atom that require a lot
of energy to break. But the molecule is only weakly
charged overall — slightly negative (red) near the
oxygen atom, slightly positive (blue) near the
hydrogen atoms. The intermolecular forces between
this molecule and its nearest neighbouring H2O
molecule are therefore weak
A liquid that is
unable to mix with
another liquid is
immiscible.
For example, oil and
water do not really
mix. Stirring them
may seem to mix
them together, but if
left to settle the two
liquids will soon
separate out.
Figure 1.13
Oil soon separates out from
water and forms a layer above it
19
Immiscible liquids can be separated
using a separating funnel.
Fractional distillation of liquid air – Air is a mixture: the gases
oxygen, nitrogen and argon are all miscible with each other.
When air is cooled to an extremely low temperature it becomes
a liquid; the elements can now be separated out. The procedure
works like the fractional distillation of crude oil:
Air needs to be in liquid form before the procedure can be
used, so it is cooled to minus 200°C by massive compression.
Under compression the gases move to their liquid state.
The mixture is then allowed to warm up slowly. The main
gases in air are boiled off at their particular boiling points: first
nitrogen (–196°C), then argon (–186°C), and finally oxygen (–183°C) are boiled and collected.
Figure 1.14
Separating oil and coloured water
20
1.4
Polymers
In this section (AQA) • reminder about polymers •
LDPE and HDPE • thermosoftening and
thermosetting polymers
Different types of poly(ethene) have different
numbers of carbon atoms in their chains —
hundreds or even thousands.
Reminder about polymers
These chains repeat the same pattern over and
over again:
Polymers are a useful type of plastic, made by
combining monomers.
For example, poly(ethene) is made by combining
ethene molecules into long chains of repeating
structural ‘units’, n number of times.
General equation for how poly(ethene) is made
from ethene:
2 carbon atoms bonded to each other as the
‘backbone’ of the chain
these carbon atoms are also bonded to 2
hydrogen atoms each.
LDPE and HDPE
Using different reaction conditions and catalysts
gives polymers distinct properties.
LDPE (low density poly(ethene)) and HDPE (high
density poly(ethene)) are made in different ways
and are useful to us in different circumstances.
21
LDPE is made under high pressure. Each LDPE
molecule is branched — it is ‘non-crystalline’.
The branches prevent each LDPE molecule from
getting close to its neighbours — the atoms in
neighbouring molecules are relatively far apart.
LDPE is therefore:
low density — relatively light for its volume …
flexible — it can be bent easily …
ideal for making shrink wrap and plastic bags.
Figure 1.16
Figure 1.15
LDPE’s branched and tangled molecules make
this a low density plastic
LDPE has a standard international
recycling number, 4. Sometimes
this is shown without letters
underneath. HDPE’s recycling
number is 2
22
HDPE is made using metal catalysts. Each HDPE
molecule is fairly straight — it barely has
branches; it is ‘crystalline’. The molecule chains
sit closer together; the intermolecular forces of
attraction between each molecule are stronger.
HDPE is therefore:
high density — it is relatively heavy for its
volume ...
strong and inflexible …
ideal for making very rigid plastics, e.g.
pipelines for domestic sewage or for natural
gas.
Figure 1.17
HDPE molecules are packed close together in long
chains. This is a very partial view of 3 molecules — a
typical HDPE molecule has thousands of carbon
(black) and hydrogen (white) atoms
Figure 1.18
Manufacturing a large HDPE pipe
23
Thermosoftening and thermosetting
polymers
Thermosoftening polymers:
are made up of tangled individual polymer
chains.
have weak intermolecular forces: the forces
that attract one chain to another chain.
melt easily when heated; after cooling again
they can re-establish the same weak
intermolecular forces and be reshaped.
Thermosetting polymers:
have cross-linked covalent bonds between the
chains.
do not melt at all when heated, due to these
cross-links.
Tip: Be careful about the words you use to
describe materials like plastics. A highly inflexible
plastic is ‘very rigid’ rather than ‘very solid’. The
scientific term ‘solid’ best describes a chemical
state — solids, liquids and gases.
Figure 1.19
Thermosoftening polymer chains are tangled up with
each other (left) but melt easily; thermosetting
polymers have cross-links (shown in black here) that
stop them from melting so easily
24
1.5
Nanotechnologies
In this section (AQA) • the nanometre scale •
nanotechnologies • fullerenes and nanotubes
The nanometre scale
There are 1,000,000,000 (one billion) nanometres
in one metre.
The double helix of a DNA molecule is about
one nanometre wide.
An individual atom is about one tenth of a
nanometre wide.
To try to get a sense of this scale …
a micrometre is one millionth of a metre. A
nanometre is one thousandth of a micrometre.
For comparison, a human hair is typically
about 100 micrometres wide (although widths
vary). So a human hair would be about
100 000 nanometres wide.
To put this another way, there are 1000
micrometres in 1 millimetre, a measure you
can see on a ruler. So, about 10 human hairs
would fit across 1 millimetre. A millimetre is
one million nanometres.
Nanotechnologies
Nanotechnologies involve creating and using
structures in a controlled way at these tiny sizes:
nanoparticles are between 1 and
100 nanometres across.
For example, gold and silver are known to be
good catalysts. Nanoparticles of these metals
are found to be much stronger catalysts than
large particles.
25
Figure
1.20
Carbon nanotubes on a ‘host’ particle about
10 micrometres wide. The nanotubes are
the hundreds of tiny white ‘strings’ across
the host’s width
26
The large surface areas found at nano scales vastly improve
catalytic properties:
the ‘insides’ of nanoparticles are
so small that most of their atoms
are on the surface.
Billions of gold nanoparticles are
therefore much more catalytic
than a large single ‘chunk’ of gold
with the same number of atoms
(the same overall volume).
The same chemical element may
therefore have differing chemical
properties depending on its size.
Today, many useful nanoparticles
are being produced by design, such
as silver nanoparticles: these kill
bacteria in clothing that is prone to
developing a smell, e.g. socks.
Figure 1.21
Individually these ‘strings’ of gold are less than 100 nm wide, and are
therefore classified by scientists as a nano-scale technology
Modern sunscreens contain
nanoparticles of titanium oxide or
zinc oxide, which absorb UV light,
protecting the skin from the Sun’s
harmful UV rays.
27
Fullerenes and nanotubes
Carbon has other unusual structures apart from
diamond and graphite (see Section 2.3).
For example, fullerenes are chemicals made up
of repeating patterns of hexagonal and/or
pentagonal rings of carbon atoms.
Nanotubes are a type of fullerene — long tubes
of carbon atoms looped in a repeating structure
of carbon rings. They are:
excellent conductors of electricity — each
carbon atom is only bonded to 3 others rather
than 4; so each atom has a free (delocalised)
electron in its outer shell.
Figure 1.22
Buckminsterfullerene — the ‘buckyball’ — was
synthesised in 1985. It has the chemical formula
C60 and is slightly more than 1 nm in diameter
Figure 1.23
A carbon nanotube is built from a repeating pattern of
hexagonal rings of 6 carbon atoms
28
extremely strong, especially in
relation to their tiny mass.
Future uses of fullerenes — a
number of potential uses of these
nano-scale technologies are being
worked on:
Buckyballs may one day act as
delivery ‘vehicles’ for drugs stored
inside their hollow ball structure
until they reach their intended
destination inside the body.
Similarly, nano-gears could be
built as part of incredibly small
machines that may be propelled
through the human bloodstream
to directly attack pathogens in the
body.
Figure 1.24
A potential structure for nano-gears as imagined by NASA scientists
Nanotubes are already being used
to strengthen objects such as
tennis racquets.
29
1.6
Practice questions
1. (all boards)
a. Oil tankers sometimes flush out
their tanks at sea causing harm to
wildlife.
Scientists can identify the source
of the oil but first they must
separate a sample of the oil from
sea water.
Suggest a way of doing this.
i. State the temperature to which air must be cooled to turn
nitrogen, argon and oxygen into liquids.
ii. Explain how pure nitrogen is obtained from liquid air.
2. (all boards)
The table below shows some properties of three materials.
a. Answer the following questions:
i. Complete the table by putting a tick in one box in each row
to show the type of bonding in each chemical.
b. Look at the data in the table
about substances in the air.
Substance
Boiling point
(ºC)
Nitrogen
–196
Argon
–186
Oxygen
–183
ii. Which of the materials in the table would you be able to
shape by bending or pressing against it?
30
3. (all boards)
a. Which of the following is a typical property of
the transition metals?
b. Choose the correct words to complete this
description of the structure of metals. The
structure of metals is a regular arrangement of
positive / negative / uncharged ions in a sea
of delocalised protons / electrons / neutrons.
c. Explain why metals are good conductors of
electricity.
4. (all boards)
Nitinol is a shape-memory alloy of nickel and
titanium and is used in teeth braces. The diagram
shows the arrangement of atoms in alloys similar
to nitinol.
Figure 1.25
Chromium is a transition metal
A. They react with cold water to give off
hydrogen gas.
B. They have low melting points.
C. They form coloured compounds.
D. They are soft and easily cut with a knife.
31
a. Explain why nitinol is less malleable than either
pure nickel or pure titanium.
b. Describe how nitinol is used in teeth braces to
force teeth into new positions.
5. (all boards)
Magnesium oxide is a white crystalline
substance. The structure of magnesium oxide is
similar to sodium chloride and is shown here.
Key: magnesium = purple, oxide = green
a. Explain why the formula of magnesium oxide is
written as MgO even though there are no
individual particles containing only one
magnesium atom joined to one oxygen atom.
b. Explain why the melting point of magnesium
oxide is 3600ºC.
c. Complete the following table for a substance
similar to magnesium oxide by suggesting
‘yes’ or ‘no’ under ‘conducts electricity’.
State of material
Conducts electricity
Solid
Liquid
Dissolved in water
6. (all boards)
a. Graphite is a material that has some properties
similar to metals, some that are similar to
simple molecular materials, and some that are
similar to giant molecular materials like
diamond.
32
7. (AQA)
a. Poly(ethene) is a thermosoftening polymer. Melamine is a
thermosetting polymer. Complete the table here, ticking boxes
to show the properties and structure of thermosoftening and
thermosetting polymers.
Figure 1.26
i. In what property does graphite
resemble a metal? State a
reason for this similarity.
b. LDPE and HDPE are both types of poly(ethene). LDPE has a
lower density than HDPE. Explain how the two different forms
of poly(ethene) are produced.
ii. Explain why graphite is soft and
slippery, with characteristics
similar to simple molecular
materials such as olive oil.
c. Scientists are working on the use of fullerenes as catalysts: for
example, to remove hydrogen from hydrocarbons as a step in
manufacturing polymers.
i. State two ways that fullerenes and graphite are similar.
iii. Explain why graphite has a high
melting point like diamond.
ii. The fullerenes used as catalysts are examples of
nanoparticles. Explain what a nanoparticle is.
iv. Give a use for graphite and
state the property that is being
used.
iii. Give a reason why nanoparticles such as fullerenes are
effective catalysts.
tap for answers
33
Chapter 2
Forces, energy and
momentum
Syllabus
AQA
OCR
Coverage
2.2.1 Forces and energy
2.2.2 Momentum
P4.3.1–4.3.11 What is the connection between forces and motion?
21st Century
P4.4.1–4.4.14 How can we describe motion in terms of energy
changes?
Edexcel
4.1–4.18 Momentum, energy, work and power
2.1
Forces, energy, work and power
In this section (all boards) • forces, energy and
work • power
Forces, energy and work
Any force that moves an object over a distance
has transferred energy: from the source of the
force (e.g. a person) to the object.
Another way of saying this is that the force has
done work on the object.
So, in moving an object, work is a measure of
energy transferred.
Figure 2.1
Work is done when a force moves an object a
distance in the direction the force acts.
The work done on an object is the energy
transferred to the object (joules, J)
35
Figure 2.2
Power
Power is the rate at which work is done. ‘Rate’
refers to time — power is the work done (energy
transferred) in a certain time:
Example
A man pushes a wheelbarrow 70 m horizontally with
a force of 60 N. What is the work done by the man
on the barrow?
W=F×d
= 60 N × 70 m
= 420 J
One watt of power = 1 joule per second
Example
The man in the example above took one minute to
push the wheelbarrow. What was the power of the
man?
P=E÷t
= 420 J ÷ 60 s
=7W
36
2.2
Gravitational potential energy; kinetic energy
In this section (all boards) • gravitational potential
energy • kinetic energy
Gravitational potential energy
Any object that is lifted up, away from the centre
of the Earth, has gravitational potential energy,
GPE.
The Earth’s gravitational field is always pulling on
the object, with a force of 10 N/kg at the surface.
But the object does not necessarily fall: there
may be a counter force keeping it in place above
the Earth, such as the blue force of the table
shown in the picture.
Figure 2.3
The mug has more gravitational potential energy on
the table than it did on the floor, which is on the
Earth’s surface
37
GPE is stored energy: it has the ‘potential’ to be
transferred into other forms of energy. It is
sometimes written Ep.
GPE is energy that comes about because work
has been done raising an object a certain
distance against the force of gravity.
So GPE relates to the weight of the object —
weight is the force of gravity holding the object
down:
From Section 7.5:
weight (a force) =
mass × gravitational field strength.
Work done = GPE gained =
weight (force) × height (distance).
Figure 2.4 (Photo: Luca Galuzzi – www.galuzzi.it)
The GPE the girl gained by jumping is equal to the
energy lost when she falls to Earth — because the
distance she moves up and down is exactly the same:
energy transferred = work done against force of
gravity = GPE
Example
A mug of mass 200 grams is lifted half a metre onto
a table. What is the GPE of the mug on the table?
Ep = m × g × h ( or W × h )
Ep = 0.2 kg × 10 N/kg × 0.5 m = 1 J
38
The mug has gained one joule of GPE:
one joule of energy was transferred in
lifting it to the table.
Tip: It doesn’t matter if the mug was
lifted directly up or slightly sideways,
distance is always measured
vertically in the GPE equation. And
do not confuse ‘W’ as in ‘weight’,
measured in newtons (W = m × g)
with ‘W’ as in ‘work’, measured in
joules (W = F × d).
Objects have GPE on and near the Earth and elsewhere too:
other planets, our Moon, asteroids … they all have a
gravitational field and exert a force that depends on their size
and mass.
When an object is thrown or lifted
upwards it gains GPE. It loses GPE
as it comes down to Earth:
GPE gain equals the work done
by the force lifting the object, e.g.
a crane, a jet engine …
GPE loss equals the work done by
the force of gravity bringing the
object down to Earth.
Figure 2.5
An object’s GPE in Mars’ gravitational field is 0.4 times that of the
same object at the same height above the Earth: Mars’ field strength
is 4 N/kg compared to Earth’s 10 N/kg
39
Example
Kinetic energy
Kinetic energy, Ek, is the energy stored by a moving object; it
relates to the mass and velocity of the object.
The roller coaster here weighs 800 kg
and is moving at 20 m/s at this point.
What is its kinetic energy?
Kinetic energy is gained by an object when a force acting on it
makes it accelerate: e.g. a
person pushing a vehicle …
the force of gravity pulling
an object to Earth …
Figure 2.6
As soon as these base jumpers
leave the cliff, their GPE (Ep)
starts to transfer to kinetic energy
(Ek). The force of gravity acts on
the jumpers, doing work on them
Figure 2.7
40
Ek = 0.5 × 800 kg × (20 m/s)2
= 160,000 J
Before dropping, the roller coaster had risen 50 m.
What was its GPE at the top of the curve? And its GPE
at the present point (assuming no other energy
losses)?
… and all of a vehicle’s kinetic energy is
converted when the vehicle comes to a stop …
so braking energy = 0.5 × mv2
GPE at the top = m × g × h
= 800 kg × 10 N/kg × 50 m
= 400,000 J
Current GPE = (Initial GPE) – (GPE so far converted
to kinetic energy)
= 400,000 – 160,000 = 240,000 J
In other words, energy is conserved. In theory, all of
the GPE becomes Ek. However, in the real world,
some energy is lost as thermal energy — in friction
and air resistance — and also sound.
Edexcel — Braking energy is equal to the kinetic
energy of the vehicle:
braking energy = braking force × braking
distance ...
Figure 2.8
The kinetic energy transferred in a car crash increases
significantly with increased velocity: because Ek relates
to velocity squared (v2)
41
2.3
Momentum
In this section (all boards) • momentum •
conservation of momentum
Momentum
A resultant force acting on an object changes the
object’s momentum in the direction of the force.
Momentum is not a measure of force (force is
measured in newtons), nor of energy (measured
in joules) …
… it can be thought of as a measure of a moving
object’s ‘ability’ to transfer energy/force if it
collides.
Momentum is a vector quantity. The momentum
of an object is given by the equation here.
Figure 2.9
The forward force of these competitors’ arm
movements gives their wheelchairs momentum in
the direction of the force
42
Example
The leading athlete in the photo here
weighs 75 kg and is moving at 2 m/s.
What is his momentum?
same/opposite directions: this law of conservation of
momentum holds.
p=m×v
= 75 × 2
= 150 kg m/s in a forwards direction
Conservation of momentum
When a moving object collides with
another object, momentum is always
conserved, as long as no external
forces interfere in this ‘closed
system’ of two colliding objects.
The total momentum of the objects
in a collision is the same before and
after the collision.
It does not matter if one object is
moving and the other not, or if both
are moving, or if they move in the
Figure 2.10
‘Newton’s cradle’ comes close to demonstrating that momentum is
perfectly conserved in a collision. However, air resistance, a force
‘outside the system’, eventually stops the balls
43
Imagine a ‘car’ of mass 1 kg moving
towards another ‘car’ of mass 3 kg
along a frictionless air track,
colliding ‘perfectly’ with the second
car so that they move on together.
The momentum before the
collision is the total momentum of
the cars:
(1 kg × 10 m/s) + (3 kg × 0 m/s)
Figure 2.11
So total momentum before
collision = 10 kg m/s
(because 3 × 0 = 0)
… and this equals total
momentum of the two cars after
the collision: 4 kg × v3
Figure 2.12
From this you can calculate v3 :
10 kg m/s = 4 kg × v3
So v3 = 10 kg m/s ÷ 4 kg
= 2.5 m/s
44
Example
In this scenario, v1 = 5 m/s, v2 = –3 m/s and v3 = –4 m/s. What is
v 4?
Working: You know total momentum
before the collision is equal to the total
momentum after ...
Momentum before collision =
(6 g × 5 m/s) + (12 kg × –3 m/s)
Momentum after collision =
(6 kg × –4 m/s) + (12 kg × v4)
30 + –36 = –24 + (12 × v4)
–6 = –24 + (12 × v4)
Figure 2.13
–6 + 24 = 12 × v4
18 = 12 × v4
v4 = 18 ÷ 12 = 1.5 m/s
Figure 2.14
45
Explosions are another type of
momentum-changing event:
Imagine that two skateboarders
with the same mass stand
together and then push each
other away.
When they are standing still their
total momentum is zero.
When they push each other away
their total momentum is still zero.
Their individual momentums are
equal in size, opposite in
direction.
Figure 2.15
The total momentum of the skateboarders is zero before and after
the collision, because momentum is a vector. They move away
from each other with the same velocity, but opposite momentums.
One is positive, one is negative: total = 0
46
2.4
Momentum and vehicle safety; changes in
momentum
In this section (all boards) •
momentum and vehicle safety •
changes in momentum
Momentum and vehicle safety
The more momentum an object has:
the more damage it does to
objects it collides with. Collision
force is highest when the object is
(a) heavy (b) moving at a large
velocity (c) stops in a short time.
the more damage it does to
objects moving in it or on it.
These objects – for example,
people travelling in a car – are
moving at the same velocity, but
their individual momentums relate
Figure 2.16
The Barringer crater in Arizona is about 1200 m wide, yet was
created by the impact of a meteorite only 50 m wide. The meteorite
was travelling at 40,000 km/h or more when it hit the ground — its
momentum was huge and it stopped very quickly
47
to their individual masses.
When a car is in a collision, the force
acting on its occupants causes their
injuries. The size of the force
depends on the speed of the car
and the time the car takes to stop.
Increasing the value of t always decreases a, and this
decreases F.
Vehicle safety features aim to
increase the time it takes for the
vehicle to come to a stop.
These reduce the force of
deceleration on the passengers
because:
F = ma, and a relates to speed
and time [a = (v – u) ÷ t ] …
F = m(v – u) ÷ t = (mv – mu) ÷ t
So the longer the car takes to
stop, the lower the forces
involved.
Figure 2.17
In a fast motorcycle accident the rider continues moving forward
even if the bike has stopped. An airbag would slow the rider down in
a longer period of time. The force on the rider is reduced; injuries
should be less severe
48
These safety features help in a
crash:
seat belts hold each occupant in
place. Modern seat belts are
designed to stretch a little,
increasing the time it takes the
wearer to come to rest,
decreasing the force on them.
air bags increase the stopping
time of the occupants’ heads.
Figure 2.18
The purple car has crumpled less than the yellow car.
This does not mean the yellow car’s accident was
necessarily ‘worse’, i.e. involved a greater change in
momentum. The yellow car’s crumple zone may have
done a better job, taking longer to crumple
front and rear crumple zones and
side impact bars also increase the
stopping time. The vehicle’s
kinetic energy is transferred in
bending and compressing the
crumple zones.
Changes in momentum
To change the velocity of an object,
a force must act on it.
49
So when a force acts on an object the
momentum of the object changes.
OCR — Change in momentum is proportional to
the resultant force acting on the object, and the
amount of time the force was acting.
Edexcel — Force is the rate of change of
momentum.
Figure 2.19
Example
Notice these two equations essentially state the
same thing:
p = F × t
so F = p ÷ t
and ‘p’ is equivalent to (mv – mu).
A motorcycle weighing 250 kg comes to a rest in 3
seconds during an accident. What force was acting
on the motorcycle if its initial velocity was 30 m/s?
F = (mv – mu) ÷ t
So F = ([250 × 30] – 0) ÷ 3
= 7500 ÷ 3
= 2500 N
50
2.5
Practice questions
1. (all boards)
A rollercoaster of mass 9000 kg accelerates
along a track to a speed of 20 m/s.
a. Show that the final kinetic energy of the
rollercoaster is about 2 MJ.
b. Calculate the work done on the roller coaster.
c. If this work was done in a time of 5.0 seconds,
what average power was developed?
d. The rollercoaster then rolls freely up through a
vertical height of 11 metres on the track. What
is the gain in its gravitational potential energy?
Assume g = 10 N/kg.
e. Calculate the final kinetic energy of the
rollercoaster, stating any assumption that you
make.
2. (all boards)
Figure 2.20
A car of mass 1000 kg is travelling north with a
velocity of 20 m/s. The driver makes an
emergency stop when a deer suddenly appears
on the road. She brings the car safely to rest in a
time of 4.0 s.
51
a. Calculate the initial momentum of
the car and state its direction.
b. Calculate the size of the force on
the car while it is stopping, and
state its direction.
c. Answer the following questions:
i. Explain, using the ideas of
change in momentum and
force, how a seat belt can
protect the occupant of a car in
the event of a severe impact.
ii. Name two other features found
in a modern car which are also
designed to protect occupants
in a similar way.
The kinetic energy is
constant
The weight is greater
than the lift
The net force on the
aeroplane is zero
The potential energy is
constant
Engine thrust is greater
than drag
b. Draw a simple free-body diagram for the aircraft showing all
the forces on it.
3. (all boards)
An aircraft of mass 50 000 kg is
preparing to land. It is travelling in a
straight line at a constant speed,
and is losing height at a steady rate.
a. Which of the following
statement(s) are true? Choose all
that apply.
Figure 2.21
52
c. Calculate the weight of the aircraft. Assume g
= 10 N/kg.
d. The speed of the aircraft is 70 m/s and it is
flying at a height of 500 m. Calculate:
i. its kinetic energy. Kinetic energy = ½ × mass
× speed2
ii. its gravitational potential energy at this
point. Give your answers in MJ.
e. After the plane lands, both the kinetic and
potential energy are reduced to zero. Explain
what happens to the energy calculated in part
(d).
4. (all boards)
The Highway Code gives the following data
relating braking distance (in metres) to speed of a
car (in miles per hour) on a flat dry road:
Speed in miles per
hour
Braking distance in
metres
20
6
30
14
40
24
50
38
60
55
70
75
A physics teacher states that braking distance
should be proportional to the square of the
speed.
a. Suggest, using the ideas of work done and
kinetic energy, why this might be true. When
the car is moving it has kinetic energy, but
when it has stopped the kinetic energy is zero.
The change in kinetic energy is equal to the
initial kinetic energy.
b. A car of mass 1500 kg is travelling at 50 mph.
53
Show that the car’s speed is about 22 m/s,
given that 1 mile = 1.6 km.
Speed = (50 miles × 1.6 km × 1000 m/km) ÷
(60 seconds × 60 minutes) = 22.2 m/s
c. Calculate the kinetic energy of the car in joules.
d. Use the stopping distance from the table to
calculate the average braking force.
5. (all boards)
In a small hydroelectric power scheme, water is
held in a reservoir and then allowed to fall
through a height of 12 m. The falling water drives
a turbine which turns an electrical generator.
a. The mass of 1.0 m3 of water = 1000 kg.
Calculate the potential energy of 1.0 m3 of
water in the reservoir. Assume g = 10 m/s2.
b. After passing through the turbine the speed of
the water is 10 m/s. Calculate the kinetic
energy in 1.0 m3 of water leaving the turbine.
c. For each cubic metre of water, how much
energy has been transferred into other forms
by the turbine?
d. 15 m3 of water fall through the turbine every
minute. Calculate the power output of
the generator if the process is 80% efficient.
6. (all boards)
An ice skater of mass 60 kg is travelling across
ice in a straight line at a constant speed of 5 m/s.
a. Calculate the momentum of the skater.
b. She accidentally collides
with another skater of
mass 90 kg who is
stationary. The skaters
cling together and move
along in the same
direction as the first
skater was travelling.
Figure 2.22
Explain why the momentum of the skaters is
conserved in this collision.
c. Calculate the speed at which the skaters move
off together.
d. Show that there is a loss of 450 J of kinetic
energy in this collision.
e. Both momentum and kinetic energy involve
mass and velocity. Explain why momentum is
conserved but kinetic energy is not, in this
scenario.
tap for answers
54
Chapter 3
Organisms and their
environment
Syllabus
AQA
OCR
21st Century
Edexcel
Coverage
B2.3 Photosynthesis
B2.4 Organisms and their environment
B4.1.1–4.1.4 How do chemical reactions take place in living things?
B4.2.1–4.2.4, 4.2.14–4.2.17 How do plants make food?
B5.1.12–5.1.14 How do organisms develop?
2.13–2.16 Organisms and energy
3.1
Photosynthesis
In this section (all boards) •
photosynthesis • leaf adaptations for
photosynthesis • glucose and starch
Photosynthesis
Plants and algae can make their
own food, using photosynthesis.
Four things are essential to the
process:
carbon dioxide
water
sunlight
chlorophyll.
Figure 3.1
How photosynthesis makes glucose in plants. Photosynthesis is
the basis of land-based life — all food chains start with plants, the
‘producers’
56
The equation for photosynthesis is:
Tip: Glucose is a carbohydrate, a ‘fuel’ for the
plant. Oxygen is a by-product that plants release
into the atmosphere
Figure 3.2
The huge green streaks here are ‘blooms’ of
phytoplankton (inset), the producers in aquatic food
chains. They are making their own food by
photosynthesis (like algae); the CO2 needed is
dissolved in the sea
Leaf adaptations for photosynthesis
Leaves are fully adapted to maximise
photosynthesis:
their palisade cells are packed with
chlorophyll — photosynthesis is impossible
without it.
57
the lower leaf has air space for
CO2 to diffuse in and O2 to diffuse
out. The stomata open for this.
xylem brings water to the leaves;
phloem takes glucose to the
plant.
Glucose and starch
Glucose is a major source of energy
for plants, and the animals that eat
those plants, such as humans.
Figure 3.3
As usual, CO2 diffusion into the leaf and O2 diffusion out of the
leaf result from random movement of these molecules: from an
area of higher concentration to an area of lower concentration
Figure 3.4
Ball-and-stick model of glucose,
C6H12O6
58
Glucose molecules are broken down
in respiration, releasing energy
needed by plant cells.
Apart from its role in respiration,
plants also use glucose to make:
oils and fats for energy storage.
amino acids, using nitrates from
the soil; the acids then make
proteins for plant growth.
chlorophyll and cellulose in cells.
starch, a carbohydrate made from
long chains of glucose.
Glucose is converted to starch for
storage all over the plant. It is
reconverted to glucose when
needed.
For example, plants need to respire
all day and all night. But at night
there is no light for photosynthesis
— glucose cannot be made.
Figure 3.5
This callery pear tree needs to respire at night just as it would in the
daytime: it gets its energy from starch, which is re-converted to
glucose for night respiration
59
There are biological benefits to
converting glucose to starch —
glucose is soluble, starch is not.
So, storing glucose would interfere
with other osmosis-driven processes
in the plant:
glucose stored in cells would tend
to dissolve in the cell’s water …
… and even more water would
move into the cell by osmosis:
from the dilute solution of water/
nutrients in the stems, to the
concentrated glucose solution in
the cells.
Some plants have specialist organs
for storing starch, e.g. tubers
(potatoes) and bulbs (onions).
Figure 3.6
Sweet potatoes (distant relatives of the potato) are full of starch —
an energy source in the human diet, as well as an energy store/
source for the plant itself
60
Tip: Textbooks often talk of plants
making their own ‘food’: in this
context ‘food’ is synonymous with
glucose and starch.
Iodine test for starch — Iodine
solution is yellowish-brown; it turns
dark blue when it reacts with starch.
The presence of starch in a plant is
proof that photosynthesis has
occurred: hence ‘the iodine test’.
To prepare a leaf for the iodine test,
the chlorophyll needs to be removed
by soaking the leaf in hot alcohol
(e.g. ethanol) for a few minutes.
Figure 3.7
The leaf on the left has been tested with iodine solution. It contains
starch because it has been photosynthesising. The leaf on the right
was taken from a tree during the night: its starch stores were reconverted to glucose after sunset and it doesn’t respond to the iodine
test
61
3.2
Factors affecting photosynthesis
In this section (all boards) • limiting
factors in photosynthesis • a simple
greenhouse • commercial greenhouses
• phototropism (OCR)
temperature — rates of chemical reactions are usually faster
at higher temperatures, and photosynthesis is no exception.
Limiting factors in
photosynthesis
Different environmental conditions
affect the rate of photosynthesis.
These limiting factors include:
light intensity — chlorophyll
absorbs light, driving the
chemical reactions of
photosynthesis.
availability of CO2 — one of the
Figure 3.8
As light intensity increases so does photosynthesis: but only up
to the point where other factors inevitably limit it
two chemicals needed for
photosynthesis, along with water.
62
These limiting factors work
in combination: they each
affect the rate of
photosynthesis, continually.
For example, a more than
warm enough plant in an
atmosphere with more than
enough CO2 will barely
photosynthesise in low light.
Sunlight is the limiting factor
here, because the other two
factors are abundant.
A UK spring scenario may
be:
Figure 3.9
Early on a frosty morning, plants
will not photosynthesise well. The
temperature needs to increase
significantly, and sunlight — even
the grey light of a cloudy day —
must reach the plant’s chloroplasts
early morning — weak sunlight, cold temperature, high CO2
concentration: plants do not photosynthesise at night so the
CO2 near them stays in the atmosphere.
early afternoon — strong sunlight, warm temperature, but
Figure 3.10
Stomata on the undersides of plant
leaves open in response to light, but
also close on hot days to stop water
loss — in which case, CO2 cannot
enter the leaf. So stomata can both
help and hinder photosynthesis,
depending on conditions
decreasing CO2 concentration, as this has been taken in by
plants all morning.
63
A simple greenhouse
Farmers want their plants to grow as quickly and as large as
possible ... and preferably all year round.
A greenhouse will affect the way
plants grow:
It is an environment that protects
plants from extreme weather.
It is an environment that is
managed by the owner.
In greenhouses plants can grow
more quickly and produce higher
yields, because some or all of the
factors involved in their growth —
warmth, water, carbon dioxide, light
— may be controlled.
However, it is not practical to grow
everything we need in greenhouses.
Major food crops like maize and
wheat are grown in huge fields.
Figure 3.11
Even a simple polytunnel greenhouse affects the plants kept inside.
It traps warmth — especially in the winter — increasing the rate of
photosynthesis
64
Figure 3.12
It would be too costly to build
greenhouses for this amount of maize; the
crop is therefore more vulnerable to
extreme weather such as drought
65
Commercial greenhouses
Commercial greenhouses control the limiting
factors: an optimum balance between
temperature, light, and extra carbon dioxide is
achieved.
Figure 3.13
Plants can photosynthesise under domestic lighting —
but this type of light is weak. In industrial greenhouses
strong artificial lights are used, emitting the
wavelengths best suited to photosynthesis
Commercial greenhouses have advantages and
disadvantages:
Advantages
Disadvantages
‘Economies of scale’
are achieved: in the
same physical space,
more can be produced.
High initial costs: e.g.
build a large
greenhouse, install
high-tech equipment,
train specialist staff.
The computercontrolled greenhouse
works 24 hours a day
with only a few staff
and is unaffected by
extreme weather.
Artifical lighting and
temperature control
use electricity, an
ongoing cost — and a
cost to the
environment unless
the energy is
sustainable.
The produce is made
ready for sale at
optimum speed and
maximum yield: a
profitable result.
The produce may not
be as natural (nutrition,
taste, texture) as an
‘organically’ grown
plant.
66
Phototropism (OCR)
The term used to describe plant growth movement is ‘tropism’.
The way plants grow in response to light is called
phototropism.It is the tips of plants that respond:
Auxins move to the side of the shoot away from the light,
making the cells on that side grow longer.
The plant now bends towards the light — the cells on the light
side are now relatively shorter.
Figure 3.14
Hydroponic vegetables are grown
directly in mineral-rich water. The
quality of the final product now
depends more on the producer than
on nature
Figure 3.16
Figure 3.15
Auxins are hormones that are key to
phototropism
Lentil sprouts lean
towards sunlight as
they grow
67
3.3
Distribution of organisms
In this section (all boards) • distribution of organisms • role of
temperature • need for nutrients • need for light • need for oxygen
Distribution of organisms
Life on Earth requires the presence of things like nutrients,
warmth, light, oxygen, carbon dioxide.
Different plants and animals need these to different degrees.
These differences explain how plants and animals are
distributed on Earth.
But first and foremost, the availability of water is crucial to
whether life can exist at all.
Water is essential to life’s key processes. To name just a few:
photosynthesis in plants ...
respiration in both plants and animal cells ...
Figure 3.17
The plains zebra is found in many parts of
Africa; it sometimes migrates hundreds
of miles in search of both food and water
proper functioning of enzymes.
68
Role of temperature
Plants and animals are adapted for their habitat’s temperature and are distributed accordingly.
If they can live in a wider temperature range they may be distributed over a larger area.
For example, we normally associate penguins with Antarctica, but their distribution is much wider.
Figure 3.18
Penguins are adapted for the cold, and there are fairly cold waters in many parts of the
Southern Hemisphere, with plenty of fish food.
69
Need for nutrients
Carnivores get nutrients from the flesh of other
animals, as do omnivores. Omnivores also eat
plants. Detritivores mainly feed on dead or
decomposing matter.
For example, the Australian Nuytsia floribunda is
a parasite. The soil conditions in Western
Australia are poor, so this tree gets many of its
nutrients from other plants nearby.
Plants get nutrients from the soil, e.g. nitrogen,
essential in building proteins. However, many
soils lack the necessary nitrate compounds …
Figure 3.20
Figure 3.19
Banana slugs are detritivores, feeding on fallen
leaves, animal droppings and fungi on the floors
of forests
The roots of this Nuytsia have extra
extensions called haustoria. These attach
themselves to the roots of nearby plants,
drawing water and nutrition from them
70
Figure 3.23
Plants use sunlight for photosynthesis. In dense
forests plants are distributed very close together,
Figure 3.21
The proteins in animal flesh are rich in nitrogen that
originated in plant roots, when these absorbed
nitrates from the ground. The Venus Fly Trap is an
unusual plant: it gets some of the nitrogen it needs
above ground, from the insects it traps
Need for light
Animals benefit from sunlight if it helps to keep
them warm. They also benefit from sunlight
because of its role in photosynthesis. After all,
food chains start with plants.
Figure 3.22
This Estonian forest is dense with trees, plants,
fungi, animals … because the ground is rich in
nutrients. This gives rise to fierce competition:
here trees grow extra tall in search of sunlight
71
but still find ways to reach the light.
Epiphytes are plants that live high up on trees in
an effort to reach sunlight.
They are not parasites: they do not take
nutrients from their host.
They obtain their water and nutrients from rain,
moisture, animal droppings, and detritus falling
from higher up the tree.
Less than 5 percent of sunlight reaches the forest
floor:
some plants can survive here, e.g. ferns and
mosses, which photosynthesise in low light.
the shady floor suits the distribution of fungi
(singular: fungus) — life forms that feed on
detritus: they do not photosynthesise.
Figure 3.24
Figure 3.25
Epiphytes thriving high on a Puerto Rican tree, where
they can reach light that barely penetrates to the
forest floor
Fungi contribute to the cycle of death and decay,
releasing enzymes that break down the dead leaf,
absorbing its nutrients
72
Need for oxygen
Animals take in oxygen from the
atmosphere using their lungs. The
oxygen available depends on the
habitat’s height above sea level:
Oxygen is about 21% of the
Earth’s atmosphere by volume ...
But this volume falls as you move
up through the atmosphere ...
At 3000 metres there is only about
70% as much air as at sea level.
(Oxygen is still 21% of this
volume.)
Some animals are adapted to low
oxygen environments. Yaks have big
hearts and lungs, to pump as much
oxygen as possible around the body.
Figure 3.26
Yaks have very few sweat glands: they do not need them. They are
distributed at altitudes around 3000 metres or higher, and would be
very uncomfortable in a lower, warmer habitat
73
3.4
Measuring distribution
In this section (all boards) • purpose of sampling • sampling
techniques • mean, median and mode • validity and reproducibility
The purpose of sampling
Scientists study the distribution and population size of
organisms for a variety of reasons, for example:
to assess the impact that a new housing scheme or road may
have had on a habitat.
to assess the impact of climate change on the local
environment.
Tip: Appreciate the difference between habitat — ‘the place
where particular plants and animals live’ — and environment —
‘the things that surround those plants/animals, e.g. the local
climate, soil conditions, availability of water, presence of other
creatures …’
Figure 3.27
This gazelle’s habitat, the desert, is a
harsh environment where there is little
water
74
Sampling techniques
A simple way to sample an area is using a
quadrat: a square frame of a known dimension
Meanwhile a line transect measures population
size along a line. A quadrat is used repeatedly at
fixed intervals along the line.
(e.g. 1 m2). The number of specimens found
within the area of the quadrat is counted.
Taking readings
Figure 3.28
Figure 3.29
A quadrat can be divided into further sub-frames to
make it easier to count the population.
A line transect can measure (a) the changing
distribution of one organism with distance, or (b)
what different organisms live in different conditions,
if those conditions change along the line
75
Other factors can be measured at the same time
along the line, e.g. soil pH, temperature, light
intensity ...
So a transect can tell us about the distribution of
organisms in different conditions, e.g. moving
from the sea (Figure 14.29).
A transect requires precise readings, not random
readings: it is used to find out how distribution
varies with distance, so measurements need to
be taken at exact intervals.
Edexcel — Other sampling instruments include
the sweep net, the pitfall trap and the pooter.
OCR — A light meter accurately measures light
intensity, e.g. in a woodland transect, to see if
there is a correlation between available light and
the distribution of organisms.
OCR — An identification key can help to identify
specimens; it is a visual reference with images of
common plants/animals in the area.
Figure 3.30
Using a sweep net
76
Mean, median and mode
Sampling only covers a small part of
the area being investigated. To
estimate an area’s total population:
The average population of the
samples taken is calculated.
This number is extrapolated:
multiplied in proportion to the total
area under investigation.
Mean, median and mode are
different types of ‘average’:
mean is the total of all the values
divided by the number of values.
median is the middle value of the
range (a single value).
mode is the value that occurs
most often.
Example
The table shows the snail samples found in a survey of a 100 m2
field. The 1 m2 quadrat was positioned nine times:
The mean number of snails found is:
(21 + 16 + 14 + 18 + 17 + 16 + 19 + 12 + 20) ÷ 9 = 17 per
quadrat.
Extrapolating, the population estimate for the field is 1700 snails
(17 × 100).
The median is the middle number:
12, 14, 16, 16, 17, 18, 19, 20, 21.
So the median number of snails per quadrat happens to
coincide with the mean number, too: 17.
The mode is 16 snails/m2: it occurs twice; every other total
occurs once.
77
Validity and reproducibility
Sampling techniques need to
ensure:
that the results are valid: as true a
representation as possible of the
situation being investigated.
that the technique is reproducible:
so that other scientists could
carry out the same survey in the
same place at a later date — to
confirm (or not), or add to, the
findings.
Sample size should be as large as
possible: e.g. to estimate snail
population in a field 100 m2, the
estimate will be more accurate after
extrapolating from a survey of 25 m2
(25% of the whole field) than from a
Figure 3.31
Using a quadrat along a transect line
survey of 5 m2 (5% of the whole
field).
78
It may need to be a random sample.
When using a quadrat, there should
be no deliberate decision about
where to position it. This ensures an
unbiased sample: not consciously
chosen by the researcher.
Make sure that conditions are
controlled, so that the experiment
could be repeated by other
scientists at a later date. These
would include:
using the same equipment
taking the same number of
readings
doing the experiment at the same
time of year, e.g. if plants are
seasonal, or animals seasonally
migrate, it is useless repeating the
experiment in a different season.
Figure 3.32
The flowers in this bed are not evenly distributed. To estimate the
total population it may be tempting to place a quadrat in certain
areas to ‘compensate’ for underpopulation, but this would be a
biased technique. Better to decide ‘blind’ where to position the
quadrat, e.g. throwing it onto the bed at random
79
3.5
Practice questions
1. (all boards)
a. Which is the correct word equation for photosynthesis?
A. carbon dioxide + glucose → oxygen + water + energy
D. low temperature.
d. The photo shows a stoma on the
underside of a leaf.
B. oxygen + water + light energy → carbon dioxide + glucose
C. carbon dioxide + water → oxygen + glucose + energy
D. carbon dioxide + water + light energy → oxygen + glucose
b. During photosynthesis:
A. light energy is absorbed by carbon dioxide in the leaf.
B. light energy is absorbed by chlorophyll in the leaf.
C. carbon dioxide is released as a by-product.
D. light energy is used to convert oxygen into glucose.
c. The rate of photosynthesis is not limited by:
A. shortage of light.
B. low oxygen concentration.
Figure 3.33
i. What is the plural of the word
‘stoma’?
C. low carbon dioxide concentration.
80
ii. Which product of photosynthesis would diffuse out of the
leaf via the stoma?
a. State the independent variable in
each of the three graphs.
iii. Describe two characteristics of carbon dioxide diffusion
into the leaf when the stoma is open.
b. What does the shape of the curve
in each graph tell you about how
the rate of photosynthesis is
affected by each factor?
2. (all boards)
In an investigation into the effects of different factors on the rate
of photosynthesis, the results shown in these three graphs were
obtained.
c. Explain why the curve in graph C
eventually falls to zero.
d. What is the name given to factors
that restrict the rate of
photosynthesis?
e. Describe the optimum conditions
for growing tomatoes in a
greenhouse.
3. (all boards)
a. Describe two uses of the glucose
produced by plants in
photosynthesis.
Figure 3.34
A.
b. The structure of a leaf is specially
adapted for maximum
photosynthesis. State and explain
two of these adaptations.
81
c. Complete the blanks to describe the four
different tissues indicated in this photo of a
leaf:
4. (all boards)
a. The glucose made in photosynthesis is
converted to starch for storage. Why does it
need to be converted from glucose?
b. The presence of starch in a leaf can be used as
an indication that photosynthesis has taken
place.
i. Which chemical is used to test for starch?
ii. What colour is a positive result in this test?
iii. So that you can see the results of the starch
test you first have to remove the green
colour (chlorophyll) from the leaf. How do
you do this?
Figure 3.35
c. Which of the following statements about the
photo is not true?
The external _______ tissue (1) allows
substances to diffuse in and out of the leaf.
These substances contribute to, or are byproducts of, photosynthesis, which occurs in
the _______ tissue (2). _______ tissue (3) brings
water and nutrients up from the roots, while
_______ (4) tissue transports food (sugars)
made by photosynthesis into the rest of the
plant.
Figure 3.36
82
A. The tree is respiring.
b. A healthy seedling was illuminated from one side as shown.
B. Glucose is providing energy for
respiration.
C. A large amount of starch is
being converted to glucose in
the tree’s cells.
D. A large amount of glucose is
being converted to starch in the
tree’s cells.
5. (OCR)
a. Green plants tend to grow
towards the light.
i. What is the general name for a
plant growth movement?
ii. What is plant growth towards
light called?
iii. What chemical is involved in
these growth movements?
Figure 3.37
i. Draw the shape of the seedling after 24 hours of
illumination from one side. On your diagram, mark the area
on the shoot where you would find the growth chemical.
ii. How does the growth chemical affect the growth of the
shoot?
83
6. (all boards)
a. Answer the following questions:
i. State three physical factors that can affect
the distribution of organisms.
ii. The photo shows a bluebell wood in
England in May. Which one of the following
factors is not a good reason to explain why
this particular species is able to thrive here?
C. There is plenty of direct sunlight all year
round.
D. Water and carbon dioxide are available in
the soil and air.
b. Answer the following questions:
i. You want to find out about the distribution of
daisies on a school field. What piece of
equipment would you use to count the
numbers of daisies in different places on the
field?
ii. Should the quadrat be positioned
deliberately or randomly to take the
readings? Explain your answer.
iii. In the diagram, how many daisies would you
record as being present in the 1 m2 frame?
Figure 3.38
A. Nitrates are available in the immediate
environment.
B. The pH of the soil is suited to this particular
species.
Figure 3.39
84
c. The following values are the number of daisies
counted in a 1 m2 frame placed randomly 11
times over the school field.
5
7
14
7
8
11
7
1
3
6
6
i. Calculate the range. Show your working.
ii. Calculate the mean. Show your working.
iii. Calculate the median. Explain your working.
iv. Calculate the mode. Explain your working.
tap for answers
85
Appendices
Appendix I
The Periodic Table
Tap to zoom
111
Appendix II
Hazard symbols
In the laboratory, safety should be at the front of
your mind. Always wear protection appropriate to
the experiment, such as goggles, a lab coat,
protective gloves.
You should also understand the meanings of
common hazard symbols: check for these and
handle the chemicals appropriately.
Corrosive. Often used to describe strong
acids and alkalis. For example, sodium
hydroxide (NaOH), also called 'caustic
soda', can burn the skin. Alkalis like this
can be just as harmful as acids.
112
Appendix III
Electrical symbols
113
Appendix IV
Exam tips
A 10-point exam preparation plan • before the
exam • during the exam
Before the exam
Plan your revision schedule – and stick to it.
How many weeks of revision do you have?
Decide how much time you will revise each
week, and divide that time between your
subjects. Your weaker subjects will need more
time than your stronger ones. A revision
notepad/book is essential.
Plan your revision method – Decide what
resources you will use for revision – your
textbook, this App, past exam papers – and how
you will use them. For example, you might
quickly read up on a topic in the morning, on the
train or bus, using the App. Then you might make
your own additional notes in the evening using
your textbook. As the exam approaches you will
want to concentrate more on actual practice,
tackling exam-style questions.
Revise in short periods of 45 minutes to 1
hour – Take a break for a drink, lunch or dinner.
Get away from your desk for 15 minutes or half
an hour – when you come back you will be
refreshed. You may want to revise 3 or 4 hours
on a Sunday afternoon: breaking this time up will
make it seem less daunting.
Give yourself an even longer break! – Time off
helps you relax and will make your concentrated
study periods more productive: your normal life
should continue! Take time to socialise with
friends, play sport, watch a film, whatever – find
a bit of time once a week to do something you
enjoy.
114
Attempt practice exam questions under exam
conditions – Attempt the whole question in one
go. If your exam paper is a 60-mark paper and
your practice question contains 6 marks, the
question should take you about 5–6 minutes.
Check your answers; clarify any mistakes or
points of confusion with your teacher – Do this
soon after you have tackled the question, while it
is fresh in your mind. Your teacher will be able to
show you the weaknesses in your answer.
During the exam
Skim the whole question paper before
attempting any questions – This should only
take you a couple of minutes. It will give you a
complete sense of what is in store, e.g. how
many marks are available for each question and
which questions you think are more difficult.
Take care to understand what the question is
looking for, based on its wording – ‘State …’ is
looking for a short factual answer; ‘Describe …’
is looking for a more detailed answer; ‘Explain ...’
is looking for deeper knowledge and
understanding; ‘Suggest …’ may be looking for
ideas that may require some creative thinking –
remember these will need to be based on
scientific evidence.
If you get stuck on a question, do not dwell on
it for more than a minute or two – otherwise
you will quickly lose valuable time! Move on to a
question that you feel confident about: this will
help you accumulate marks and maintain your
rhythm; you can always go back to the tough
question towards the end of the exam.
Short notes, spider diagrams and sketches
can help you gain marks if you are running out
of time – In the last few minutes of the exam, you
can still show the examiner your knowledge and
understanding of the question in a more visual
way, rather than using full sentences and
paragraphs.
115
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© Damian Bohle 2013
ISBN: 978-0-9574703-8-5
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