Download Chemical Reactions (Part One)

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Safety precautions
When scientists carry out experiments, they have to take
safety precautions. This is to make sure that they minimize the
risk of harming themselves, other people and the environment.
What kind of safety
precautions should
you take in the lab?
What kind of safety
precautions should
chemists working in
industrial labs take?
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What are hazard symbols?
When working in a lab, how can you tell which chemicals are
safe and which are dangerous?
Hazard labels are used on containers and vehicles that
contain dangerous chemicals.
toxic
irritant
harmful
corrosive
flammable
These hazard symbols show why the chemical is dangerous.
Why is it important that symbols are used, rather than words?
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What do hazard symbols mean?
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What do hazard symbols mean?
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Everyday reactions
Chemical reactions happen all around us, not just in the lab.
cooking
making materials
sticking
burning
rusting
living
Can you think of more everyday chemical reactions?
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How can you spot a chemical reaction?
Lots of changes happen in chemical reactions.
You might recognize that a chemical reaction is happening
because the substances involved:
 change colour
 give off a gas
 get hot
 get cold
 give out light
 make smells
 form solids.
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Chemical reactions are hard to reverse
Chemical reactions are usually difficult to reverse.
For example, magnesium burns in oxygen to form magnesium
oxide. It is not at all easy to ‘un-burn’ the magnesium once it
has been burnt.
magnesium
+
oxygen

magnesium oxide
Many reactions need an input of energy to get them started.
This is called activation energy.
Many reactions (like the burning of magnesium) also release
energy once the reaction has started.
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Odd-one-out
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Non-useful chemical reactions
Chemical reactions in food cause
it to decay, making it unsafe to eat.
Chemical reactions like these are
a nuisance, but other reactions
can even be dangerous.
The reaction between iron and
oxygen causes the metal to become
rusty, which weakens the structure
and makes it dangerous to use.
Factories sometimes produce waste gases that pollute the
atmosphere. These gases cause acid rain and might
contribute to the greenhouse effect.
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Useful chemical reactions
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More useful chemical reactions
Chemists carry out chemical reactions to produce new
substances that improve people’s lives.
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The chemistry of food
There are many chemical reactions involved in our food: how
many can you think of?
 cooking
 ripening fruit
 food going mouldy
 making fertilizers and pesticides
 photosynthesis for plants to grow
 digestion when you eat food
 fermentation to make bread, beer and wine.
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Cooking
Cooking involves chemical reactions. Changes in colour, taste
and texture are due to the molecules in food joining together in
new ways.
Cooking an
egg changes
its texture from
runny to firm.
Eggs contain a protein called albumen. The protein molecules
are long chains of amino acids folded into a ball shape.
When eggs are heated, some of the proteins break apart
and the molecules unfold. These molecules then join to other
nearby protein molecules until they are all linked in a network.
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Making bread and alcohol
Yeast is a living organism that carries out the chemical
reactions that are used for making bread and alcohol.
Yeast uses oxygen from the air for aerobic respiration.
sugar
C6H12O6
+
oxygen  carbon dioxide
6O2
6CO2
+
water
6H2O
What effect do you think the carbon dioxide gas has on the
bread?
Yeast can also carry out respiration without oxygen. This is
called anaerobic respiration, or fermentation. This is used to
produce the alcohol in beer and wine.
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Ripening fruit
The ripening of fruit is a complex collection of chemical
reactions. Take apples as an example:
 Starch is broken down into sugars, increasing sweetness.
 Acids are neutralized,
making the apples less sour.
 Chlorophyll (green) changes
to anthocyanin (red).
 Pectin, a chemical that makes
apples hard, is broken down,
making the apples softer.
Why do food producers and supermarkets need to know
about the conditions and reactions involved in ripening fruit?
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What causes ripening?
The ripening of all fruit and vegetables involves similar chemical
reactions.
The speed of ripening is affected by
the temperature and by the presence
of a chemical called ethene, C2H4.
Food scientists can tell producers
and supermarkets the best
conditions for slowing down or
speeding up the ripening process
so that fruit and vegetables arrive
in the shops perfectly ripe.
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Food spoilage
The changes that take place as food ‘spoils’ are also caused
by chemical reactions.
For example, peeled apples turn brown when exposed to
the air because they react with oxygen in the air.
Knowing what causes this reaction
can help to slow it down.
Placing sliced apples in water
prevents browning because the
apples are no longer exposed to air.
The chemical ascorbic acid (vitamin C), which is found in
lemon juice, is also known to prevent the browning reaction.
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Chemistry and cleaning
Grease and oil on clothing, cutlery, skin and hair are difficult
to remove with just water, because oil and water do not mix.
Adding soap allows the oil and water to mix.
Soap molecules
have two ends:
one end is
attracted to the
water molecules;
the other end is
attracted to the
oil molecules.
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head of molecule
bonds to water
molecules
tail of molecule
bonds to oil
molecules
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Chemistry and cosmetics
The ingredients in cosmetics like makeup, shampoo and fake
tan are made by chemical reactions. Many of the ingredients
in these products are made from crude oil.
Is it sustainable to use ingredients like these?
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Chemistry and medicine
The medicines that we take are all made by chemists. Many
medicines started out as natural products, which came from
living things:
 aspirin – from the bark of willow trees
 penicillin – from mould
 morphine – from poppies
 digoxin – from foxgloves.
Do you know what these
medicines are used for?
What do you think might be the advantages and disadvantages
of using drugs from plants and moulds?
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Chemistry and computers
Materials scientists are chemists who research new materials
for things like computers.
screen – to
give out light
casing – made of tough,
mouldable plastic
paints and inks – for the
letters on the keys
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electric
wires –
to carry
electricity
silicon chips – to
store and transfer
information
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Reactants and products
In a chemical reaction, one or more new substances are
always formed.
The starting substances used in a reaction are the reactants.
The new substances formed in a reaction are the products.
reactants
products
The arrow means ‘change into’. In a chemical reaction, the
reactants change into the products.
It is often difficult to reverse a chemical reaction and change
the products back into the reactants.
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Word equations
A word equation is a quick, shorthand way of writing a chemical
reaction.
There are always three parts to a word equation:
 the names of the reactants
 an arrow
 the names of the products.
What is the word equation for hydrogen reacting with oxygen
to form water?
hydrogen
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+
oxygen

water
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Reactant or product?
In this chemical reaction, which substances are the reactants
and which substances are the products?
magnesium
+
copper

oxide
magnesium
oxide
+
substance
reactant or product?
magnesium oxide
product
magnesium
reactant
copper oxide
reactant
copper
product
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copper
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Write the word equations
1. Magnesium burns brightly in oxygen to form magnesium
oxide.
+
magnesium
oxygen

magnesium oxide
2. Calcium hydroxide reacts with hydrochloric acid to
form calcium chloride and water.
calcium
hydroxide
+
hydrochloric
calcium

acid
chloride
+
water
3. Sodium reacts with hydrochloric acid to form sodium
chloride and hydrogen.
sodium
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+
hydrochloric
sodium

acid
chloride
+
hydrogen
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Mass during a reaction
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Why doesn’t the mass change?
In chemical reactions, no atoms can be made or destroyed.
Chemical reactions just change how the atoms are bonded
together.
+
+
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What happens to the mass?
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Why did the mass increase?
In the reaction between magnesium and oxygen, the mass
increased.
This is because the magnesium atoms joined up with oxygen
atoms.

+
What is the word equation for this reaction?
magnesium
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+
oxygen

magnesium oxide
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What happens to the mass?
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Why did the mass decrease?
The mass of the copper carbonate decreased
as it was heated because it decomposed to
form copper oxide and carbon dioxide.
What is the word equation for this reaction?
Why did the mass decrease?
The mass decreased because the carbon
dioxide gas escaped out into the air.
Can you calculate the mass
of carbon dioxide that was
produced in the reaction?
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Chemical reactions – true or false?
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Glossary
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Anagrams
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Multiple-choice quiz
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