Download PART 2 – CHEMISTRY

PART 2 – CHEMISTRY
Text 1.
What is Chemistry?
Chemistry is the study of substances. The chemist wants to know what substances are made of, how they
act, and how they change. He tries to find out why chemical changes take place and how they can be
controlled.
Chemical changes occur everywhere. To learn what happens in chemical changes, it is necessary to
understand some of the principal ideas of chemistry. Everything in the world is made up of one or more of
103 basic substances called chemical elements. Carbon, iron, oxygen, and mercury are familiar elements.
Elements consist of tiny bits of matter called atoms. Each element has its own kind of atom which differs
from the atoms of all other elements.
Atoms can join together to form larger chemical units called molecules. For example, oxygen atoms
usually combine in groups of two to form oxygen molecules. When atoms of two or more different
elements combine, they form a substance called a chemical compound. Water is a compound made up of 2
elements – hydrogen and oxygen. A molecule of water consists of two atoms of hydrogen and one of
oxygen.
A chemical change takes place whenever elements unite to form a compound. Because of this chemical
change, a compound has properties (characteristics) that are different from the elements that make it up.
The properties of common salt, a compound, are different from the properties of sodium and chlorine, the
elements that make it up. Sodium is a soft, silvery-white metal that can be cut with a knife. Chlorine is a
poisonous, yellow-green gas. When sodium and chlorine combine chemically, they form salt – a grainy,
white substance used to season food. Chemistry is divided into inorganic and organic chemistry, in
relation to the different substances that are studied.
(World Book Encyclopedia)
Exercise. Finish the sentences with the right word.
1. Everything in the world is made up of one or more basic substances called __________________
.
2. _____________________ are tiny bits of matter that form an element.
3. Atoms join to form larger units called _____________________ .
4. A ____________________ is a substance made up of 2 or more different elements.
5. The __________________ of a compound can be very different from the single elements it is
made of.
6. Chemistry is divided into two parts, _____________________ and _____________________ .
Text 2.
The Atom and the Elements
When a lump of rock salt is crushed into smaller grains, each of these is still salt. These grains may be
ground into a very fine powder but the chemical composition remains unchanged. This division can
continue up to the point where we have one molecule of salt invisible to the naked eye or even the most
powerful optical microscope. This molecule consists of two atoms, one of sodium, the other of chlorine.
These two atoms are bound together to form sodium chloride (common salt) but by chemical reactions
they may be separated and in different combinations with other atoms, may change into different
substances.
The atom is the smallest particle which can take part in a chemical reaction and cannot be divided
chemically. It consists of three elementary particles, protons which have a positive charge, electrons with
a negative charge and neutrons which have no electric charge. The nucleus is composed of protons and
1
neutrons. Around this, electrons orbit at high speed. The atomic number is the number of protons in the
nucleus. The mass number is the total number of protons and neutrons in the nucleus. For example, the
atomic number of fluorine is 9. This means that there are 9 protons in the nucleus and 9 electrons in orbit
around it. The mass number is 19 which is the number of protons plus neutrons. Considering the mass
number and atomic number of the elements, it is important to note an exceptional case. It may happen that
in an element's atom, the number of neutrons may differ accounting for a difference in mass number.
These variations of the same element are called isotopes.
Atomic Shells or Energy Levels. According to present atomic theory, electrons orbit around the nucleus
of the atom in a series of concentric spherical spaces called shells. Each shell has a precise amount of
energy which is called the energy level. These energy levels are definable in a precise but complex way.
As one moves out from the nucleus, the shells increase in size. An atom has a maximum of seven of these
shells. The first shell nearest to the nucleus has a maximum of 2 electrons. The others may have up to
eight or more electrons.
The chemical properties of the elements depend on the number of electrons present in the outer shell or
energy level.
Stable and Unstable Atoms
Let's look at why and how elements combine to form the molecules of every substance around us. If there
are 2 electrons in a single shell surrounding the nucleus or 8 electrons in the outermost shell in the case
where the atom has more than one shell, then the atom is said to be stable. This means that the atom does
not react with any other kind of atom and thus remains isolated and inert. For example, this is so in the
case of the rare gases, argon, neon, helium etc. meaning that they cannot form compounds.
If the outer energy level or shell is incomplete, having less than the numbers stated above, the atom is said
to be chemically unstable. These atoms tend to react with other atoms until they reach an external energy
level of 2 or 8 electrons, thus becoming a stable compound.
Chemical Bonds. The force of attraction between one atom and another may create molecules which have
two types of chemical bond. If one atom gives away electrons to another which receives them, the bond is
called ionic. On the other hand, if the electrons are shared by both atoms, the bond is called covalent.
An atom of hydrogen has only one electron in its single shell. To reach a state of stability, two atoms
combine to form a molecule H2 in which the atoms share two electrons. So this bond is covalent. Atoms of
different elements may also combine in this way. For example, in the case of water H 2O, an atom of
oxygen which has six electrons in its outer shell shares electrons with two atoms of hydrogen, thus
creating a stable compound with 8 electrons in its outer shell.
The situation is different in the case of common salt (sodium chloride) which consists of one atom of
sodium combined with one atom of chlorine to form the compound NaCl. The sodium atom has only one
electron in its outermost shell which it releases to the chlorine atom becoming Na+. The chlorine atom has
seven electrons in its outer shell. By receiving one electron it stabilizes as it now has 8 electrons and
becomes Cl-. The bond is therefore called ionic.
Valency. The number of electrons in the outer-most shell of an atom determines the valency of the
particular element. Looking at written formulas of compounds, we note that some atoms are connected by
a single tie, some by two and others by as many as six. These ties show the combining power or valency
of the elements. Hydrogen is taken as the unit because an atom of hydrogen has the lowest combining
power of all the elements. Substances which have a single tie like hydrogen are called univalent elements.
An atom of oxygen combines with two of hydrogen, thus oxygen forms part of a large group of elements
having a valency of 2 which may be called diads or divalent.
Exercise. Say whether the following statements are true or false, then correct the false ones.
1. _____ Atoms consist of three particles; protons, electrons and nucleus.
2. _____The number of protons in the nucleus of an atom gives us its atomic number.
2
3. _____ An isotope is an element which has a different mass number due to the number of protons
it contains.
4. _____ Electrons orbit around the nucleus of an atom in a series of energy levels.
5. _____ An atom can have a maximum of ten shells: each of which can contain up to eight
elements.
6. _____ Atoms with a complete external shell are said to be stable.
7. _____ Unstable atoms tend to remain isolated and inert.
8. _____ In an ionic bond, electrons are shared by both atoms.
9. _____ Sodium Chloride (NaCl) is a good example of an ionic bond.
10. _____ the number of ties that an atom has is called its valency.
Text 3.
Matter
Matter, anything that has mass and occupies space, consists not only of things you can see and touch but
also of such things as air, which you cannot see. Matter exists in three phases: solids, liquids, and gases. A
solid is matter with a rigid shape and a fixed volume that does not change much with temperature. A
liquid has a fixed volume but not a fixed shape, and it conforms to the shape of its container; its volume
may change somewhat with changes in temperature and pressure. In contrast with solids or liquids, a gas
has neither fixed volume nor shape, as it expands to fill its container completely; its volume is very
sensitive to temperature and pressure. Matter is composed ultimately of different kinds of atoms.
Molecules are identifiable units of matter consisting of two or more atoms combined in a definite ratio.
When the atoms are the same we have an element; when they are different, we have a compound. A
compound, then, is a matter that consists of identifiable units containing atoms of different elements
combined in specific ratios. A great deal of the research done in chemistry involves the study of
transformations of one or more compounds into others.
A compound is different from a mixture. The elements in a compound lose their individual chemical
characteristics, and the compound has new characteristics. In a mixture, each of the constituents retains its
identity. A cup of coffee with sugar and milk is a mixture of many substances, as are a soft drink, a piece
of concrete, and a coin.
Mixtures can be homogeneous or heterogeneous. A homogeneous mixture has the same composition
throughout the mixture. If you stir sugar into a glass of water, the sugar dissolves and sugar molecules are
distributed uniformly, that is, homogeneously, throughout the water. This is an example of a solution, a
homogeneous mixture of two or more substances. The material dissolved is the solute, and the medium in
which it is dissolved is the solvent. In a sugar/water solution, sugar is the solute and water the solvent. In
contrast, a mixture of solid grains of sand and salt is heterogeneous, since particles of each component of
the mixture remain separate and can be observed as individual substances.
In either a homogeneous or a heterogeneous mixture, the components can be separated into pure
substances by physical means, that is, without changing the specific atom ratios within the particles.
The components of a mixture can also be separated by chemical means, but this involves changing the
chemical nature of one or more of the constituents. This is often done in chemical analysis, where the
components of a mixture are transformed into new substances that can be observed or separated by
physical means.
Each chemical substance has a set of physical properties, properties that can be measured and observed
without changing the atom ratios within the substance. Such properties include colour, the temperature at
which a substance melts or boils, density, and physical state at room temperature. Substances may
undergo physical changes.
3
In contrast with physical properties, the chemical properties of a substance are those that it exhibits when
it undergoes a change in atom arrangements or in atoms ratios. This change is often brought about by
contact with another substance. When gasoline burns in an automobile engine or metals rust and corrode,
their chemical composition changes.
No two pure substances have the same combination of chemical and physical properties under the same
conditions, so we can use these differences to identify substances. Many of the physical properties of
oxygen and nitrogen are very similar; both are colourless gases at room temperature, for example.
However, a burning match will go out if it is put into a flask of nitrogen gas, but it will burn brightly in
pure oxygen. The two gases clearly have different chemical properties.
Kotz & Purcel, Chemistry and Chemical Reactivity, Hartcourt Brace Publishers.
Exercise. Answer the following questions.
1. What are the three phases in which matter can be found? How are they different?
2. How is a compound different from a mixture?
3. How is a homogeneous mixture different from a heterogeneous one?
4. In a solution of water and salt, which is the solute and which is the solvent?
5. What are some of the physical properties that can be measured and observed in a chemical
substance?
6. What are the chemical properties of a substance?
7. Can two pure substances have the same combination of chemical and physical properties under
the same conditions? Why or why not?
GRAMMAR – PART 4
Cohesion Markers
Cohesion markers are linking words which tell you how sentences, or parts of sentences,
are related to each other. They are signals that instruct you how to make sense of the text. Each of these
signals has a meaning associated with it.
When we read something in detail It is important to understand them.
Cohesion markers are words which are connected with something already said (anaphoric linkers) or
something that will be said later (cataphoric linkers).
They help us identify the relationship between the parts of a passage. They are:
the article "the"
identifiers
(this, that, these, those)
personal pronouns
(he, she, it, they, him, her, them, etc.)
possessive pronouns
(his, hers, theirs, etc.)
relative pronouns
(who, that, which, whom, whose)
the adverb of place "where"
(= in which)
implied
(other, such)
repetitions or variations of words (synonyms, antonyms, etc.)
Relative Pronouns
Personal
Non personal
Subject
who/that
which/that
Object
whom*/that
which/that
*whom is very formal and it is not much used except for literary texts
4
Possessive
whose
of which
As you can see from the chart, the choice relative pronoun depends on various elements. In fact, the first
question you will ask yourself is: "Does it concern a person or a non-person?" And then: "Is it a subject or
an object? "
Examples
l. The biologist who / that discovered this element won the Nobel Prize.
(person / subject)
2 The girl that / whom we met at the party is a friend of mine.
(person / object)
3 This lab, which is being rebuilt, is not ready for experiments yet.
(non-person /subject)
4 There isn't anything that / which I wouldn’t do for her.
(non-person /object)
Exercise. Complete this article from about "foot-and-mouth" disease with the appropriate cohesion
markers.
France stepped up its measures against food-and-mouth disease yesterday. In fact _______ banned the
export and movement of livestock after the virus was identified in the carcasses of nine flocks of sheep
imported from Britain.
The Government, ________ President is Lionel Jospin, ordered the 15-day ban along with a suspension of
horse racing and other measures as Britain came under increasing attack for low quality agriculture
________ sent sickness to the Continent.
_________ measures, _________ apply to sheep, cattle, goats, pigs and horses, followed confirmation that
foot-and-mouth antibodies had been found among British sheep on nine farms and livestock depots in
central and northern France.
_________ animals were slaughtered last week under stringent measures __________ included the
destruction of all 20,000 sheep imported since January from Britain and 30,000 French animals with
__________ _________ had been in contact.
The Farm Minister, _________ introduced emergency protective measures, said that foot-and-mouth
disease is an evil that has been imported from the United kingdom _________ the situation at this stage is
truly worrying.
(The Times, March 6, 2001)
Text 4.
Chemical nomenclature
To find information about a particular substance, you must know its chemical formula and name. The
names and formulas of compounds are essential vocabulary in chemistry.
The naming of substances is called “chemical nomenclature”. Many important substances that have been
known for a long time, such as water, H2O, and ammonia, NH3, have individual, traditional names. For
most substances, we rely on a systematic set of rules that lead to an informative and unique name for each
substance, based on its composition.
The rules for chemical nomenclature are based on the division of substances into different categories. The
major division is between organic compounds and inorganic ones.
Organic compounds are compounds that contain the elements carbon and hydrogen in chemical
combination. Organic compounds are produced by living plants and animals and can be synthesized in the
laboratory. The typical names of organic compounds are carbohydrates, lipids, proteins, and nucleic acids.
5
Inorganic compounds are compounds that do not have the elements carbon and hydrogen in chemical
combination.
Let us consider three categories of inorganic substances: ionic compounds, acids, and binary molecular
compounds. In naming an ionic compound, the cation (positive ion) is named first and then the anion
(negative ion), i.e. BaBr2 is named barium bromide.
Cations formed from metal atoms have the same name as the metal, i.e. Na + is named “sodium ion”. If the
metal can form cations of differing charges, the charge is given using Roman numerals, i.e. Cu+ is named
“copper (I) ion”. Cu2+ is named “copper (II) ion”. Monoatomic anions have names ending in -ide, i.e. H- is
named hydride ion. Polyatomic anions containing oxygen and another element (oxyanions) have names
ending in -ate or -ite, i.e. nitrate (NO3-) or nitrite (NO2-).
The name of an acid is related to the name of its anion. Anions whose names end in -ide have associated
acids that have the hydro- prefix and an -ic ending, i.e. hydrochloric acid (HCl). Anions whose names end
in -ate have associated acids with an -ic ending, whereas anions whose names end in -ite have acids with
an -ous ending, i.e. chloric acid (HClO3) and chlorous acid (HClO2).
The procedures used for naming binary molecular compounds are similar to those used for naming ionic
compounds.
Exercise. True or False. Mark if the following statements are true (T) or false (F) according to the text:
then correct the false ones.
______ 1. The names and formulas of compounds are very important in Chemistry.
______ 2. All of the most important substances have individual, traditional names.
______ 3. The division of substances into different categories is at the basis of the rules for
chemical nomenclature.
______ 4. The principal division used to distinguish substances is between organic and
inorganic compounds.
______ 5. Inorganic compounds, such as carbohydrates, lipids and proteins, contain carbon
and hydrogen in chemical combinations.
______ 6. In naming an ionic compound, the anion is always named before the cation.
______ 7. Roman numbers are used to distinguish between ions having differing charges.
______ 8. Nitrate (NO3) is a typical polyatomic anion or oxyanion containing oxygen plus
another element.
______ 9. The name of an acid is related to the name of its cation.
______ 10. The procedures for naming binary molecular compounds are different from those
used for naming ionic ones.
Text 5.
Chemical reactions
The essence of chemistry is the study of chemical reactions, the combination of the elements and their
compounds to give new compounds. Chemists have used reactions to produce many of the materials Teflon, nylon, Dacron, Kevlar, polystyrene, and PVC, among others – we use every day. Cancer
chemotherapy agents are produced by chemical reactions, as are experimental drugs to treat AIDS. Plants
and animals are chemical reaction factories. Your automobile operates on the energy produced in a
6
combustion reaction, and so does the generating plant that produces the electricity you use. Many of these
reactions are quite complex and difficult to understand. However, there are certain principles that govern
all chemical reactions.
In any chemical change, matter is conserved. Although the atoms involved are rearranged into different
species in the course of a reaction and the number of molecules may change, the total number of atoms of
each kind in the reactants and products must be the same. Thus a balanced chemical equation shows the
relative amounts of products and reactants, the amounts being indicated by stoichiometric coefficients.
Stoichiometry is the study of mass relations in chemical reactions, and its guiding principle is the
conservation of matter.
Many important chemical reactions occur in water. Ionic compounds that contain certain ions can dissolve
in water to a significant extent. In doing so, they break up or dissociate into their ions. The resulting
solution conducts electricity, so the dissolved ionic compounds are called electrolytes.
Acids ionize to provide H3O+ ions in water, while bases provide OH- ions. Some ions do not enter directly
into reactions involving ionic compounds and so are called spectator ions.
Since many reactions occur in solution, the concentration of material in the reaction medium must be
defined.
A very convenient unit of concentration is molarity.
Molarity (m) = moles of solute divided by liters of solution
A titration is a way to carry out a reaction in a very precise manner and to use it for analysis. In an acidbase titration, an acid of known concentration is added to a base of unknown concentration (or vice versa)
until the number of moles of H3O+ that can be supplied by the acid is exactly equal to the number of moles
of OH- that can be supplied by the base. This is called the equivalence point. From the known
stoichiometry of the reaction that occurs, one can find the amount of unknown acid or base.
Oxidation-reduction reactions (often called redox reactions) involve the transfer of electrons between
compounds. A compound is said to be reduced if the oxidation number of one of its atoms is reduced by
acquiring electrons from another species, the reducing agent. Conversely, a compound is oxidised if the
oxidation number of one of its atoms is increased because that compound has transferred one or more
electrons to an oxidising agent.
Rates of Chemical Reactions
Some chemical reactions are so fast that they appear to be instantaneous, others take a few seconds, and
some reactions seem to take a very long time. In order to follow a reaction in the lab, we need an
observable or measurable change. Changes we can measure include:
1. A change in pH
2. a change in temperature
3. a change in colour
4. a change in mass
5. the disappearance of a reactant
6. the appearance of a precipitate
7. a volume of gas given off.
In order for substances to react they must come into contact with each other.
In a liquid, the particles are always moving around, and so will collide with each other and anything else
in the liquid. If a collision is to cause a reaction to take place, the collision must have enough energy and
be in the right direction. In a similar way, not all collisions result in a reaction. A certain energy barrier
(threshold) must be reached. This is called the activation energy. Each different reaction has its own
activation energy.
At the start of a reaction there is the greatest concentration of particles of reactants present, and therefore
there is a much greater chance of the reactant particles colliding with each other. The more collisions there
are, the faster the reaction will be.
As the reaction proceeds, the reactants are being used up completely. When this happens there will be no
more collisions between reactants and so no more reaction.
In order to alter the rate of a reaction, either the number or the energy of collisions must be changed.
7
Heating a solution adds energy to the particles: they will move faster and the reaction will be quicker.
Because they are moving faster, they will hit each other harder. This means that a greater number of
collisions will reach the activation energy: therefore the rate of reaction will be greater.
Pressure will have an effect only if the reaction involves gases.
In a reaction involving a solid and a liquid or a solid and a gas, the reaction takes place on the surface of
the solid: the smaller the pieces the solid is broken into, the faster the rate of the reaction.
The rates of some reactions can be altered by adding other chemicals to the reaction mixture. A substance
that alters the rate of a reaction, without altering the reaction in any other way, and without being used up
during the reaction, is called a catalyst.
Some reactions can go in both directions, depending on the conditions. In chemistry this is called a
reversible change and it is shown by an arrow going in both directions.
If a reversible reaction is enclosed in a sealed system so that no chemicals can enter or leave, a situation
can occur in which the reaction goes both back and forth at the same time. A balance is set up where the
rate of one is equal to the rate of the other: this is called a chemical equilibrium.
Exercise. Answer the following questions about the text
1. When does a chemical reaction occur?
2. What is the most important factor to know about any chemical reaction?
3. What is stoichiometry?
4. What are electrolytes?
5. What is molarity? In what kind of reactions is it important?
6. What is the difference between a reduced and an oxidised compound?
7. Do all chemical reactions take place in the same amount of time? Explain.
8. What is the threshold or activation energy?
9. When does a reaction end?
10. How can we change the rate of a reaction?
11. What is a catalyst?
12. What is a reversible reaction? How can we put it into equilibrium?
GRAMMAR – PART 5
Coherence Markers or Linkers
Of time
After, after that, before, when, while, meanwhile, then, afterwards, at first, finally, in the end, as soon as,
(just) as/the moment that, until/till, as long as, since, by the time that
To include, exclude, give alternatives
And, but, both……. And (sia, sia), as well as (oltre a, e anche), or , or else (oppure), either… or (o….o)
neither….nor (nè…nè)
How…. Way (modo)
As (come, modo), as if/as though, (come se), like (come)
Es. You may use it as a computer or as a calculator.
8
It looks like a computer but it isn’t.
He’s quite good as a teacher, but as a person…….
Function (scopo)
To, in order to, in case
Es. He opened the windows to/in order to let some fresh air in.
Can you buy a bottle of milk, just in case we run out.
Break the glass in case of fire.
To add or contrast
To reinforce or add to an idea:
What’s more…., on top of that, in addition = per di più
Besides, furthermore, moreover = inoltre
To contrast or concede:
However = ma, tuttavia, comunque
Still, and yet, even so, nevertheless, nonetheless, though = eppure, tuttavia
All the same, in any case, anyway, anyhow = comunque, ad ogni modo
In spite of that/despite, I spite of the fact, despite the fact = nonostante ciò
Afterall = dopotutto
Although, though, even though/if = sebbene
Such as = Per esempio
While, whereas = mentre
On the contrary = al contrario
On the other hand = d’altra parte
To conclude or show consequence:
In conclusion, therefore, thus
Exercise. Cloze Test. Put a correct sequential link into this lab report.
________ a quantity of toothpaste was weighed and its weight recorded, __________ it was transferred to
a 250ml conical flask and a small amount of deionised water was added. __________ the flask was
swirled to disperse the toothpaste in the solution. ____________ 50ml of standardised hydrochloric acid
was pipetted into the flask. A glass funnel was __________ inserted into the flask and the contents were
heated for about 15 minutes or ___________ the reaction was complete.
Exercise. Cloze Test. Put the words in the box below into the spaces in the text.
for example
it
on the other hand
only
or
very few
others
similarly some
there
they
while
Strong and weak acids and bases.
___________ acids are strong and ____________ are weak. A strong acid reacts with metals and bases
faster than a weak acid. The strength of the acids does not refer to the fact that __________ are more
concentrated __________ more diluted. A strong acid is completely ionized in solution. Hydrochloric acid
9
solution contains hydrogen ions and chloride ions. __________ are no un-ionized molecules of hydrogen
chloride.
_________________ , a weak acid like acetic acid solution contains mainly molecules of acetic acid.
______________ (only 1 %) of the molecules ionize .
Hydrochloric acid, sulphuric acid and nitric acid, _______________, are strong acids, _______________
acetic acid and citric acid are examples of weak ones.
________________, the strength of a base is related to the number of ionized molecules in solution.
Ammonia in solution is called ammonium hydroxide. __________ is a weak alkali, ____________
slightly dissociated into ions.
Exercise. Cloze Test: Put the words in the box below into the correct spaces in the text.
a
an anyway because
how however
the
their that this which whose
A study has revealed __________ GMO corn can be dangerous to monarch butterflies, __________ it
produces a natural pesticide that can kill __________ caterpillars. ________ laboratory investigation has
revealed a high level of mortality among green lacewings __________ had eaten cornborers which had
lived on Bt corn. _________ dangerous it is in ________ actual cornfield, __________, is not known.
__________, the European Commission has suspended ________ authorization for genetically modified
crops. ________ action worried farmers ________ profits depend on the corn market.
Text 7.
The Chemistry of Food
Generally speaking our food can be divided into three main chemical groups: carbohydrates, fats and
proteins. Certain additional substances not included in these categories are also vital to the nutritional
system, even though, strictly speaking, they are taken in with our food but are not really foods and have
different functions, for example, water, vitamins and mineral salts.
Carbohydrates
Carbohydrates are a large group of organic chemical compounds with the basic formula C m(H2O)n. They
all consist of combinations of only three elements; carbon, hydrogen and oxygen. Carbohydrates are
divided into three main groups:
1.Monosaccharides cannot be hydrolised (split) into smaller carbohydrate molecules; they usually have 5
or 6 carbon atoms. They are soluble and easily digested.
Glucose (C6H12O6) is present in the blood and also found in fruit juices and honey. Glucose is available
commercially and may be used as an emergency food in illness because it is easily digested or absorbed
directly into the bloodstream via a drip-feed. It is also used extensively in brewing.
Fructose (C6H12O6) found in honey and fruit juices, is even sweeter than glucose.
2. Disaccharides are sugars made up of two monosaccharides or simple sugars.
Sucrose or cane-sugar (C12H22O10) is obtained from sugar-cane in tropical regions and sugar-beet in more
temperate climates. It is white, crystalline, sweet and soluble in water. Sucrose is built up of one molecule
of glucose and one of fructose:
Maltose (C12H22O11) is produced by the enzymic hydrolysis of starch.
10
Lactose, a white tasteless sugar found in milk, is less sweet than cane-sugar but is more suited to an
infant's digestive system.
3. Polysaccharides are very complex carbohydrates made up of chains of hundreds or thousands of simpler
sugars. Their general formula is (C6H10O5)n.
Starch, consisting of amylase and amylopectin, is the most important vegetable polysaccharide. The main
dietary sources of starch are cereals, legumes and tubers. Pure starch is a white powder used in the food
industry as a thickening agent and is used to stiffen textiles and paper. Just as animals and man store fats
in various parts of the body as a food reserve, so plants store sugars. Storage is facilitated by conversion
into starch. When needed, plants can recreate sugars from these starch reserves. Stores of foods in roots
and seeds like potatoes, corn and nuts are in the form of starch.
Cellulose, the most abundant polysaccharide in the plant kingdom, has many industrial uses as a natural
source of material in the making of textiles, paper, ropes and plastics. It has no food value for humans. It
is very difficult to break down chemically. Herbivorous animals eat a lot of cellulose and are able to break
it down, having in some cases a secondary digestive system or the help of friendly bacteria.
Glycogen is only in animal cells and that is why it is also called animal starch. It is a source of energy
stored in the liver and muscles.
Lipids
Many foodstuffs contain fats of one kind or another. If they are liquids, they are called oils and are of
vegetable origin. They may be solid like beef or mutton fat or somewhere between, like butter, and are of
animal origin. All fats and oils, whether animal or vegetable, have the same basic chemical origin. They
are all compounds of glycerine and fatty acids. Glycerine or glycerol, as it is more correctly called, is a
trihydric alcohol (i.e. it has three OH-groups) and its compounds with acids are called esters.
Lipids are fatty acids commonly formed by the reaction of a fatty acid with glycerol. They are soluble in
alcohol but not in water. Lipids are the main constituents of plant and animal waxes, fats and oils. Their
main biological function is to store energy. This reserve of energy lasts much longer than sugars because
the body consumes them more slowly.
Palmitic and stearic acids are found in hard fats such as beef fat whereas oleic acid is found in oils such as
palm and coconut oil. Oleic acid contains a double bond and is therefore called unsaturated. This is a
characteristic of oils and soft fats.
Soft fats and vegetable oils can be hardened Treatment with hydrogen which attaches to the double bond,
transforms them into saturated fats or saturates as they are often called. This is how margarine is produced
from unsaturated vegetable oils.
The main groups of lipids are triglycerides, phospholipids and steroids.
Triglycerides are the principle constituents of vegetable oils and animal fats. They are composed of a
molecule of glycerol combined with three of a fatty acid such as stearic, palmitic or oleic acid.
Phospholipids are the basic material of all cell membranes.
Steroids are lipids without fatty acid components. Steroids include cholesterol and the sex hormone
testosterone. Too much cholesterol in the bloodstream can cause circulation problems which may lead to
atherosclerosis.
Exercise. Answer the following questions.
1. What elements are at the basis of all carbohydrates?
2. What are the two monosaccharides and in what foods can we find them?
3. From what plants do we obtain sucrose?
4. Where do we find lactose and what is it used for?
5. In what foods do we find starch?
6. Do animals store starch? If so, where?
11
7. In what forms can we find lipids? What is their origin?
8. Are lipids soluble in water?
9. Why do we need lipids? How are they different from sugars?
10. How are steroids different from other fats?
Proteins
The entire animal world is built of proteins. Our muscles, internal organs, skin, hair and nails are all
composed of them. Proteins are chemical compounds made up of a chain of amino acids. Each amino acid
has carbon, nitrogen, oxygen and hydrogen in it. It is interesting to note that, although there is a large
variety of proteins, the constituent elements are present in remarkably constant proportions.
The molecular structure of proteins is very complex. The average protein molecule is made up of more
than a hundred amino-acid molecules. Amino acids are water-soluble molecules containing a basic
amine group (-NH2) and an acid carboxyl (-COOH) group.
When two or more amino acids are joined together they are known as peptides. Proteins are made up
of interacting polypeptides. Polypeptides are folded or twisted peptide chains consisting of more than
three amino acids. Many different proteins are found in the cells of living organisms but they are nearly all
made up of the same 20 amino acids joined together in different combinations. Eight of these, which are
called the essential amino acids, cannot be made by humans and must be obtained from the food intake.
Proteins in the human diet are mainly of animal origin but vegetable proteins are an essential part of
this food chain as it is from them that animals obtain their supplies. Animals cannot synthesize proteins
from their constituent elements. Plants make their own amino acids and can therefore make their own
proteins. Animals have to use plant proteins which they use as the basic material, breaking them down into
amino acids and then rebuilding them into more complex animal proteins.
Food rich in proteins is necessary to keep the body healthy. Foods such as eggs, milk, meat, fish and
cheese consist of proteins as well as essential amino acids. Some foods do not have them and are therefore
not such good sources of protein. Since humans cannot store amino acids, food rich in protein should be
eaten every day. Protein deficiency can cause severe health problems such as kwashiorkor, a disease of
children under five years causing a swollen abdomen and retarded growth. It may even cause mental
retardation and brain damage.
Enzymes are an important group of complex proteins which act as catalysts in many biochemical
reactions. A catalyst is a molecule that controls the rate of a chemical reaction but is itself not used up in
the process. They are very important in maintaining the organism’s metabolism. Nearly all the chemical
processes taking place in the body are regulated by enzymes. For example, enzymes in the digestive tract
break down the large protein molecules, carbohydrate molecules and fat molecules of foodstuffs into
smaller ones. Others help to transfer these small molecules from the intestine into the bloodstream.
Enzymes regulate the process of the storage and release of energy in the organism.
Enzymes have several characteristics that are important to the chemistry of living cells. These
characteristics are due to their protein nature. First, enzymes are made inactive by heat. Second, the action
of a enzymes can be blocked by certain compounds (called poisons). Third, enzymes are specific in their
activity because they catalyse one particular reaction.
Scientists affix the –ase ending to names of enzymes, i.e. maltase that catalyses the digestion of the sugar
maltose into two glucose molecules.
Exercise. True or False. Mark if the following statements are true (T) or false (F) and then correct the false
ones.
_____ 1. Proteins contain carbon, nitrogen, oxygen, and hydrogen which are generally in constant
proportions.
_____ 2. Two or more amino acids that are joined together are known as polypeptides.
12
_____ 3. Proteins are made from twenty basic amino acids, eight of which cannot be made by humans.
_____ 4. Plants synthesize their own amino acids and proteins.
_____ 5. Animals break down plant proteins and then rebuild them into simpler animal proteins.
_____ 6. Humans can easily store amino acids, so it is not necessary for us to eat food rich in protein
every day.
_____ 7. Protein sufficiency can slow down growth and may even cause brain damage.
_____ 8. Enzymes act as catalysts but they are not very important for an organism’s metabolism.
_____9. Every chemical reaction that takes place in our body is regulated by enzymes.
_____10.Enzymes can be made inactive by heat or even blocked by poisons.
OTHER READINGS
Text. 8
Food Analysis
Chemical analysis plays an important role in the food manufacturing industries, both in developing
new processes and in maintaining quality in existing processes.
Some techniques of analysis are: colorimetry, thin layer chromatography and gas chromatography.
These techniques can provide qualitative and quantitative information on the components of food.
Gas Chromatography
In the areas of fresh food, food processing and beverages the analysts usually need gas
chromatography. One of the most important aspects of food science is the optimization of plant
growing and animal management. Chemicals here are used to control pests, diseases and weeds, but
residual amounts of these toxic chemicals in food must be monitored. Gas chromatography can
analyse these residues.
For example, thanks to its sensitivity and selectivity it can detect chlorinated pesticide residues.
Environmental contamination, ranging from petroleum to mercury, can also be detected. Gas
chromatography has been used in a lot of ways in the laboratories associated with large scale food
processes, for example to investigate flavours and off-flavours, to monitor foodstuffs for
decomposition products, to detect adulteration in foods through cheaper substitutes by separating the
complex mixture or to measure preservatives like sorbic and benzoic acid. Without doubt we can
state that chemical analysis in food and drink processes is essential to quality control in production.
The PH Indicator
But simpler tests can also be used. For example, you can test food with a pH indicator. When tested
directly or shaken in water the food may show an acidic, alkaline or neutral nature. The pH scale of
1-14 is used for comparing acidity and alkalinity. Universal indicators can show where the pH of a
liquid lies on the scale.
Testing for Nitrogen
Another example can be testing for nitrogen in food. If nitrogen is present, all or most of the it will
be combined with carbon and hydrogen in proteins. However, when the food sample is heated with
soda-lime, the proteins break down. Ammonia gas is released. Ammonia contains nitrogen
combined with hydrogen (NH3). Ammonia is an alkaline gas. It turns red litmus paper blue or gives
a color on pH paper which shows well above 7.
Exercise. Answer the following questions
1. What techniques may be used to analyse food?
2. Why does food need to be analysed?
13
3. What can we find in food through analyses?
4. Are all types of food analysis difficult to perform?
Text 9.
Safe Laboratory Practices: Personal Protective Equipment
a. Eye protection: Wear appropriate eye protection (this applies to all persons, including visitors) where
chemicals are stored or handled. Avoid use of contact lenses in the laboratory unless necessary; if contact
lenses are used, inform supervisor so that special precautions can be taken. Protectors must provide
adequate protection against particular hazards for which they are designed, be reasonably comfortable
when worn under the designated conditions, fit snugly without interfering with the movements or vision of
the wearer, and be durable, easy to disinfect and clean, and kept in good repair.
b. Skin protection: Wear appropriate gloves when the potential for contact with toxic chemicals exists.
Inspect gloves before each use, wash them before removal, and replace periodically. Match glove material
to the hazard: such materials as nitrile, neoprene, natural rubber, PVC, latex, and butyl rubber vary widely
in chemical resistance. What may be safe with one chemical may prove harmful with another. Glove
thickness may be as important as glove material in some cases. Evaluate physical properties of the glove
material. Maximize comfort and dexterity. Ensure a safe grip. Measure proper size and length.
c. Respiratory protection: Use appropriate respiratory equipment when engineering controls are unable
to maintain air contaminant concentrations below the action levels.
d. Hearing protection: Exposure to high noise levels can cause hearing loss or impairment, and it can
create physical and psychological stress. There is no cure for noise-induced hearing loss, so prevention of
excessive noise exposure is the only way to avoid hearing damage. Use ear plugs or earmuffs.
e. Foot and leg protection: Most workers who suffered impact injuries to the feet were not wearing
protective footwear. Safety shoes should be sturdy and have an impact-resistant toe.
f. Other protective equipment: Provide and use any other protective equipment I and/or apparel as
appropriate. Do not wear personal protective clothing or equipment in non-laboratory areas. Remove
laboratory coats immediately on significant contamination with hazardous materials.
Exercise. Say if the following statements are true (T) or false (F); then correct the false ones.
_____ 1. Anyone entering the lab needs to use the correct type of protection for their eyes.
_____ 2. The use of contact lenses does not create a problem in the lab.
_____ 3. Eye protection should be comfortable, fit loosely, and be long-lasting.
_____ 4. Gloves must be worn when there is a possibility of contamination by toxic chemicals.
_____ 5. The material a glove is made of is of no importance to its chemical resistance.
_____ 6. How thick a glove is could be just as important as its composition.
_____ 7. Respiratory protection is necessary only when danger levels are exceeded.
_____ 8. Although excessive noise exposure may lead to hearing loss, it may be cured, so prevention is
not very important.
_____ 9. Most head injuries are due to impact because the wearer’s shoes were not sturdy enough.
_____ 10.Any further protective devices or clothing should be furnished when they are necessary.
14
Text 10.
The Microscope
Since its invention, the microscope has been one of the most important tools available for scientific
research and discovery. Without it, many of the mysteries of the origins of life and problems of disease
would have been impossible to resolve. The microscope is an optical instrument which is used for various
types of study which are impossible by the naked eye alone. It is possible to see very fine detail in the
structure of a specimen by providing a magnified image. It can be used to make measurements of lengths,
areas, angles etc. and to determine optical properties of an object such as the refractive index, phase
change and reflectance.
Light interacts with matter in various ways. The most important are reflection, refraction, absorption,
diffraction, polarization and fluorescence. Reflection is the main way in which our eyes receive light from
the objects around us. In microscopy it is used for the study of opaque objects. In the study of biological
specimens with the microscope, absorption is the commonest interaction between light and matter. The
object is made visible because it reduces the amount of light passing through it. But because many
organisms are transparent, it is often necessary to use staining techniques to make structures under study
more evident.
Light normally contains waves of different frequencies. These waves have all possible phase relationships
and vibrate in all possible planes. If the light is filtered with a polarizer, the light vibrates in a single plane.
Polarized light is used to identify minerals and crystals because, depending on their structure and
orientation, they transmit light of a certain wavelength.
There are several types of microscope. The simple microscope has only one lens system and may be
designed to be used in the hand. The compound microscope has a more complex system of lenses. The
basic structure and components of most of them are very similar. There is a stand which carries the stage
at right angles to the optical axis, a tube carrying the objectives and the eyepieces and an illumination
system consisting of a condenser and a mirror or a lamp. The tube was provided originally with a rack for
coarse and fine focusing but nowadays it is usually the stage which is moveable to obtain focusing. A
basic instrument offers a wide range of magnifications ranging from about xl0 to xl500. This
magnification is calculated by multiplying the power of the objective with that of the eyepiece. The
objectives are mounted on a rotating turret which facilitates quick changes. A typical set of objectives may
consist of a x l0,x 20, x 40 and x 100. The most common eyepieces are x 5 and x 10.
Stereomicroscopes are used when it is important to have a high quality of image which is erect and with a
full perception of depth. They are binocular instruments in which the image is observed by both eyes from
slightly different viewpoints. This instrument is not to be confused with instruments having twin eyepieces
but a single objective system. They are obviously not suitable for photomicrography or video attachments.
However many modem microscopes are fitted with a trinocular head which allows binocular vision and
photomicrography to take place simultaneously.
The electron microscope is an instrument that uses a beam of electrons instead of light rays. The electron
lens is an arrangement of electromagnetic coils that control and focus the beam. Electrons are not visible
to the naked eye, so a special screen is used to form the image. As the wavelength of the electron beam is
much shorter than that of light, much greater magnification and resolution are possible than with the
optical microscope. The most powerful electron microscopes can produce magnifications of x7 million. It
has made possible the observation of very small organisms such as viruses and even certain large
molecules.
Exercise. Multiple choice. Choose the best words to fill in the spaces.
1. The microscope is an _____________ instrument.
a. topical
c. angular
b. optical
d. sensible
2. Light can interact with matter by _____________ .
a. reflection
c. refraction
15
b. diffraction
d. all of the above
3. ______________ is the main way our eyes receive light.
a. reflection
c. refraction
b. diffraction
d. none of the above
4. We use polarized light to identify _____________ .
a. Chemicals
c. lipids
b. Minerals
d. all of the above
5. A basic microscope may magnify from _____________________.
a. x10 to x100
c. x10 to x1500
b. x10 to x20
d. x20 to x40
6. An electron microscope uses _________________________ .
a. a beam of electrons
c. a special screen
b. electromagnetic coils
d. all of the above
16
17