Ask the students: what happens when you burn a piece of paper, boil an egg, melt butter, prepare mango
squash, dry clothes under the Sun? What kind of changes are they?
Explain that we are surrounded by different kinds of changes called reactions. Some changes can easily be
reversed. These are called physical changes. No new substance is made during a physical change. Physical
changes are usually easy to reverse. Melting ice to form water is a physical change, and the water can be
easily frozen back into ice.
However, some reactions cannot be easily reversed. Such reactions are called chemical reactions: metals rust,
plants grow and then die and decay, cooking food. Some chemical reactions are very fast, such as explosions,
or very slow, such as buildings being eroded by acid rain.
Demonstrate some simple chemical reactions in class such as burning a candle, a piece of paper, etc. Have
the students examine the products. Explain that during a chemical reaction a new substance is formed. The
new substance has very different properties from the starting substance.
Start the lesson by explaining different types of chemical reactions.
Combination or synthesis
Write the following equation on the board:
hydrogen + oxygen ‡ water
Ask the students what has happened. Explain that when two or more substances combine to form one new
substance, a combination or a synthesis reaction takes place. For example, when hydrogen and oxygen
combine together, they form a new substance called water. When elements combine like this, they form a
Write the following equation on the board:
calcium carbonate ‡ calcium oxide + carbon dioxide
Explain to the students that sometimes a single chemical compound is broken down into two or more
different substances. We say that the compound has decomposed. When calcium carbonate is heated strongly
it decomposes into calcium oxide and carbon dioxide.
Energy changes in chemical reactions
Ask the students: what kind of a reaction is frying an egg? Is it a reversible reaction?
Frying an egg is a one-way reaction. Once the egg has been fried it is impossible to change it back into a raw
egg. The burning of fuel is another example of a one-way reaction. Many chemical reactions belong to this
category and are called irreversible reactions. Can the students think of other irreversible reactions?
Ask the students: are all reactions irreversible?
Explain that there are some reactions that can be reversed. A simple example is the physical change when
ice is heated to form water. The water can be changed back into ice.
Formula of a compound
Ask the students: how can we represent a chemical compound on a piece of paper?
Explain that we can write the names of compounds in the form of a formula which tells us what elements
it contains and in what proportions. If we know the symbols or short form names of the elements and the
number of atoms of each element that combine to form the compound, we can write its formula. For example,
we know the symbol of hydrogen is ‘H’ and the symbol of oxygen is ‘O’. To write the formula of water we can
O. From this formula we can make out the elements that are in the compound and also the number
of atoms of each. The formula of a compound tells us what elements it contains and in what proportions.
Discuss the uses of writing the names of compounds in the form of formulae. Explain that the formula of a
compound can be used to:
• Write the name of a compound in shorthand.
• Compare the relative masses of different molecules.
• Find the mass of a compound.
Additional activity 1
Find the formula of the following compounds:
Sodium chloride, copper sulphate, calcium carbonate, calcium hydroxide
Fill in the table below.
Name of the compound
Number of atoms of each element
Writing a word equation
Explain that we can represent chemical reactions in the form of equations.
For example, when a piece of solid magnesium is placed in some dilute hydrochloric acid, a chemical reaction
happens. During the reaction a gas is given off. This gas is called hydrogen. The solution remaining contains
The above description tells us what is happening during a chemical reaction. There is a much easier way of
writing all this information down. It can be written as a word equation.
magnesium + hydrochloric acid ‡ magnesium chloride + hydrogen
The reactants are on the left of the arrow. These are the substances that are added together at the beginning
of the reaction. The products are on the right of the arrow. The products are the substances that are made
during the reaction. If heat is provided to perform the reaction, the word ‘heat’ can be written above the
arrow. Consider the example of the decomposition of calcium carbonate to calcium and carbon dioxide:
calcium carbonate ‡ calcium oxide + carbon dioxide
Help the students practice writing chemical equations with examples from the student’s book.
Additional activity 2
Write down the word equations for the following chemical reactions:
1. When sodium is put in a bowl of cold water, it immediately reacts giving off a gas. This gas is tested
by putting a lighted splint near it. The gas ‘pops’, indicating that it is hydrogen. The solution that is
left turns the universal indicator purple. An alkali called sodium hydroxide has formed.
2. Green copper carbonate is heated in a test tube. The gas given off turns lime water cloudy. It is carbon
dioxide. A black solid is left in the test tube. This is copper oxide.
Breaking down compounds using electricity
Set up an electrolytic cell in the laboratory and show the students how it works. Ask them how they think
the bulb glows when the circuit is complete. Ask the students whether they have heard the term electrolysis.
Explain that electro means electricity and lysis means breakdown. So electrolysis means breaking down of
chemical compounds using electricity. A substance which conducts current and undergoes decomposition is
called an ‘electrolyte’. Two wires or plates at which the current enters and leaves the electrolyte are called
electrodes. The electrode at which the current enters the electrolyte is called the anode and that by which it
leaves, is called the cathode.
The apparatus consisting of a vessel, electrolyte, electrodes, in which electrolysis takes place, is called an
electrolytic cell or voltameter.
When two electrodes are dipped into an electrolyte and connected to a battery, positive ions (cations) are
attracted towards the negative cathode and the negative ions (anions) towards the positive anode. This migration
of ions in both directions is the process by which an electric current flows through the electrolyte.
Ask the students: what is air made up of?
Explain that air is a mixture of gases. Draw a pie chart on the board and explain the percentage composition
Ask the students: is the composition of air always the same at every place?
Explain that the composition of air is not always exactly the same. For example, the percentage of water
vapour in the air is much greater on a humid day than on a dry day. Besides the air over a busy city also
contains harmful gases such as carbon monoxide and sulphur dioxide, which come from car exhausts and
Explain that air is essential for life. All animals and plants, including humans, are built from compounds
containing carbon, nitrogen, hydrogen and oxygen.
Nitrogen is used to make compounds called nitrates. These are added to the soil as fertilizers. Plants take
in nitrates and water from the soil through their roots. They use these to build up the compounds they need
for their roots, stems and leaves. Plants also use carbon dioxide in the air for photosynthesis. Oxygen is
required by living things for respiration.
Separating gases from the air
Ask the students: can the gases in the air be separated? How do we know that air is a mixture of gases?
Discuss the constituents of air and explain how they can be separated by the method of fractional
Additional activity 3
Make a list of the steps that are involved in separating the constituents of air:
• The air is cooled down till it becomes liquid.
• The liquid air is slowly warmed up.
• All the substances in it have different boiling points, so they boil off one by one, and are collected
Explain and discuss the various uses of the gases in air:
• Oxygen is used by deep sea divers, by astronauts, by fire fighters and in hospitals for patients with
• Nitrogen is used for making fertilizers, for freezing things.
• Argon is used to fill ordinary light bulbs.
• Neon is used in advertising signs.
• Krypton and xenon are used in powerful lamps.
• Helium is very light, so it is used to fill balloons.
Compounds and mixtures: p 42
1. An element is a substance that is made up of only one kind of atom.
2. Iron and sulphur
3. Forming a compound from its elements is called combination or synthesis.
4. a) A mixture of copper and sulphur would be formed, where both would retain their individual
b) A compound copper sulphide would be formed.
5. A mixture is an example of a physical change. No new substance is formed in a physical change
and the components of a mixture retain their individual properties. The change is usually easy to
Compounds are examples of a chemical change. In a chemical change a completely new substance is
made and the change is usually difficult to reverse.
More about compounds: p 43
1. Chlorine, calcium, sodium, potassium
2. a) hydrogen oxide
3. Water is a liquid and used to put out fires. It is made from hydrogen and oxygen which are gases and
explosive in nature.
4. a) Calcium is a metal, phosphorus is a non-metal, oxygen is a gas.
b) They are combined together by a chemical reaction to form a compound which does not harm
5. A formula is a simple way of telling us which atoms and how many atoms combine to make a
Breaking compounds: p 44
1. Decomposition is the breaking up of a single compound into two or more simpler products. Energy
is needed to break the chemical bonds holding the compound together.
2 a) When heat is used to break down a compound, it is called thermal decomposition.
b) When copper carbonate is heated, it changes from a green to a black powder. Carbon dioxide
gas is given off.
‡ CuO + CO
3. a) A photographic film has a thin coating of a compound called silver chloride.
b) When a photograph is taken, light falls on to the silver chloride coating and breaks it down to
produce black crystals of silver. Chlorine gas is given off. The black shapes on a photographic
negative contain small amounts of silver.
4. a) Living things can break down chemical compounds by digestion and respiration.
b) Enzymes help break these compounds down.
Breaking compounds using electricity: p 45
1. A charged particle is called an ion.
3. a) The negative electrode is called the cathode.
b) The positive electrode is called the anode.
c) An electrolyte is a liquid that allows electricity to flow through it.
4. Set up an electrolytic cell. The electrolyte is a copper chloride solution with two carbon electrodes
dipped in it. Connect the electrodes to a battery with the help of copper wires.
In the solution, copper chloride splits into positive copper ions and negative chloride ions. The
) ions are attracted towards the anode. The positive (Cu
) ions are attracted towards the
cathode where they collect electrons and become uncharged, and form a layer of copper. At the same
time, the chloride ions lose electrons at the anode and form chlorine gas.
Breaking compounds using electricity: p 46–47
1. A chemical reaction that gives out energy in the form of heat is called an exothermic reaction.
2. a) Dropping water onto anhydrous copper sulphate.
b) Striking a match.
4. An exothermic reaction gives out energy in the form of heat. An endothermic reaction needs to take
in energy to keep it going.
5. Cooking is a good example of an endothermic reaction.
A mixture called air: p 48–49
1. a) nitrogen, oxygen, carbon dioxide
c) i) oxygen and nitrogen
ii) carbon dioxide
2. a) oxygen
b) carbon dioxide, argon
Match burns. Energy transferred
as heat and light.
3. a) It does not burn.
b) It does not react easily with other elements.
c) It would allow the wire filament to become hot and give out light without burning.
4. a) It helps patients with breathing problems.
b) It is used by welders for producing very hot flames to melt metals.
5. The gases in air can be separated by fractional distillation. After separation each gas maintains its
properties. This proves that air is a mixture.
Test yourself: p 50–51
1. i) compound ii) mixture
2. a) B
4. a) sodium, chlorine gas
c) cell or a battery
d) This is an example of a chemical change. Sodium chloride has completely different properties as
compared to sodium and chlorine gas.
5. a) It is a chemical change because potassium carbonate breaks down into potassium chloride, water
and carbon dioxide.
b) Exothermic reaction
6. a) Air is liquefied by cooling and compressing it.
b) i) –186°C
c) i) nitrogen
d) nitrogen, argon
f) Fractional distillation is a physical process because the gases which are cooled and liquefied can
be heated and made to change into gaseous form without any chemical change taking place.
Workbook 2, chapter 5
1. a) mixture
2. a) i) C
b) i) E
ii) The atoms have not combined completely.
3. b) HCl
4. a) C
c) i) A
ii) magnesium oxide
d) i) B
ii) It has lost weight after heating.
5. a) Electrolysis means the breaking down of chemical substances by electricity.
b) and c)
d) When an atom gains electron(s), it is called a negative ion.
When an atom loses electrons, a positive ion is formed.
6. a) An endothermic reaction absorbs energy. An exothermic reaction releases energy.
b) Exothermic reactions
respiration in animals and plants
frying an egg
striking a match
plants using sunlight energy to make food
a firecracker exploding
sucking a mint to cool your mouth
7. a) i) nitrogen
v) carbon dioxide
c) The gases in air can be separated by fractional distillation. Air is first liquefied then distilled. Each
gas has a different boiling point. As the temperature of the liquid is raised, each gas is collected
as it turns to vapour.
noble gases, etc.
1. Dissolve some baking soda in a little water. Then test the solution with red and blue litmus paper.
Add a few drops of vinegar to the solution and test it again. What happens when the vinegar is added
to the baking soda? Is this a chemical change? How do you know?
2. Heat some sugar in a test tube. Does a chemical change take place? What makes you think so?
3. Moisten a teaspoon of quicklime (dry slaked lime) with water. Does a chemical change take place?
How do you know?
Multiple Choice Questions
1. Elements are made up of
A one kind of atom only
B one or more kind of atoms
C two atoms combined chemically
D two atoms combined physically
2. The elements copper and chlorine join together to form a compound. This compound
A has the properties of copper
B has the properties of chlorine
C is not like the elements copper or chlorine D becomes a new element in the periodic table
3. Which of the following statements is always true when a compound is made?
A A solid is formed
B An energy change takes place
C A gas is released
D It needs heating to start the reaction
4. Which of the following is not a chemical change?
A rusting of iron
B burning of magnesium in air
C melting of ice
D heating sugar strongly
5. The formula for hydrochloric acid is
6. The breaking down of compounds into two or more simpler compounds or elements is called
7. What must a compound have in order to break down by electrolysis?
8. A chemical reaction that gives out energy in the form of heat is called
A an exothermic reaction
B a chemical reaction
C an endothermic reaction
9. Which one of the following gases in air is a noble gas?
C carbon dioxide
10. The process used to separate the gases in air is called
C fractional distillation
Unit flow chart
What is a magnet?
What is the theory for magnets?
How magnets are made?
The uses of magnets
Aims and learning objectives
• To identify magnetic materials
• To understand the theory of magnetism
• To use the concepts of magnetic field, a permanent magnet and an electromagnet
• To describe how magnetism can be induced in a piece of iron or steel
• To explain the difference between a permanent and a temporary magnet
• To describe the shape and direction of the magnetic field around an electromagnet and how the strength
of the magnetic field can be changed
• To describe some uses of magnets and electromagnets
• To understand that magnetism is used to store information
Nearly every one knows something about magnets. You have probably used a magnet to pick up pins, tacks,
and other things made of iron or steel. We do not know when and how magnetism was discovered, but
different stories are told about how a kind of rock was discovered which would attract pieces of iron towards
itself and hold on to them.
For many years, people thought that magnetism was a quality unique to one kind of rock. It was interesting,
but it was not really useful. At last, someone discovered that a piece of iron would act like a magnet if it was
rubbed on the rock. This was followed by an even greater discovery.
If a magnet made of iron was set on a piece of wood floating in water, the magnet would turn until it
pointed towards the north and south. This was the first compass. The rock used to make the iron magnet in
the compass was called lodestone. Scientists have found that lodestone is made of a kind of iron ore called
The compass is one of the most important inventions ever made, but for a long time no other uses for
magnets were discovered. Magnets were so weak that they could move only small pieces of iron. Finally
scientists discovered how to make much stronger magnets by using electric current. They also learned how
to use electric current to make electromagnets, where the magnetic force could be turned on or off. From
then on, many new uses for magnets were found.
Telephone receivers, loudspeakers and speedometers, all have magnets in them. So do electric bells and
buzzers. Magnets are found in every electric motor or generator. Doctors often use magnets to get tiny bits
of iron out of a person’s eyes or throat. In these and many other ways, magnets are used every day.
Ask the students: what can magnets do? What materials does a magnet attract?
Test different things, to find out whether they are attracted by a magnet. Ask the students to make a list of
the things that are attracted by a magnet. What materials are all of these things made of?
Can the students feel the force of a magnet when it attracts a material? Can they feel its force when the
material is pulled off the magnet? Did they have to touch a material with the magnet to attract it?
Explain that a magnet has a force because it can attract materials that are magnetic in nature. You cannot
see this force. You can only see what it does. You cannot even feel the force unless a magnet is near a
Poles of a magnet
Show the students a bar magnet. Ask them where the force of the magnet is strongest.
Explain that the closer the material is to the magnet, the stronger the force seems to be. But where is the
force? Is it inside the magnet? Is it outside the magnet? Or both?
Additional activity 1
Ask the students to spread some tacks evenly on a sheet of paper placed on a wooden board. Then lay a
bar magnet on the tacks. Lift the bar magnet.
To what parts of the magnet do most of the tacks stick?
Repeat the experiment using a U-shaped magnet.
What happens? Where is the force of a magnet strongest? How do you know?
Explain that the tacks cling to both magnets. However, the students should notice that the much of the
magnetic pull seems to come from two points near the ends. These are the poles of a magnet. Explain that
the force of a magnet is strongest at the poles.
Finding the poles of a magnet
If you hang a bar magnet using a thread, it swings round until it lies roughly north-south. This effect gives
the poles their names north pole and south pole.
Bring the ends of two identical bar magnets together. You will feel that there is a force between the poles:
if the poles are the same, they repel (push each other apart). If the poles are different, they attract each
How can we make magnets?
Additional activity 2
One magnet can be used to make another magnet.
Stroke a nail or knitting needle with one end of a bar magnet, but be careful to stroke it only in one
direction and to lift your hand between strokes. Now test it to see whether it has become a magnet. What
do you discover? How do you think you could make the magnet stronger? Try and see what happens.
Explain that pieces of iron and steel become magnets if you place them near a magnet. The magnet induces
magnetism in both metals. The magnet attracts the pieces of metal because the poles nearest each other are
different. When the pieces of metal are pulled away, the steel keeps its magnetism, but the iron does not.
The steel has become a permanent magnet and will keep its magnetism for a long time. The iron was only
a temporary magnet.
Ask the students: why can iron and steel be made into magnets?
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