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1. Chapter 7
Chemical Reactions and Equations

2. A. Objective Type Questions
• 1. Melting of ice
• 2.Cracking of an egg
• 3.Presence of both air and water
• 4.Carbon dioxide
• 5.Evolution of a gas

3. B. Short Answer Type Questions
1. A physical change is that in which only the
physical properties of a substance undergo
change. There is nochange in the chemical
composition of the substance. Eg : melting of
ice.

5. 3

6. 4.
• Measures to prevent browning of fruits and vegetables are as
follows:
• 1) Brushing the surface of the fruit with juice of citrus fruits
• 2)Soaking of some vegetables in water. Example – Cut potato can
be soaked in water to prevent its browning.
• 3)Adding certain preservatives such as sulphur dioxide.

7. 5.
• (a) It is possible to get the pure substance from the
mixture.
• (b) The substances that can get decomposed on heating
cannot be separated by evaporation. This can be done
only by crystallization.

8. C. Long Answer Type Questions
1. To prepare crystals of copper sulphate, we first prepare an
aqueous solution of copper sulphate. Then we add 2-3 drops of
sulphuric acid into the solution. The solution is then filtered using a
filter paper. The filtrate is concentrated by heating it till the first
crystal appears in the solution. Keep the setup undisturbed for 6-8
hours.
Let the concentrated filtrate cool down. We will observe that copper
sulphate crystals are formed. Sulphuric acid is added to remove
water of crystallisation to produce anhydrous copper sulphate.

9. 2.
• When a candle burns, both physical and chemical changes occur.
• (a) Physical changes in burning candle: On heating, candle wax melts and form liquid wax. This is a
physical change. As liquid wax can again turn into solid wax on cooling, so it is a reversible change.
• (b) Chemical changes in burning candle: When you light a candle, the wax present near the wick will
melt. The wick absorbs the liquid wax. The liquid wax vaporises due the heat produced by the flame.
The wax vapours near the flames burn and gives new substances like carbon dioxide, carbon soot,
water vapour, heat and light. This is an irreversible or chemical change.

10. 3.
• 3. In a neutralisation reaction, an acid and a base
react together to form a new substance i.e. salt and
water.
• Since the new substances are formed after the
reaction and the reaction is irreversible, so it is a
chemical change.

11. 4.
• In coastal areas, air contains hi gh percentage of
moisture as it is near to the coast, while in deserts
the air is dry and hot. As the percentage of
moisture in air is more in coastal area as compared
to desert, rusting becomes faster.

12. 5. This is because, the acid reacts with
sodium carbonate thereby producing
carbon dioxide gas. The bubbles
produced are of carbon dioxide gas only.

13. 6.
• Alloying can increase or reduce the strength, hardness,
electrical and thermal conductivity, corrosion resistance,
or change the colour of a metal. For ex: iron can undergo
rust, but its alloy stainless steel does not.
• Similarly, pure gold is soft but when alloyed with copper
and silver, it becomes hard and stronger.