Fill in the blanks.
1. Melting of ice-cream is reversible change.
2. Ripening of fruits is a chemical change.
3. Breaking of glass is a physical change.
4. Expansion on heating is maximum in gas and minimum in solid.
B. Multiple choice questions
1. Which of the following is a reversible change?
ans. Melting of an ice-cream.
2. In which of the following, property of expansion and contraction of metals is utilized?
ans. All of the above
3. Baking of cake is an example of
ans. Both chemical change and irreversible change
4. Expansion gaps are kept in railway tracks because of
ans. Expansion of rails in hot weather.
5. Most of chemical changes are
ans. Irreversible
C. Very short answer questions (one word)
1. A change in which no new substances are formed
ans. physical changes
2. Name one reversible change
ans. Melting ice.
3. Squeezing of rubber ball is a chemical change.
ans.False
4. Growth of child is a fast change.
ans. False
5. Rusting of iron is fast or slow change.
ans. Slow
D. Short answer questions.
1. Define
a. Reversible Change:
Changes that can be reversed when the cause of the change is removed. For example melting of ice.
b. Irreversible Change:
the changes that cannot be reversed. For example spoiling of food
c. Physical Change:
The change in which no new substances are formed. For tearing of paper.
d. Chemical Change:
The changes in which new substances with different properties are formed. For example burning of candle.
2. What happened when ice is heated? Is it a reversible or irreversible change?
Ans. When the ice is heated it changes into water. Is it a reversible change as it can be reversed when the cause of the change is removed.
3. Burning of a paper is reversible or irreversible change? Why?
ans. Burning of a paper is irreversible as we cannot get back the paper.
4. What is the effect of heating and cooling on substances?
ans. Heating leads to expansion while cooling leads to contraction of substances.
5. Do you agree" all physical changes are reversible"? If not, give the correct statement.
ans. No, there are some physical changes that are irreversible. For example tearing of paper. When the paper is torn, the size and shape of the paper changes, but the molecules of the paper remains same.
6. Breaking of a glass tumblers and burning of paper are changes in which we do not get the original substance back. What is the difference between theses two changes?
ans. No new substance is formed after breaking of a glass tumblers. It has properties of glass but it is broken into pieces
Burning of a paper is a chemical change. Paper turns into ashes after it burns and there has no properties of paper.
E. Long answer questions
1. Give examples to explain the difference between changes that can or cannot be reversed.
ans. Changes are reversible as well as irreversible. A balloon becomes larger when we inflate it with air. It comes to its original shape when the air is let out. In this changes we can see that the change gets reversed when the cause of the change is removed.
There are also changes where the substances cannot be brought back to their original state. An inflated balloon if pricked, it bursts. This bursted balloon cannot be brought back to its original shape. Likewise, chapatis cannot be brought back to the original dough and dough into flour.
2. Differentiate between a physical and a chemical change.
ans. Physical Change:- Changes in which no new substances are formed are called physical changes, e.g. breaking of a glass, freezing of water, melting of ice cream,
Chemical Change:- Changes in which new substances with different properties are formed, are called chemical changes, e.g. cooking of food, burning of a substance,
3. When a candle burns, both physical and chemical changes occur. Explain this.
ans. On heating candle wax melts and form liquid wax. It is physical change since it again turns into solid wax on cooling. When candle is burnt in air, the mass of the product is increased due to atmospheric oxygen. It is chemical change.
4. Write three different applications of expansion and contraction of solid.
ans. 1. Iron tools is fixed to wooden handle:-
The iron blade has a ring in which the wooden handle is to be fixed. Normally, the wooden ring is made bit smaller than than the wooden handle. To fix handle ring is heated. Heating expands the iron ad the ring becomes slightly larger in size. The handle is inserted into the ring which easily fits in the ring tightly after contraction.
2. Fixing Metal rim over the cart-wheel:-
Metal rim is heated until it is red-hot and forced on to the wooden wheel. Rims are made slightly smaller and when they are heated, they expand and fit onto the wheel. There after, cold water is poured over the rim to cool it. On cooling rim contracts and fits onto the wheel.
3. Joints in railway tracts:-
While laying railway tracks gaps are left between rails to allow expansion during summer. If no gaps are left, the rails will bend in summer due to expansion.
5. Write two application of expansion and contraction of gases.
ans. If you keep an inflated balloon in the sun, you will see the size of the balloon increase this shows the expansion of the gas in the balloon because of heat of the sun.
and if an inflated balloon is tied at the mouth of a bottle and the bottle is left in cold water, you will see the size of the balloon would shrink because of the air inside the balloon contracts on cooling. This is contraction of gas.
Like wise, automobile tyres get frequently punctured or burst during summer because of expansion of air in the tube. That's why it is advised to fill less air in tyre during summer or let some air out of the tyre.
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