The depressing action takes place:1) physical and reversible changes2) Physical and non-reversible changes3) Chemical and reversible changes4) Chemical and non-reversible
Answers
Page No 65:
Question 1:
In a physical change
(a) the molecules of the substances do not change.
(b) the molecules of the substances change.
(c) atoms forming the molecules undergo changes.
(d) new substances are formed.
ANSWER:
(a) The molecules of the substances do not change.
Physical changes are reversible, the molecules of the substance remain exactly the same before and after the physical change.
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Question 2:
Growth in plants and animals is
(a) a physical change.
(b) a chemical change.
(c) neither a physical change nor a chemical change since it is a life process.
(d) either a physical change or a chemical change depending on how much the growth is.
ANSWER:
(b) a chemical change
Growth occurs due to the intake of food. The substances present in food are used to nourish the cells and allow them to grow/divide. Hence, growth is a chemical change in which the molecules present in the food undergo changes to form molecules that enhance growth.
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Question 3:
If two substances are brought in contact, it will result in
(a) a physical change only.
(b) a chemical change only.
(c) either a physical change or a chemical change, depending on the conditions.
(d) a chemical change if they are heated together otherwise, a physical change.
ANSWER:
(c) either a physical change or a chemical change, depending on the conditions.
When two substances are mixed together, it can result either in a physical or chemical change. This depends on the substances that are mixed and also on the conditions.
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Question 4:
Chemical changes are
(a) always irreversible.
(b) mostly irreversible.
(c) always reversible.
(d) mostly reversible.
ANSWER:
(a) always irreversible.
Chemical reactions always result in forming new substances with different molecules. It is not easy to get back the original substances. Hence, the chemical changes are permanent and irreversible.
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Question 5:
Which of these is an irreversible change?
(a) melting of ice
(b) melting of wax
(c) boiling of water
(d) burning of wax
ANSWER:
(d) burning of wax.
In the process of burning of wax, it is neither the solid wax nor the liquid wax that is burnt. Wax vapours are burnt giving off heat and light. Once burned, it is not possible to get back the wax. Burning of wax is a chemical process and hence, is irreversible.
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Question 6:
Which of these is a reversible change?
(a) milk turning into curd
(b) burning of wax
(c) melting of wax
(d) rusting of iron
ANSWER:
(c) melting of wax.
The melted wax gets solidified after cooling. The melting of wax, therefore, is a reversible change.
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Question 7:
Which of the following is a physical and reversible change?
(a) heating of iron and sulphur to form iron sulphide
(b) melting of ice to form water
(c) weathering of rocks
(d) cooking of rice
ANSWER:
(b) melting of ice to form water
The melting of ice to give water is a physical and reversible change. The water can be frozen to again form ice.
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Question 8:
Which of these is a physical and irreversible change?
(a) burning of a paper
(b) evaporation of water
(c) breaking of a glass tumbler
(d) lighting of an electric bulb
ANSWER:
(c) breaking of a glass tumbler
The glass tumbler or its broken pieces are made of glass and there are no new molecules created, so its is a physical change. However, you cannot get back the glass from the broken pieces so, the change is irreversible.
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Question 1:
Every change has a cause. True or false?
ANSWER:
True.
A substance continues to remain either in its physical state or chemical state until and unless it is acted by an external factor like heat, motion, wind etc. So, every change has a cause.
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Question 2:
Every change can be classified as either physical or chemical. True or false?
ANSWER:
True.
Sometimes a change in a substance may result in the formation of new substance and sometimes may not. Depending on this, every change can be classified as either physical or chemical.
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Question 3:
Is dissolving sugar in water a physical change or a chemical change?
ANSWER:
Sugar dissolving in water is a physical change. The sugar can be obtained back by evaporation of water and the water by condensation of water vapour.
Page No 66:
Question 1:
Give one example for each of the following types of changes:
(a) physical changes.
(b) chemical changes
(c) reversible changes
(d) irreversible changes
ANSWER:
(a) Physical changes:
Example: Change in the size of the pencil or eraser with continuous use.
(b) Chemical changes:
Example: Burning of wood.
(c) Reversible changes:
Example: Salt dissolving in water.
(d) Irreversible changes:
Example: Curdling of milk.
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