Physical and Chemical Changes
(ICSE Understanding Chemistry)
EXERCISES
A. Multiple choice questions.
1. A change may alter the following properties of a substance
a. physical. [ ]
b. chemical. [ ]
c. either physical or chemical. [✓]
d. none of these. [ ]
2. Which of the following is an irreversible change?
a. dissolving sugar in water. [ ]
b. melting the butter. [ ]
c. stretching an elastic. [ ]
d. cutting a paper. [✓]
3. Which of the following is a chemical change?
a. folding a paper. [ ]
b. burning a paper. [✓]
c. cutting a paper. [ ]
d. writing on a paper. [ ]
4. In which of the following a new substance is formed?
a. melting o ice. [ ]
b. boiling of water. [ ]
c. formation of sugar solution. [ ]
d. formation of iron sulphide. [✓]
5. A chemical change involves
a. change in state only. [ ]
b. change in colour only. [ ]
c. change in composition. [✓]
d. none of these. [ ]
B. True or False.
1. A physical change can usually be reversed. [T]
2. New product is always formed during a chemical change. [T]
3. Formation of curd is an example of a physical change. [F]
4. Formation of water vapour is an example of chemical change. [F]
5. Boiling or frying an egg is an irreversible change. [T]
C. Fill in the blanks.
1. Growth of a tree is a/an irreversible change. (reversible/irreversible)
2. Respiration is a chemical change. (physical/chemical)
3. Process when a substance changes from liquid to gaseous state below its boiling point is called evaporation. (evaporation/boiling)
4. Process when a substance changes from solid to gaseous state is called sublimation. (melting/sublimation)
5. Formation of iron oxide is called rusting. (rusting/burning)
D. Match the following.
=
|
1. Evaporation of naphthalene balls. |
(a) Fermentation. [3] |
|
2. Reddish brown layer on iron. |
(b) Sublimation. [1] |
|
3. Making of idli batter. |
(c) Physical change. [4] |
|
4. Melting of an ice cube. |
(d) Cooking. [5] |
|
5. Making dal. |
(e) Rusting. [2] |
E. Name the following.
1. A change that is useful to us.
= Desirable Change.
2. A change that is temporary.
= Reversible Change.
3. A change in which the chemical composition of the substance remains the same.
= Physical Change.
4. Process where a substance is broken down by microorganisms to form a new substance.
= Fermentation.
5. Process where a liquid separates into a semi-solid mass.
= Curdling.
F. Diagram based questions.
1. Name the change (physical/chemical) occurring in the following figures.
2. Name the process taking place in the following figures.
=
Fig. (a) - Chemical Change; Melting;
Fig. (b) - Physical Change; Evaporation;
Fig. (c) - Chemical Change; Baking;
G. Give reasons for the following.
1. Inflating of a balloon is a reversible change while bursting of a balloon is an irreversible change.
= Inflating of a balloon is a reversible change we can come to the previous state of the balloon by releasing the air from the balloon. On the other hand, we can not get the balloon at its previous state once it get burst. That is why bursting of balloon is an irreversible change.
2. Boiling of milk is a physical change while curdling of milk is a chemical change.
= Boiling is a physical change because we can get the previous state of milk only just cooling it. On the other hand, curdling of milk is a chemical change because in this process milk is transformed into a totally new substance i.e. curd. Curd is different from milk and it cannot be changed back to the original form of milk again.
3. Dissolving sugar in water is a physical change but dissolving quick lime in water is a chemical change.
= Dissolving sugar in water is a physical change because we can get the original form of sugar by boiling the solution. Water would get evaporated and sugar will be remained as its previous form. On the other hand, dissolving quick lime in water is a chemical change because it will produce a totally different substance called calcium hydroxide. Calcium hydroxide is different from quick lime and it cannot be changed back to the original form of quick lime.
H. Differentiate between the following.
1. Slow and fast changes.
=
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Slow Changes |
Fast Changes |
|---|---|
|
These are the changes that take place slowly over a
period of time. These changes may occur over a few hours to months or years. |
These are the changes that take place quickly in a
short span of a few seconds to a few minutes. |
|
Example :- Growing of a child into a adult,
germination of a seed, changing of milk into curd, etc. |
Example :- Bursting of a balloon, bursting of a
cracker, lighting of a bulb, etc. |
2. Reversible and irreversible changes.
=
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Reversible Changes |
Irreversible Changes |
|---|---|
|
These are the changes that can be reversed to obtain
the original form again. |
These are the changes that cannot be reversed to
obtain the original form. |
|
These are temporary changes. |
These are permanent changes. |
|
No new substance is formed. |
New substances are formed. |
|
Examples :- Melting of ice, inflating of a balloon,
moulding of a playdough. |
Examples :- Burning of wood or paper, frying of egg,
baking of cake. |
3. Natural and man-made changes.
=
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Natural Changes |
Man-Made Changes |
|---|---|
|
These are the changes that occur naturally without
any human interference. |
These are the changes that occur due to human
activities to meet their requirements. |
|
These changes are usually permanent. |
These changes can be reversible or irreversible. |
|
Examples :- Colour change in fruits and flowers,
ripening of fruits, wrinkling of skin, growth of plants and animals, changes
during sunset and sunrise. |
Examples :- Cutting of trees, burning a paper, boiling
an egg, construction of roads, making of dams. |
4. Desirable and undesirable changes.
=
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Desirable Changes |
Undesirable Changes |
|---|---|
|
These are the changes that are desirable or useful
to human beings. |
These are the changes that are harmful to human
beings. |
|
Examples :- Ripening of fruits, formation of curd
from milk, cooking of food. |
Example :- Spoiling of food, rusting of iron objects. |
5. Physical and chemical changes.
=
|
Physical Changes |
Chemical Changes |
|---|---|
|
These are the changes that alter the physical
properties of a substance. The change can be in the size, shape, colour,
appearance or state of a substance. |
These are the changes that alter the chemical
properties of a substance. They change the internal structure and chemical composition
of a substance. |
|
No new substances are formed. |
New substances are formed. |
|
Little or no energy is absorbed or released. |
Large amount of energy is absorbed or released. |
|
These are usually temporary and easily reversible
changes. |
These are always permanent and irreversible changes. |
|
Example :- Melting of butter, changing of water into
ice, moulding the play-dough, inflating a balloon. |
Examples :- Frying an egg, cooking of food, burning
of paper, rusting of iron, curdling of milk, making dosa/idli batter. |
6. Boiling and evaporation.
=
|
Boiling |
Evaporation |
|---|---|
|
Process in which a substance changes from liquid to
gaseous state at the boiling point. |
Process in which a substance changes from liquid to
gaseous state below the boiling point. |
|
Fast process. |
Slow process. |
|
Occurs at the boiling point only. |
Occurs at all temperatures. |
I. Very short answer questions.
1. Name the type of change in the following examples.
(i) Growth of a child.
= Slow Changes.
(ii) Melting of ice.
= Physical Changes.
(iii) Ripening of fruit.
= Desirable Changes.
2. Name the changes that are temporary and can be reversed.
= Reversible Changes.
3. Give one difference between natural and man-made changes.
= Natural changes are usually permanent and irreversible. On the other hand, man-made changes can be reversible or irreversible.
4. What happens during evaporation?
= During evaporation, when a substance is heated, it changes from a liquid to a gaseous state below its boiling point.
5. Name the process involved in the formation of water drops on the outer surface of a glass full of ice.
= Condensation.
6. What is condensation?
= Condensation is a process in which a substance changes from a gaseous state to a liquid state on cooling.
7. Name the type of change which occurs when salt is dissolved in water.
= Physical change.
8. Name the chemical formed when iron gets exposed to moist air.
= Iron Oxide (Fe₂O₃).
9. What is fermentation?
= Fermentation is a chemical process in which a substance is broken down by microorganisms like bacteria, yeast to form a new substance. In this process, heat and carbon dioxide are released.
10. What happens when quicklime is dissolved in water?
= When quicklime is dissolved in water, a new substance called calcium hydroxide is formed due to the chemical reaction of calcium oxide and water.
CaO + H₂O → Ca(OH)₂ + Heat
J. Short answer questions.
1. State two changes that we see around us.
= Two changes that we see around us are -
i) Burning of wood giving heat and light.
ii) Rusting of iron leading to corrosion.
2. List one example each of reversible and irreversible change.
= One example of reversible change - Melting of ice.
One example of irreversible change - Burning of wood.
3. List four properties of a substance that may be altered during a physical change.
= The four properties of a substance that may be altered during a physical change are - shape, size, colour and appearance.
4. List two characteristic of a chemical change.
= Two characteristics of a chemical change are -
i) Large amount of energy is absorbed or released.
ii) There are always permanent and irreversible changes.
5. Why is rusting an undesirable change?
= Rusting is an undesirable change because it leads to corrosion of iron. We can not obtain iron in its original form.
6. Define sublimation. What type of change is it?
= Sublimation is a process in which a substance changes directly from a solid to a gaseous state, without passing through the intermediate liquid state. It occurs faster at a higher temperature.
It is a physical change.