ICSE Living Science Chemistry for Class 8 Solutions Chapter 2 Physical and Chemical Changes

Check Your Progress (Page 15)

Write true or false. Correct the false statements
Question 1.
Stretching a rubber band is a physical change.
Answer:
True.

Question 2.
The ripening of fruits is a physical change.
Answer:
False. The ripening of fruits is a chemical change.

Question 3.
Energy is either absorbed or evolved during a chemical change.
Answer:
True.

Question 4.
Boiling (vaporization) refers to the change of a liquid into its solid state at its boiling point.
Answer:
False. Boiling (vaporization) refers to the change of a liquid into its gaseous state at its boiling point.

Question 5.
The total mass of the substances involved in a chemical change remains constant.
Answer:
True.

Think And Answer (Page 16)

All physical changes can be reversed. Do you agree?
Answer:
All changes of state are physical changes. For example, when water is cooled, it freezes to form ice. When left in the open, ice melts and changes back to water. Similarly, when water is boiled, it changes to water vapour (steam), and when water vapour is cooled, it condenses back to liquid water. This means that the change of state in all these cases is temporary and reversible

A. Tick the most appropriate answer

Question 1.
Which of the following is/are characteristic (s) of a physical change?
a. temporary change
b. reversible
c. formation of no new substance
d. all of the above
Answer:
d. all of the above

Question 2.
Which of the following is not a chemical change?
a. rusting of iron
b. cooking of food
c. digestion of food
d. freezing of water
Answer:
d. freezing of water

Question 3.
Which of the following is a chemical change?
a. burning of magnesium ribbon in air
b. freezing of water
c. evaporation of water
d. tearing of paper
Answer:
a. burning of magnesium ribbon in air

Question 4.
Which of the following refers to the change of state from solid to liquid?
a. boiling
b. freezing
c. melting
d. evaporation
Answer:
c. melting

Question 5.
Which form of energy is absorbed in photosynthesis?
a. heat
b. sound
c. electrical
d. light
Answer:
d. light

B. Fill in the blanks.

Question 1.
A change is temporary and reversible.
Answer:
A physical change is temporary and reversible.

Question 2.
In a chemical change, the original substance loses its own and properties.
Answer:
In a chemical change, the original substance loses its own composition and properties.

Question 3.
A change is generally associated with the evolution or absorption of energy.
Answer:
A chemical change is generally associated with the evolution or absorption of energy.

Question 4.
When a chemical change takes place with the evolution of heat energy, the change is said to be .
Answer:
When a chemical change takes place with the evolution of heat energy, the change is said to be exothermic.

Question 5.
Water while freezing releases energy.
Answer:
Water while freezing releases heat energy.

C. Write true or false. Correct the false statements.

Question 1.
In a physical change, the chemical composition of the original substance is altered.
Answer:
False; In a chemical change, the chemical composition of the original substance is altered.

Question 2.
The melting of wax is a physical change.
Answer:
True

Question 3.
All changes of state are physical changes.
Answer:
True

Question 4.
Condensation refers to the change of state from liquid to gas on cooling.
Answer:
False; Condensation refers to the change of state from gas to liquid on cooling.

Question 5.
A chemical change is permanent and irreversible.
Answer:
True

Question 6.
Mass is created during a chemical change.
Answer:
False; Mass is neither created nor destroyed during a chemical change.

D. Classify the following changes into physical and chemical changes.

1. Melting of wax
2. Magnetization of iron
3. Digestion of food
4. Burning of a candle
5. Sublimation of iodine
6. Rusting of iron
7. Stretching a rubber band
8. Burning of a piece of magnesium ribbon
Answer:
Physical changes :
Melting of wax
Magnetization of iron
Sublimation of iodine
Stretching a rubber band

Chemical changes :
Digestion of food
Burning of a candle
Rusting of iron
Burning of a piece of magnesium ribbon

E. Answer the following in short

Question 1.
What is a physical change?
Answer:
A physical change is a temporary change in which no new substance is formed. This change can be reversed by reversing the conditions.

Question 2.
What is a chemical change?
Answer:
A chemical change is a permanent change in which the original substance loses its own composition and properties and gives rise to one or more new susbtances with different compositions and properties.

Question 3.
What is an endothermic change?
Answer:
When a chemical change takes place with the absorption of heat energy, the change is said to be endothermic.

Question 4.
What is an exothermic change?
Answer:
When a change takes place with the evolution of heat energy, the change is said to be exothermic.

Question 5.
What happens to the intermolecular force of attraction between the molecules in a change of state from solid to liquid?
ribbon
Answer:
In a change of state from solid to liquid or from liquid to gas, the intermolecular force of attraction between the molecules decreases.

Question 6.
What happens to the intermolecular space between the molecules in a change of state from liquid to solid?
Answer:
In a change of state from liquid to solid or from gas to liquid, the intermolecular space decreases.

F. Answer the following in detail.
Question 1.
Write the characteristics of physical changes.
Answer:
The following are the characteristics of a physical change :

  1. Generally, the change is temporary and can be reversed by changing the conditions.
  2. No new substance is formed. So, there is no change in the mass of the substance undergoing a physical change.
  3. There is no change in the chemical composition of the original substance.
  4. There may or may not be a change in the physical properties such as state, shape, size or colour of the original substance,
  5. No significant change in energy is involved.

Question 2.
Write the characteristics of chemical changes.
Answer:
The following are the characteristics of a chemical change.

  1. One or more new substances are formed with composition and properties different from the original substances.
  2. The change is permanent and irreversible.
  3. An energy change is generally involved in a chemical change. Energy is either absorbed or evolved during a chemical change.
  4. The mass of the substances undergoing a chemical change is generally altered.

Question 3.
Give the .main differences between physical changes and chemical changes.
Answer:
Physical changes :

  • No new substance is, formed. However, some physical . properties of the original substance may change.
  • The change is temporary and reversible.
  • There is no change in the mass of the substance undergoing a physical change.
  • No significant change in energy is involved.

Chemical changes :

  • One or more new substances with different properties and compositions from the original substance(s) are formed.
  • The change is permanent and irreversible.
  • There is a change in the mass of the substance undergoing a chemical change. However, the total mass of the substances involved in a chemical change remains constant.
  • Energy is either absorbed or evolved during a chemical change.

Question 4.
Explain the change of heat energy in a physical change.
Answer:
So far you have learned that in a physical change, two types of processes occur most often.One of the processes is change of state and the other is the dissolving of a substance in a liquid. Both these processes involve energy changes, mainly a change in heat energy. This is because in a change of state from solid to liquid or from liquid to gas, the intermolecular force of attraction between the molecules has to be decreased so that the intermolecular space increases. This is possible only when the solid or the liquid absorbs some heat energy.

Similarly, in a change of state from liquid to solid or from gas to liquid, the intermolecular force of attraction between the molecules has to be increased so that the intermolecular space decreases. This can happen when the liquid or the gas releases some heat energy.

Question 5.
Explain energy changes in a chemical change.
Answer:
In a chemical change of substances, the molecules and atoms of the substances rearrange themselves to form new substances. This rearrangement involves some energy changes. Energy is released in some chemical changes while in others it is absorbed. Energy in the form of heat and light is released in chemical changes, which involve the burning of a substance. Energy in the form of sunlight is absorbed in the process of photosynthesis, it is being a chemical change.

Think And Answer

Question 1.
Sublimation of camphor is a physical change. Explain.
Answer:
Camphor sublime, that is, when heated these substances directly
change into gaseous state, without passing through the liquid state. The vapour formed can be cooled to get back the solid. Thus, the change is reversible. And no new subtances are formed. Therefore, sublimation of solid is a physical change.

Question 2.
The burning of paper is a chemical change. Why?
Answer:
If you bum a piece of paper, new substances such as carbon dioxide, water vapour, smoke and ash are produced. The molecules of paper and oxygen combine and undergo change to form the molecules of these new substances. The change is permanent and cannot be reversed. You cannot combine ash, water vapour, etc. to form the original paper again. Thus, the burning of paper is a chemical change.

Question 3.
The melting of wax is a physical change. Why?
Answer:
During the burning of a candle, some wax melts and solidifies back to wax on cooling. Therefore, this shows that the melting of wax is a physical change.

Question 4.
The burning of a piece of magnesium ribbon in air is a chemical change. Explain.
Answer:
Magnesium bums in the air to form a new compound, magnesium oxide, which cannot be changed back to magnesium. This is a permanent and an irreversible change. Thus, the burning of magnesium ribbon in the air is a chemical change.

Living Science Chemistry Class 8 ICSE Solutions

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