ICSE Living Science Physics for Class 8 Solutions Chapter 1 Matter
Think and answer
Question 1.
What happens to the arrangement of molecules of solids on melting ?
Answer:
When a solid melts, the kinetic energy of its molecules increases. So, the molecules start moving faster, change their positions easily and slide over one another. As a result, the intermolecular space between the molecules increases and the solid gets converted to its liquid form.
Check Your Progress
Answer the following.
Question 1.
Differentiate between atoms and molecules.
Answer:
Atom :
- An atom is the smallest unit of element.
- It may or may not have an independent existence.
Molecule :
- A molecule is the smallest unit of matter.
- It has an independent existence.
Question 2.
How does the kinetic energy of molecules in a substance depend on its temperature ?
Answer:
The kinetic energy of molecules is directly proportional to the temperature of the substance.
Question 3.
Describe the arrangement of molecules in the three different states of matter.
Answer:
Arrangements of molecules in solids :
Molecules in solids are very tightly packed, thus the intermolecular spaces between them are very small. Hence, the intermolecular force of attraction is very strong in solids. The strong intermolecular force of attraction does not allow molecules in solids to move away from their fixed positions.
Arrangements of molecules in liquids :
The molecules in a liquid are loosely packed. Therefore, intermolecular spaces in a liquid are more than that in a solid. Thus, the intermolecular force of attraction in a liquid is weaker as compared to that in a solid. Because of the weak intermolecular force of attraction molecules in a liquid can change their positions and can also slide over one another.
Arrangements of molecules in gas :
The intermolecular spaces in a gas are very large as compared to that in a solid or a liquid.
Thus, the intermolecular force of attraction is negligible in a gas. Due to this negligible intermolecular force of attraction, molecules in a gas are able to move independently and occupy the entire space in a container.
Question 4.
Explain the properties of liquids on the basis of kinetic theory of matter.
Answer:
- The properties of liquids are given below.
- The molecules in a liquid are loosely packed. Therefore, intermolecular spaces in a liquid are more than that in a solid. Thus, the intermolecular force of attraction in a liquid is weaker as compared to that in a solid.
- Because of the weak intermolecular force of attraction, molecules in a liquid can change their positions and can also slide oyer one another. This enables a liquid to flow. So, liquids do not have a definite shape, they take the shape of the container in which they are kept.
- Although liquids do not have a definite shape, they have a definite volume. This is because there exists substantial intermolecular force of attraction, which holds the molecules together.
- Liquids have larger intermolecular spaces than solids. Therefore, they may be compressed slightly.
- On heating, liquids expand more than solids because of the weaker intermolecular force of attraction.
Question 5.
Differentiate between the force of cohesion and the force of adhesion.
Answer:
The force of attraction between molecules of the same kind (within a substance) is called the force of cohesion whereas the force of attraction between molecules of different kinds (of different substances) is called the force of adhesion.
A. Tick the most appropriate answer.
Question 1.
Molecules of a solid
a. vibrate at their positions.
b. move only when heated.
c. move because they are loosely bound.
d. do not move at all.
Answer:
a. vibrate at their positions.
Question 2.
The force of attraction between two similar types of molecules is called
a. adhesion.
b. cohesion,
c. electrostatic force
d. conduction.
Answer:
b. cohesion
Question 3.
The temperature at which a solid changes into its liquid state is called
a. boiling point
b. freezing point
c. condensation point
d. melting point
Answer:
d. melting point
Question 4.
How does the kinetic energy of a molecule in a substance change with a decrease in its temperature?
a. It remains the same
b. It increases
c. It decreases
d. It becomes twice
Answer:
c. It decreases
Question 5.
The state of matter, in which the kinetic energy of the molecules is so small that they cannot overcome the intermolecular force of attraction between them, is –
a. solid
b. liquid
c. gas
d. plasma
Answer:
a. solid
Question 6.
In convection currents,
a. hot molecules rise up
b. cold molecules sink down
c. molecules actually move from their place
d. all of these
Answer:
d. all of these
B. Fill in the blanks.
Question 1.
The force of attraction between molecules of a substance is called ………….
Answer:
The force of attraction between molecules of a substance is called intermolecular force of attraction.
Question 2.
The intermolecular force of attraction between the molecules ………… with increase in temperature.
Answer:
The intermolecular force of attraction between the molecules decreases with increase in temperature.
Question 3.
In solids, intermolecular spaces are very ……………..
Answer:
In solids, intermolecular spaces are very small.
Question 4.
The molecules of a gas are ………………. packed.
Answer:
The molecules of a gas are loosely packed.
Question 5.
During melting of a solid, its ……………….. remains constant.
Answer:
During melting of a solid, its temperature remains constant.
Question 6.
The process of water changing into its vapour at 50°C is called ………………
Answer:
The process of water changing into its vapour at 50°C is called evaporation.
C. Write true or false. Correct the false statements.
Question 1.
An atom is defined as the smallest unit of matter that has an independent existence, and carries the physical and chemical properties of that matter.
Answer:
False.
Correct : A molecule is defined as the smallest unit of matter that has an independent existence, and carries the physical and chemical properties of that matter.
Question 2.
The intermolecular force of attraction increases when intermolecular space increases.
Answer:
False.
Correct : The intermolecular force of attraction increases when intermolecular space decreases.
Question 3.
If the molecules in a substance have very large kinetic energies, which can completely overcome the intermolecular force of attraction between them, then the substance is a liquid.
Answer:
False.
Correct : If the molecules in a substance have very large kinetic energies, which can completely overcome the intermolecular force of attraction between them, then the substance is a gas.
Question 4.
When a gas is cooled, its molecules lose energy and slow down.
Answer:
True.
Question 5.
The force of attraction between the molecules of water and that of glass is the force of cohesion.
Answer:
False.
Correct : The force of attraction between the molecules of water and that of glass is the force of adhesion.
Question 6.
The process of change of state from gaseous state directly to solid state is called sublimation.
Answer:
False.
Correct : The process of change of state from gaseous state directly to solid state is called deposition.
D. Match the columns.
| Solid to liquid | a. least intermolecular spaces |
| Liquid to gas | b. least intermolecular forces |
| Solid | c. melting |
| Gas | d. mode of heat transfer |
| Conduction | e. evaporation |
Answer:
| Solid to liquid | a. least intermolecular spaces |
| Liquid to gas | b. least intermolecular forces |
| Solid | c. melting |
| Gas | d. mode of heat transfer |
| Conduction | e. evaporation |
E. Define the following terms.
- Freezing point
- Condensation
- Element
- Condensation point
- Vaporization
- Fusion
Answer:
- Freezing point – The temperature at which a liquid freezes to become a solid is called the freezing point of the liquid.
- Condensation – The process of conversion of vapour or gas into its liquid state, at a particular temperature, on cooling is called condensation.
- Element – An element is defined as a substance that cannot be subdivided into two or more simpler substances by any physical or chemical method.
- Condensation point – The temperature at which a vapour or a gas changes to a liquid is called the condensation point of the gas.
- Vaporization – The process of conversion of a liquid into its gaseous state, at a particular temperature, on
heating is called vaporization. - Fusion – The process of conversion of a solid into its liquid state, at a particular temperature, on heating is called fusion.
F. Answer the following in short.
Question 1.
Define intermolecular force and intermolecular spaces.
Answer:
Intermolecular force : Intermolecular force can be defined as the force of attraction which the molecules of matter always exert on each other.
Intermolecular spaces : It is the distance between the molecules of matter.
Question 2.
Why do liquids expand more than solids on heating?
Answer:
On heating, liquids expand more than solids because of weaker intermolecular force of attraction between their molecules.
Question 3.
Name three factors that can help you decide whether a substance is a solid, liquid or a gas.
Answer:
The three factors that decide the state of the matter are the strength of intermolecular force of attraction, intermolecular space and the kinetic energy of the molecules.
Question 4.
What are adhesives ?
Answer:
Adhesives are the substances that have the capacity to join and hold different types of material together by adhesion.
Question 5.
How can you explain the process of freezing, using kinetic theory of matter ?
Answer:
When the temperature of a liquid is decreased, its molecules lose energy and slow down. So, the intermolecular forces of attraction between them increase, and they come close to each other. With continued cooling (during freezing), the intermolecular spaces decrease to such an extent that the liquid changes into its solid state.
Question 6.
The freezing point of glycerol is 17.8°C and its boiling point is 290°C. Find its melting point and condensation point.
Answer:
Freezing point of glycerol = 17.8 °C Boiling point of glycerol = 290 °C
Melting point of glycerol = Freezing point of glycerol = 17.8 °C
Condensation point of glycerol = Boiling point of glycerol = 290°C
G. Answer the following in detail.
Question 1.
On the basis of the kinetic theory of matter, explain why ?
a. solids have a Fixed shape and a fixed volume.
Answer:
According to kinetic theory of matter, in solids, molecules have strong force of attraction that holds the molecules in a fixed place, so the solids have fixed shape and due to fixed shape, solids are rigid and hard. So, they have fixed volume.
b. liquids have a fixed volume but not a fixed shape.
Answer:
According to kinetic theory of matter, molecules of liquid have lesser intermolecular force of attraction, they do not stay at a fixed place, so they do not have fixed shape but they have definite volume as though the molecules of a liquid are loosely packed but there still exists substantial intermolecular force that holds the molecules together to have a definite volume.
c. gases have neither a fixed volume nor a fixed shape.
Answer:
According to kinetic theory of matter, due to large intermolecular distance, there is less or negligible force of attraction between the molecules in a gas. Therefore, the molecules are free to move. So, they neither have fixed shape nor fixed volume.
Question 2.
State the main postulates of the kinetic theory of matter.
Answer:
The main postulates of the kinetic theory of matter are as follows :
- Matter is made up of very small particles called molecules which are in a state of constant motion. Due to this motion they posses kinetic energy.
- The motion of particles and their kinetic energy is directly proportional to heat. When the substance is heated, the velocity of particles also increases. As a result their K.E. also increases and when the substance is cooled, it loses its heat and velocity of particles decreases. As a result, K.E. also decreases.
- Molecules of matter always exert force of attraction on each other. This force of attraction is called intermolecular force of attraction.
- Intermolecular force of attraction is inversely proportional to the intermolecular distance.
Question 3.
Explain conduction of heat in solids on the basis of the kinetic theory of matter.
Answer:
The mode of transfer of heat energy from one molecule to another molecule in the direction of heat flow, from higher temperature to lower temperature without actual movement of molecules from their mean position is called conduction.
According to kinetic theory of matter, when we heat some solid, the molecules get heat energy, the kinetic energy of molecules increases, they vibrate about their mean position with greater amplitude. As a result, the molecules collide with next molecules and transfer part of heat energy to them and so on. In this way, the heat gets transferred from hotter part to colder one.
Question 4.
Demonstrate the process of boiling with the help of an activity.
Answer:
The process of conversion of a liquid into its gaseous state, at a particular temperature, on heating is called boiling. The following activity demonstrates the process of boiling of water.
Materials required : A beaker filled with water, a thermometer, a clamp stand, a Bunsen burner, a tripod stand and a wire gauze.
Procedure :
Place the beaker filled with water on the wire gauze and the tripod stand. Suspend the thermometer with the help of the clamp stand and lower it into the beaker. Start heating the beaker with the help of Bunsen burner. Observed what happens to the water and record the temperature after every 30 seconds.
Observations :
- The temperature of the water rises till it reaches the temperature of 100 °C.
- The water starts getting converted to steam, when its temperature reaches 100 °C.
- The temperature recorded by the thermometer remains fixed at 100 °C till the water is completely converted into steam.
Conclusion :
When heat is supplied to water, the temperature of the water rises till it reaches 100 °C.
The water then starts getting converted into steam at a constant temperature of 100 °C. This process is known as boiling and the temperature of 100 °C is known as the boiling point of water.
Think and answer :
Question 1.
Which change of state of matter is responsible for the formation of clouds ?
Answer:
Condensation is responsible for the formation of clouds.
Question 2.
Why does kinetic energy of the molecules of a substance increase when its temperature is raised ?
Answer:
When temperature of a substance increases, the molecules gain heat energy. They start moving faster. As the kinetic energy of the molecules is directly proportional to their velocities, therefore, their kinetic energy increases with increase in temperature.
Question 3.
Why does water curve downward when placed in a glass tube ?
Answer:
The force of adhesion between the molecules of water and glass is responsible for the downward curve of water placed in the glass tube. Due to this force, the water molecules cling to the walls of the glass tube and travel up along these walls as far as the forces of gravity and cohesion between the molecules of water allow them.
Living Science Physics Class 8 ICSE Solutions