Matter

The inter-molecular force is maximum in:

1. solids
2. gases
3. liquids
4. none of the above

Answer

solids

Reason — When inter-molecular force is very strong and kinetic energy is very less, the inter-molecular space is reduced and matter exists as a solid.

The inter-molecular space is maximum in:

1. liquids
2. solids
3. gases
4. none of the above

Answer

gases

Reason — In gases, inter-molecular force is negligible and kinetic energy is very high. Due to this inter-molecular space is maximum in gases.

The molecules can move freely anywhere in

1. gases
2. liquids
3. solids
4. none of the above

Answer

gases

Reason — The inter-molecular force of attraction between the molecules is least and the inter-molecular space is maximum in gases so the molecules can move freely anywhere.

The molecules move only within the boundary in

1. liquids
2. gases
3. solids
4. none of the above

Answer

liquids

Reason — The inter-molecular forces, although weak, are sufficient to keep the molecules of the liquid within the boundary of the vessel.

The temperature at which a liquid gets converted into its vapour state is called its

1. melting point
2. boiling point
3. dew point
4. freezing point

Answer

boiling point

Reason — The constant temperature at which a substance changes from the liquid state to its vapour state by absorbing heat is called its boiling point.

Rapid conversion of water into steam is an example of

1. evaporation
2. freezing
3. melting
4. vaporization

Answer

vaporization

Reason — Vaporization is a rapid and violent process where liquid changes into vapour at constant temperature by absorbing heat.

Evaporation takes place from the

1. from the surface of liquid
2. throughout the liquid
3. from the mid-portion of the liquid
4. from the bottom of liquid

Answer

surface of liquid

Reason — Evaporation is a surface phenomenon and the molecules on the surface have highest kinetic energy so the molecules get easily escaped and liquids get converted into gaseous state.

Boiling takes place from

1. from the surface of the liquid
2. throughout the liquid
3. from mid-portion of liquid
4. from none of the above

Answer

throughout the liquid

Reason — Boiling is not a surface phenomenon and boiling takes place at a fixed temperature when the whole liquid reaches that fixed temperature.

Identify the correct statement.

(1) The molecules of water move faster than ice.

(2) The molecules are more closely packed in ice than water.

(3) The molecules of water vibrate to and fro only but those of ice are free to move within the boundary of the vessel in which it is kept.

(4) The molecules have more intermolecular space in ice than in water.

1. (1), (2) and (3)
2. (2), (3) and (4)
3. (1) and (2)
4. all are correct

Answer

(1) and (2)

Reason —

(1) Correct because water is in liquid state, so its molecules have more kinetic energy and move faster than those in solid ice.

(2) Correct as ice is in solid state where intermolecular spaces between the particles are very less than those of water which is in liquid state.

(3) Incorrect since in ice (solid), particles vibrate in fixed positions (to and fro) but in water (liquid), molecules are free to move within the vessel.

(4) Incorrect because water has more intermolecular space than ice.

Assertion (A) : We often pour tea in a saucer to cool it faster.

Reason (R) : Evaporation takes place at all temperatures.

1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true and R is not the correct explanation of A
3. Assertion is true but reason is false
4. Assertion is false but reason is true

Answer

Both A and R are true and R is not the correct explanation of A

Explanation

Assertion (A) is true because the rate of evaporation increases if the area of surface exposed increases so pouring tea into a saucer increases the surface area, which enhances evaporation, allowing the tea to cool more quickly.

Reason (R) is true because evaporation is a surface phenomenon which occurs at any temperature, not just at the boiling point.

But here reason does not explain the assertion.

Assertion (A) : The molecules in a liquid can move within the boundary of the vessel.

Reason (R) : A liquid has a definite volume and a definite shape.

1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true and R is not the correct explanation of A
3. Assertion is true but reason is false
4. Assertion is false but reason is true

Answer

Assertion is true but reason is false

Explanation

Assertion (A) is true because according to molecular model of liquids, the liquid molecules can move about freely within the boundary of the vessel in which the liquid is kept.

Reason (R) is false because liquids do not have a definite shape, but they have a definite volume.

Fill in the blanks:

(a) Evaporation takes place at ............... temperatures.

(b) ............... process is just the reverse of melting.

(c) ............... is a process that involves direct conversion of a solid into its vapour on heating.

(d) The temperature at which a solid converts into a liquid is called its ............... .

(e) The smallest unit of matter that exists freely in nature is called ............... .

(f) Molecules of a substance are always in a state of ............... and so they possess ............... .

(g) Inter-molecular space is maximum in ............... less in ............... and the least in ............... .

(h) Inter-molecular force of attraction is maximum in ............... , less in ............... and the least in ............... .

Answer

(a) Evaporation takes place at all temperatures.

(b) Freezing process is just the reverse of melting.

(c) Sublimation is a process that involves direct conversion of a solid into its vapour on heating.

(d) The temperature at which a solid converts into a liquid is called its melting point.

(e) The smallest unit of matter that exists freely in nature is called molecule.

(f) Molecules of a substance are always in a state of motion and so they possess kinetic energy.

(g) Inter-molecular space is maximum in gases less in liquids and the least in solids.

(h) Inter-molecular force of attraction is maximum in solids, less in liquids and the least in gases.

Write true of false for each statement :

(a) The temperature of a substance remains unaffected during its change of state.

(b) Ice melts at 100°C.

(c) Water at 100°C has more heat than steam at 100°C.

(d) Evaporation of a liquid causes cooling.

(e) Water evaporates only at 100°C.

(f) Boiling takes place at all temperatures.

(g) Evaporation takes place over the entire mass of the liquid.

(h) The process of a gas converting directly into solid is called vaporization.

(i) At high altitudes, water boils above 100°C.

(j) The melting point of ice is 0°C.

Answer

(a) True

(b) False
Correct Statement — Ice melts at 0°C.

(c) False
Correct Statement — Steam at 100°C has more heat than water at 100°C.

(d) True

(e) False
Correct Statement — Water evaporates at all temperatures.

(f) False
Correct Statement — Boiling takes place at a fixed temperature which is 100°C.

(g) False
Correct Statement — Evaporation takes place only at the surface of the liquid.

(h) False
Correct Statement — The process of a gas converting directly into solid is called deposition or solidification.

(i) False
Correct Statement — At high altitudes, water boils below 0°C.

(j) True

Match the following:

Answer

Define the term matter. What is it composed of?

Answer

A matter is something that has mass, occupies space and can be perceived by our senses. It is composed of tiny particles called molecules.

State three properties of molecules of matter.

Answer

The three properties of molecules of matter are

1. They are very small in size.
2. They have spaces between them.
3. They possess kinetic energy so they are in constant motion.

What do you mean by inter-molecular spaces? How do they vary in different states of matter?

Answer

The space between the molecules of matter is called inter-molecular space. In solids the inter-molecular space is the least so the molecules are tightly packed. In liquids the spacing is more than solids where molecules can move freely but within the boundary of the vessel. In gases the inter-molecular space is the most so the molecules can move freely anywhere.

What is meant by inter-molecular forces of attraction? How do they vary in solids, liquids and gases?

Answer

The molecules of matter exert a force of attraction on each other known as inter-molecular forces of attraction. This force of attraction is maximum in solids, less in liquids and the least in gases.

Which of the following are correct?

(a) Solids have definite shape and definite volume.

(b) Liquids have definite volume but no definite shape.

(c) Gases have definite volume but no definite shape.

(d) Liquids have both definite shape and definite volume.

Answer

The correct options are

(a) Solids have definite shape and definite volume.

Reason — In solids the inter-molecular space is the least and the molecules remain in their fixed positions due to strong inter-molecular force of attraction so solids have definite shape and definite volume.

(b) Liquids have definite volume but no definite shape.

Reason — The molecules in liquid are less closely packed, their positions are not fixed and they move freely. Although the inter-molecular force of attraction is weak as compared to solids but it is sufficient to keep them within the boundary of the vessel. So liquids have definite volume but no definite shape.

A liquid can change into vapour state

(a) at a fixed temperature and

(b) at all temperatures

Name the processes involved in the two cases.

Answer

The processes involved in the two cases are —

(a) Boiling

(b) Evaporation

State

(a) the melting point of ice and

(b) the boiling point of water

Answer

(a) The melting point of ice is 0°C.

(b) The boiling point of water is 100°C.

What is evaporation?

Answer

Evaporation is the process by which a liquid changes into vapour at all temperatures from the surface of liquid.

State three factors which affect the rate of evaporation of a liquid.

Answer

Three factors which affect the rate of evaporation of a liquid are —

1. The temperature of liquid — If temperature of liquid is high, rate of evaporation is also high.
2. The area of exposed surface — Rate of evaporation increases with increase in the area of exposed surface.
3. The presence of moisture or humidity — Evaporation is faster in dry air than in humid air.

Why are volatile liquids such as alcohol and spirit stored in tightly closed bottles?

Answer

Rate of evaporation depends upon the nature of liquid. Volatile liquids like alcohol and spirit have low boiling point, so they evaporate much faster than water. So they are stored in tightly closed bottles to avoid their evaporation.

Why is cooling produced on evaporation of a liquid?

Answer

During evaporation process a liquid changes into gas by absorbing heat from the surroundings. This results in fall of temperature of the surroundings. So cooling is produced on evaporation of a liquid.

Give two applications of evaporation.

Answer

The two applications of evaporation are —

1. Water kept in earthen pots get cooled in summer. Water gets evaporated through the pores of pot by absorbing heat from the water present inside the pot leading to cooling.
2. Tea in saucer gets cooled faster than in cup because the rate of evaporation increases with increase in surface area.

What do you mean by sublimation? Explain with an example.

Answer

Sublimation is the process by which a solid when heated, directly changes into vapour without first changing into liquid.

For example, when ammonium chloride is heated it directly changes into vapour without changing into liquid.

Why does the size of naphthalene balls decrease when left open?

Answer

Naphthalene balls when left open, directly change into vapour without heating due to sublimation. Hence, with time they become small in size.

Discuss the three states of matter: solid, liquid and gas on the basis of molecular model.

Answer

Molecular Model of Solids

The molecular model of solids is shown in the figure below:

1. Each solid is made up of very tiny particles called molecules that are in a rigid arrangement.
2. The inter-molecular space is very small incase of solids.
3. The molecules in a solid can only vibrate to and fro about their mean positions. They do not leave their positions.
4. The molecules in a solid are closely packed due to strong inter-molecular force of attraction between them.
5. Solids have definite shape and volume.

Molecular Model of Liquids

The molecular model of liquids is shown in the figure below:

1. Each liquid is made up of very tiny particles called molecules that are not in a rigid arrangement.
2. The inter-molecular spaces in liquids are more than that in solids.
3. The molecules in a liquid are less closely packed and their positions are not fixed as they are free to move within the boundary of the vessel in which the liquid is kept.
4. Although the inter-molecular force of attraction is weak as compared to solids but it is sufficient to keep them within the boundary of the vessel.
5. Liquids have definite volume but no definite shape.

Molecular Model of Gases

The molecular model of gases is shown in the figure below:

1. Gases are made up of very tiny particles called molecules that are not in a rigid arrangement.
2. In gases the inter-molecular space is maximum and the force of attraction is least as compared to solids and liquids.
3. The position of these molecules are not fixed. The molecules can move randomly anywhere due to weak inter-molecular forces.
4. Gases have neither definite shape nor definite volume.

What do you mean by 'the change of state'? Write the flowchart showing the complete cycle of change of state.

Answer

The process of change from one state to another state either by absorption or rejection of heat at a constant temperature is called change of state.

Flowchart showing the complete cycle of change of state is shown below:

Differentiate between melting point and boiling point, giving atleast one example of each.

Answer

Describe the process of condensation and sublimation with examples.

Answer

The change from vapour state to liquid state at a constant temperature on releasing heat (or on cooling) is called condensation.

Example — Steam (gas) at 100°C condenses to water (liquid) at 100°C on cooling (i.e. by rejecting heat).

Sublimation is the process by which a solid when heated directly changes into its vapour (gaseous state) without first changing into liquid.

Example — When camphor (solid) is heated it directly changes into its vapours (gaseous state).

Explain the terms melting and melting point.

Answer

The change from solid state to liquid state on heating at a fixed temperature is called melting.

The temperature at which a solid changes into liquid without further increase in temperature is called the melting point of the solid.

Example — Ice (solid) at 0°C melts to form water (liquid) at 0°C by absorbing heat.

Describe an experiment to demonstrate that a substance absorbs heat during melting without change in its temperature.

Answer

The following experiment demonstrates that a substance absorbs heat during melting without change in its temperature —

1. Take a test tube with some wax in it.
2. Clamp the test tube in a vertical stand and place the test tube in a beaker of water.
3. Place the beaker on a wire gauze which is kept on a tripod stand.
4. Clamp a thermometer in the same vertical stand and insert the thermometer in the test tube such that the bulb of the thermometer is inside the wax as shown in the figure above.
5. Heat the beaker over the flame of a burner.
6. Record the temperature of the wax from the thermometer after every minute.

Observations:

Conclusion: From the above observations, we can conclude that wax melts at 55°C during which heat is supplied but the temperature does not rise. Hence, wax absorbs heat during melting without change in its temperature.

Explain the terms vaporization and boiling point.

Answer

The change from liquid state to gaseous state on heating at a constant temperature by absorption of heat is called vaporization.

The temperature at which a liquid changes into vapour without further increase in temperature is called the boiling point of the liquid.

Example — Water (liquid) at 100°C changes into steam (gas) at 100°C by absorbing heat.

Describe an experiment to demonstrate that water absorbs heat during boiling at a constant temperature.

Answer

The following experiment demonstrates that water absorbs heat during boiling at a constant temperature —

1. Take a beaker and pour some water in it.
2. Place the beaker on a wire gauze placed over a tripod stand.
3. Clamp a thermometer in a vertical stand and insert it in the beaker.
4. Heat the beaker over the flame of a burner.
5. Record the temperature of water after every minute.

Observation — The temperature of water gradually rises and when it reaches 100°C, water starts boiling. Once the water starts boiling, its temperature does not rise any further, although heat is still being supplied.

Conclusion — From the above experiment we can conclude that, at 100°C water begins to boil and changes into steam by absorbing the heat being supplied to it without further rise in temperature.

Wet clothes dry more quickly on a warm dry day than on a cold humid day. Explain.

Answer

Evaporation depends upon the presence of moisture or humidity and temperature. Rate of evaporation is faster in dry air than in humid air. It also increases with increase in temperature. So wet clothes dry more quickly on a warm dry day than on a cold humid day.

Explain with an example that when a liquid evaporates, it takes heat from its surroundings.

Answer

If some alcohol (or spirit) is poured on some cotton (or wool) wrapped around the bulb of a thermometer, the reading of the thermometer falls. The reason for this observation is that, to evaporate, alcohol requires heat which it absorbs from the cotton wrapped around the thermometer (it's surroundings) producing a cooling effect that is recorded by the thermometer as a fall in temperature.

Explain why in hot summer days water remains cool in earthen pots?

Answer

Water seeps out on the surface through the pores in the earthen pot and it evaporates. The heat required for evaporation is taken from water inside the pot which therefore gets cooled.

Describe an experiment to demonstrate the process of sublimation.

Answer

The process of sublimation is demonstrated by the following experiment —

1. Take some ammonium chloride and powder it.
2. Keep the powder in a china dish.
3. Cover the china dish with an inverted funnel as shown in the figure.
4. Close the end of the funnel with a piece of cotton.
5. Place the china dish on a wire mesh kept on a tripod stand and heat it by a burner.

Observations — We will observe that vapours of ammonium chloride are formed in the inverted funnel above the dish. These vapours upon rising, get cooled and change to solid ammonium chloride which gets deposited on the inner walls of the inverted funnel.

Conclusion — Ammonium chloride on heating changes directly into gaseous state and when these vapours are cooled they get converted back into solid state. This demonstrates the process of sublimation.

Some ice is taken in a beaker and its temperature is recorded after each one minute. The observations are listed below:

From the above observations what conclusion do you draw about the melting point of the ice?

Answer

From the above observations it is concluded that the melting point of ice is 0°C.

At 0°C ice starts melting by absorbing heat without any rise in its temperature. After 5 minutes, when the whole ice melts into water, the temperature of this water starts rising further.

Water in a dish evaporates faster than in a bottle. Give reason.

Answer

Rate of evaporation increases with increase in area of the exposed surface. So water in a dish evaporates faster than in bottle as the area exposed in dish is more than that in bottle.

A certain quantity of water is heated from 20°C to 100°C. Its temperature is recorded after each 1 minute. The observations are:

What conclusion do you draw from the above table about the boiling point of water? Explain.

Answer

From the above table it is concluded that the boiling point of water is 100°C.

At 100°C, water begins to boil and changes into steam by absorbing the heat being supplied to it without further rise in temperature.

A patient suffering from high fever is advised to put wet cloth strips on his forehead. Why?

Answer

A patient suffering from high fever is advised to put wet cloth strips on his forehead because when water from cloth strips evaporates it takes away heat from the body of the patient thus reducing the temperature of the body.

When we put some acetone or petrol on our palm, it feels cold. Give reason.

Answer

Acetone or petrol evaporates quickly when applied to the palm so the liquid particles absorb heat energy from the skin to overcome intermolecular forces and escape into the air and this absorption of heat from the palm causes a cooling sensation.

On a humid day, you sweat more and feel less cool in comparison to a breezy day where sweating makes you feel much cooler. Give reason.

Answer

On a humid day, the air already contains a high amount of moisture, so the rate of evaporation of sweat from the skin is slower. Since evaporation causes cooling, less evaporation means less cooling effect. On a breezy day, the dry moving air enhances the evaporation of sweat, which draws more heat from the body and makes us feel cooler.