Matter

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 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.

(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.

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.

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:

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

liquids

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

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.

vaporization

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

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.

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.

(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.

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

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.

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.

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.

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.

The processes involved in the two cases are —

(a) Boiling

(b) Evaporation

(a) The melting point of ice is 0°C.

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

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

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.

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.

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.

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.

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.

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

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.

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.

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 because when water from cloth strips evaporates it takes away heat from the body of the patient thus reducing the temperature of the body.

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, 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.

(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