NCERT Class 9 Science Textbook for Blind and Visually Impaired Students made Screen Readable by Professor T K Bansal.

Do we always need to heat or change pressure for changing the state of matter?
Can you quote some examples from everyday life where change of state from liquid to vapour takes place without the liquid reaching the boiling point?

Water, when left uncovered, slowly changes into vapour.
Wet clothes dry up.
What happens to water in the above two examples?

We know that particles of matter are always moving and are never at rest. At a given temperature in any gas, liquid or solid, there are particles with different amounts of kinetic energy. In the case of liquids, a small fraction of particles at the surface, having higher kinetic energy, is able to break away from the forces of attraction of other particles and gets converted into vapour. This phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation.

1.5.1 FACTORS AFFECTING EVAPORATION

Let us try to understand this with the help of an activity.

Activity 1.14

• Take 5 mL of water in a test tube and keep it near a window or under a fan.
• Take 5 mL of water in an open china dish and keep it near a window or under a fan.
• Take 5 mL of water in an open china dish and keep it inside a cupboard or on a shelf in your class.
• Record the room temperature.
• Record the time or days taken for the evaporation process in the above cases.
• Repeat the above three steps of activity on a rainy day and record your observations.
• What do you infer about the effect of temperature, surface area and wind velocity (speed) on evaporation?

You must have observed that the rate of evaporation increases with:

• an increase of surface area:
We know that evaporation is a surface phenomenon. If the surface area is increased, the rate of evaporation increases. For example, while putting clothes for drying up we spread them out.

• an increase of temperature:
With the increase of temperature, more number of particles get enough kinetic energy to go into the vapour state.

• a decrease in humidity:
Humidity is the amount of water vapour present in air. The air around us cannot hold more than a definite amount of water vapour at a given temperature. If the amount of water in air is already high, the rate of evaporation decreases.

• an increase in wind speed:
It is a common observation that clothes dry faster on a windy day. With the increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding.

1.5.2 HOW DOES EVAPORATION CAUSE COOLING?

In an open vessel, the liquid keeps on evaporating. The particles of liquid absorb energy from the surrounding to regain the energy lost during evaporation. This absorption of energy from the surroundings make the surroundings cold.

What happens when you pour some acetone (nail polish remover) on your palm? The particles gain energy from your palm or surroundings and evaporate causing the palm to feel cool.

After a hot sunny day, people sprinkle water on the roof or open ground because the large latent heat of vaporisation of water helps to cool the hot surface.

Can you cite some more examples from daily life where we can feel the effect of cooling due to evaporation?

Why should we wear cotton clothes in summer?

During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool. Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation.

Why do we see water droplets on the outer surface of a glass containing ice-cold water?

Let us take some ice-cold water in a tumbler. Soon we will see water droplets on the outer surface of the tumbler. The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets.

Questions

Q1. Why does a desert cooler cool better on a hot dry day?

A1. On a hot summer day, the humidity in the air is very low, and hence a lot of evaporation takes place and causing cooling.

Q2. How does the water kept in an earthen pot (matka) become cool during summer?

A2. The fine pores on the body of the matka allows water to seep out of it. This on evaporation causes cooling.

Q3. Why does our palm feel cold when we put some acetone or petrol or perfume on it?

A3. When acetone is poured on our palm, it evaporates by absorbing heat from our palm. That is why we feel cold.

Q4. Why are we able to sip hot tea or milk faster from a saucer rather than from a cup?

A4. Since the surface area of a saucer is more than that of a cup, more evaporation is able to take place from the saucer than from a cup. Which is why we are able to sip tea or milk more easily in it.

Q5. What type of clothes should we wear in summer?

A5. We must wear light colored cotton clothes in summer.