Chemistry - Matter in our Surroundings

EVAPORATION -

The phenomenon of change of liquid into vapours at any temperature below its boiling point is called evaporation.

Surface Phenomenon - the process only occurs at the surface of the liquid.

HOW DOES EVAPORATION HAPPEN?

in liquids → small fractions of particles on the surface have higher kinetic energy (exposed to sunlight? - heat) → break away from the forces of attraction of other particles and converted into vapour.

FACTORS AFFECTING EVAPORATION

→ increase of surface area - evaporation is a surface phenomenon → ithe greater the surface area of the container, the greater the number of liquid molecules present at the surface. the larger the number of liquid molecules are at the surface, the larger the number of molecules that will break away from the liquid and become a gas at a given point of time, which contributes to an increase in the rate of evaporation. example - we spread out clothes while drying it.

→ increase of temperature - when there is more temperature, then the rate of evaporation is higher → more number of particles get enough kinetic energy to go into vapour state. this increase in energy makes it easier for the liquid molecules to overcome the intermolecular forces of attraction (that holds the liquid together) and escape into the atmosphere as a gas.

→ decrease in humidity - (humidity - amount of water vapour present in air) - air around us cannot hold more than a definite amount of water vapour at a specific temperature → if the amount of water vapour in air is high, then the rate of evaporation decreases.

→ increase in wind speed - when there is more wind speed, then the rate of evaporation is higher → particles of water vapour move away with the wind which decreases humidity. example - clothes dry faster on a windy day.

HOW DOES EVAPORATION CAUSE COOLING?

particles of liquid keeps on absorb energy from its surroundings to regain the energy lost during evaporation → absorption of energy from the surroundings make the surroundings cold.

example - when you pour acetone on your palm → particles gain energy from your palm/ its surroundings and evaporate which causes the palm to feel cool.

LATENT HEAT OF VAPORISATION

Latent heat of vaporisation is the heat energy required to change1 kg of a liquid to gas at atmospheric pressure at its boiling point.

example 1 - on a hot sunny day people sprinkle water on the roof / open ground → large latent heat of vaporisation helps to cool the hot surface → heat energy = latent heat is absorbed from the body → makes the body cool → cotton a good absorbent of water → helps to absorb sweat and expose it to the atmosphere for easy evaporation

example 2 - when we take ice-cold water in a tumbler → water droplets formed on the outer surface of the tumbler → water vapour in the air → comes in contact w/ the cold glass of water → loses energy → gets converted to liquid state (water droplets)

QUESTIONS -

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

It is because the temperature is high and less humid on a hot dry day, enabling better evaporation. High levels of this evaporation provide better cooling effects.

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

An earthen pot is porous in nature. These tiny pores facilitate the penetration of water and hence their evaporation from the pot surface. The process of evaporation requires energy which is contributed by water in the pot as a result of which water turns cooler.

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

Acetone, petrol, and perfume are volatile substances that evaporate when they come in contact with air. Evaporation is facilitated as it uses energy from the palm, hence leaving a cooling effect on our palms.

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

A saucer has a larger surface area than a cup, promoting quicker evaporation. Hence, the tea or milk in a saucer cools down faster.

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

In summer, it is preferred to wear light-coloured cotton clothes because light colour reflects heat and cotton materials have pores that absorb sweat, facilitating evaporation, and hence causing a cooling effect on the skin.