Class 9 science Solutions English Medium

Terms for changing states: 

(A) Fusion: The process of melting, that is, change of solid state into liquid state is known as fusion.

(B) Solidification: The process of changing liquid state into solid is called solidification. 

(C) Vaporisation: The process of changing liquid state into gas is called vaporisation.

(D) Condensation: When gases get converted their state into liquid form is known as condensation. 

(E) Sublimation: Sublimation is the change of gaseous state directly to solid state without going through liquid state, and vice versa.

• Phenomenon of changing of a liquid into vapours at any temperature below its boiling point is called evaporation.
• Humidity is the amount of water vapour present in air.

atmosphere (atm) is a unit of measuring pressure exerted by a gas. The unit of pressure is Pascal (Pa):
1 atmosphere = 1.01 × 105 Pa. The pressure of air in atmosphere is called atmospheric pressure.

The atmospheric pressure at sea level is 1 atmosphere, and is taken as the normal atmospheric pressure.

Evaporation: 

Phenomenon of changing of a liquid into vapours at any temperature below its boiling point is called evaporation.

• 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.

How a liquid get converted into Vapour or get vapourised: 

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.

The Factors which influence the rate of Evaporation: 

1. An increase of surface area

2. An increase of temperature

3. A decrtrease in humadity

4. An increase in wind speed 

1. 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.
2. An increase of temperature: With the increase of temperature, more
number of particles get enough kinetic energy to go into the vapour state.
3. 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.
4. 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.

Understand this main point during evaporation: 

In every evaporation 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. 

Same fiction can apply for when we pour some acetone (nail polish) or petrol on our palm. 

Q:- Why does our palm feel cold when we put some acetone or petrol or perfume on it?

- The particles of acetone or petrol gain energy from your palm or surroundings and get evaporated and this evaporate absorbation of heats from palm causeing the palm to feel cool.

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

- A saucer increases the surface area for tea or milk cause faster evaporation, this evaporation causing coolness so we able to sip easily. 

Q: 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.