IX A Science 21/4/21 Topic taught and Homework
Topic taught
Effect of change of Pressure
By applying pressure, we can bring the particles of matter close to each other thereby, increasing the force of attraction among the particles.
When we compress and decrease the temperature of a gas, the gas changes into a liquid.
Dry Ice – Carbon dioxide in solid form is known as Dry Ice. It can directly turn into gas by decreasing the pressure to 1 atmosphere.
Evaporation
Evaporation
We already know that –
Particles of matter are never at rest
Particles of matter possess different amounts of kinetic energy
The particles of liquids have more kinetic energy. Therefore, they are able to overcome the forces of attraction and convert into vapor without any external forces.
Evaporation – The phenomenon of change of a liquid into vapors at any given temperature below its boiling point is called Evaporation. Evaporation is different than boiling, as shown in the figure below.
Evaporation is different than boiling
Factors Affecting Evaporation
Condition Rate of Evaporation Reason
Increase in Surface Area Increases Particles have more space and thus can evaporate easily
Increase in temperature Increases Kinetic energy among the particles increases
Increase in humidity Decreases Water content in air increases and so evaporation decreases
Increase in wind speed Increases Water vapours are blown away by winds allowing more evaporation
How evaporation causes cooling?
The process of evaporation uses the energy of the liquid particles. Therefore, the particles absorb energy from the surroundings in order to compensate the energy that is being lost in the process of evaporation. This results in cooling of the surrounding area.
Evaporation causes cooling
For Example:
Our palms feel cool when we put some acetone (nail paint remover) on it
People sprinkle water on their roofs or ground on sunny days to cool the area
We are able to sip hot tea faster in a saucer than in a cup
Why people wear cotton clothes in summer?
We perspire more in summer. As the sweat evaporates it takes energy from our body surface and keeps our body cool. Cotton can absorb the sweat easily and exposes it to the atmosphere causing evaporation to take place easily. This, in turn, keeps us cool in summer days.
Why water droplets appear on the surroundings of a glass with ice-cold water?
There are water vapours present in the air. When they come in contact with the walls of the glass that has ice-cold water in it they condense. As a result, their state changes from gaseous state to liquid state thus forming tiny water droplets on the walls of the glass.
Classwork
Which gas is called dry ice? Why?
Homework
1. What do you understand by the term ‘latent heat of fusion’? How much is the latent heat of fusion of ice?
2. Why do the doctors advise to put strips of wet cloth on the forehead of a person having high
3. Look at the following figures and suggest in which of the glass containers, i.e., A, B, C or D, the rate of evaporation will be the highest? Explain.
