Matter In Our Surrounding

Tutormate > CBSE Syllabus-Class 9th Chemistry > Matter In Our Surrounding

06 Evaporation

  • The process of a liquid changing into vapour or gas even below its boiling point is called evaporation.
  • Evaporation of a liquid can take place even at room temperature, though it is faster at higher temperatures.
  • The body is cooled by spraying perfume on the body or even acetone and water.
  • The only difference lies in the rate at which coolness is felt.
  • In the case of acetone, the part of your body that is in contact with the liquid will cool the fastest.
  • This is because the evaporation rate of acetone is higher than that of water or perfume.

Factors Affecting Evaporation:

There are four factors which affect evaporation.

  • Temperature: The rate of evaporation increases on increasing the temperature of the liquid.
  • Surface Area of the Liquid: If the surface area is increased, the rate of evaporation increases.
  • Humidity of Air: Humidity is the amount of vapour present in the air. When the humidity of air is low, then the rate of evaporation is high, and water evaporates more readily.
  • Wind Speed: With the increase in wind speed, the particles of water vapour move away with the wind.

Cooling Caused by Evaporation:

  • The process of evaporation that is accompanied by natural cooling is based on the principle that in order to change its state, matter must either gain or lose energy.
  • If change of phase takes place from liquid to gas, molecules of matter require energy to overcome their potential energy by their kinetic energy.
  • The liquid takes this energy from its surroundings.
  • During energy transfer in most cases, an increase or decrease in temperature of the substance takes place, depending on whether the energy is being transferred from the substance to the surroundings or vice versa.
  • Phase change does not result in observable heat transfer even though there is an increase in temperature of the substance till the boiling point is attained during evaporation.
  • The molecules of the substance absorb heat energy continuously from the surroundings and thus cool the surroundings till they reach the boiling point.
  • Thereafter they begin to break free from the liquid and turn into vapour.
  • Since there is no change in temperature till the evaporation process is complete i.e. the entire liquid gets converted into vapour, the amount of energy required for this phase change is called the latent heat of vaporization, where the word ‘latent’ means ‘hidden’, meaning this heat will not change the temperature reading on a thermometer.
  • Scenario 1:
    • Pouring some acetone (nail polish remover) on your palm results leaves the hand to feel cold.
    • This is due to the fact that to change from the liquid to the vapour state, spirit requires latent heat of vaporisation.
    • The spirit takes this heat of vaporisation from our hand.
    • Hence, the particles gain energy from your palm or surroundings and evaporate causing the palm to feel cool.
  • Scenario 2:
    • After a hot sunny day, people sprinkle water on the roof or open ground to cool hot surface.
    • This is because of the large heat of vaporization of water which helps to cool the hot surface.
  • Scenario 3:
    • During hot summer days, water is usually kept in an earthen pot to keep it cool.
    • The earthen pot has a larger number of extremely small pores the water continuously keeps seeping through these pores.
    • This water evaporates continuously and takes the latent heat required for vaporisation from the earthen pot and the remaining water.
    • In this way, the remaining water loses heat and gets cooled
  • Scenario 4:
    • Wear cotton clothes in hot summer days to keep cool and comfortable.
    • Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation.
    • A fan increases the rate of evaporation of sweat (or moisture) from our skin and makes us feel and comfortable.
  • Scenario 5:
    • Sweating enables us to cool our bodies.
    • Perspiration is essentially evaporation of water from our body that takes energy from our body in the process and thus results in the lowering of our body temperature.
  • Scenario 6:
    • An air cooler is more effective on hot, dry days.
    • The working of an air cooler is based on the principle of evaporative cooling.
    • The temperature is high and humidity is low on a hot, dry day so the evaporation rate is higher.
    • Energy from air is taken by water which gets converted to vapour making the air cooler.

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