Evaporation causes cooling


 
 
Concept Explanation
 

Evaporation causes cooling

Evaporation: conversion of a liquid into its vapour either by exposing the liquid to air at the room temperature or by heating it below its boiling point is called ‘evaporation.’ For example: take common salt or sodium chloride in a porcelain dish and place it on water bath. After some time, the water from the porcelain basin has evaporated, leaving behind sodium chloride as residue.

Explanation for the process of evaporation: As the particles of liquid are constantly moving, they collide with each other. They also collide with particles of air when left exposed. In doing so, some of the particles at the surface of the liquid acquire extra energy and overcome the intermolecular forces of attraction that pull them back and keep them with the bulk of the liquid. These highly energetic particles leave the surface and go into vapour.Evaporation causes cooling: when the surface particles take up energy and escape into air as gas. This results in fall of temperature of liquid. The liquid is thus cooled.

  • Fill an earthen pitcher with water and note the temperature. Now cover it with lid and keep for 3 to 4 hours and now note the temperature. The temperature have fallen i.e. water in the pitcher has become colder. The fine pores in the pitcher allow water particles to come on the outer surface from where the particles escape into air. The heat for evaporation comes from bulk of water in the pitcher. This makes the water colder.
  • Place a few drops of spirit on your palm. You will feel cold. This is because heat required for evaporation is drawn from your palm.
  • During summers we perspire profusely. The sweat draws heat from our body and evaporates away. This makes us feel cool. Cotton, being a good absorber of water, absorbs sweat and exposes it to air. This is why we prefer to wear cotton clothes in summer.
  • Sample Questions
    (More Questions for each concept available in Login)
    Question : 1

    Which of the following are correct :

    (a) The highly energetic particles of liquid leave the surface and go into vapour.

    (b) The low energetic particles of liquid leave the surface and go into vapour.

    (c) As the particles of liquid are constantly moving, they collide with each other.

    Right Option : C
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    Explanation
    Question : 2

    During evaporation, particles of a liquid change to vapours only ____________________

    Right Option : A
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    Explanation
    Question : 3

    Before the doctor gives an injection, he applies spirit at that site. At that time our skin feels very cool because :

    Right Option : A
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    Explanation
     
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