The terms melting, boiling and evaporation are the processes associated with the change of state of a substance.
What do we mean by ‘change of state’?
Transformation of a substance into another form is called change of state. Example: Ice melts into water if heated.
What happens in the process of Melting?
Melting or Fusion is a process by which a substance in the solid state is converted into liquid state. To simplify, when a solid is heated, it melts down.
How does solid change into Liquid?
It happens because a substance while changing into liquid state absorbs heat without a rise in temperature. A substance changes from a solid to liquid state at a fixed temperature. This temperature is known as melting point of the solid and remains constant throughout the process of melting. The quantity of heat required to change unit mass (1gm) of the substance at its melting point from solid to liquid state without changing the temperature is termed as latent heat of fusion of solid.
What happens in the process of Boiling or Ebullition?
A liquid if heated continuously under a given superincumbent pressure releases vapour from its surface. Eventually vaporisation takes place throughout the mass of the liquid in a rapid and vigorous way. This stage is known as boiling of the liquid. If the superincumbent pressure does not change, the temperature of the liquid remains constant as long as it’s boiling. This constant temperature is called boiling point of a liquid.
Factors which govern boiling point:
- The boiling point varies depending upon the increase or decrease in the superincumbent pressure on the liquid.
- The presence of any dissolved impurity increases the boiling point. Boiling point of a solution is always greater than that of the pure solvent.
- The boiling point also depends on the material of the boiler, its roughness and the degree of cleanliness of its inner surface to some extent.
What is Evaporation?
Evaporation is the gradual and slow change of a substance from liquid to a vapour state, which takes place at the surface of the liquid at all temperatures.
Factors which govern Evaporation:
- The temperature of the liquid.
- The nature of the liquid.
- The removal of air over the liquid surface.
- The pressure of air.
- The area of exposed surface.
- The pressure of vapour in contact with the liquid.
- The word ‘latent’ means hidden.
- The word ‘superincumbent’ means lying or resting on or above something.
- Melting point of solid such as naphthalene can be determined by two methods; cooling curve method and capillary tube method. It should be noted that naphthalene expands on melting.
- The melting and boiling point of different substances varies from each other.
- The change of a substance from the liquid to the vapour or gaseous state is called vaporisation.
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