Page 65 - ICSE Chemistry 6

intermolecular forces between the molecules. Then the molecules break free and begin to move around
even faster. At this stage ice changes into water. This process is mel ng. In other words, the process of
conversion of a solid into its liquid state is known as mel ng.
Mel ng point: The en re conversion process occurs at a constant temperature. The constant temperature
at which a solid changes into a liquid is called its mel ng point. The mel ng point of a solid is a measure
of the strength of the intermolecular forces between its molecules.

Change of a Liquid to a Solid

Freezing or solidifi ca on: When water is kept in a freezer, Thirst for Knowledge
it loses heat. Due to loss of heat, the energy of molecules
decreases, their movement slows down and they come • The numerical value of mel ng point
closer to each other. In some me, a stage is reached when and freezing point is the same.
the movement of molecules becomes so slow that water • The mel ng point of ice and the
freezing point of water is 0°C.
changes into ice. This process of conversion of a liquid into
a solid is called freezing or solidifi ca on.
Freezing point: Freezing or solidifi ca on occurs at a constant temperature. The constant temperature at
which a liquid changes into a solid is called its freezing point.

Change of a Liquid to a Gas

Vaporiza on: The process by which a liquid changes into its gaseous or vapour state is called vaporiza on.
Vaporiza on is of two types—evapora on and boiling.
Evapora on: Evapora on occurs at all temperatures. During evapora on, a liquid gains heat energy from
its surroundings and changes into vapour state. When water (liquid) is
heated, it changes to water vapour or steam (gas).

Boiling: When water is heated, its molecules gain heat energy and start
moving away from each other. As temperature increases, a stage is reached
when the molecules gain so much energy that they leave the surface and
change into water vapour or steam (gas). This process of change of a liquid
to its gaseous state on hea ng is called boiling. Fig. 3.8: Boiling of water

Boiling point: Similar to mel ng and freezing, boiling also occurs at a constant temperature. The constant
temperature at which a liquid changes into a gas is called its boiling point.

Table 3.3: Diff erences between Boiling and Evapora on
Boiling Evaporation

• Boiling occurs at a given temperature called boiling • Evaporation takes place at all temperatures lower
point of the liquid. than the boiling point of the liquid.
• It is a rapid process. • It is a slow process.
• It occurs throughout the liquid. • It takes place only from the surface of the liquid.
• There is no cooling caused during boiling. • Cooling is caused during evaporation.

• It occurs only when the liquid is heated. • It takes place on its own.

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