Page 66 - ICSE Chemistry 6

Thirst for Knowledge

A substance that exists in the gaseous state under normal condi ons of temperature and pressure is
called a gas. For example, oxygen, hydrogen and nitrogen are some gases.

A substance that exists as a solid or a liquid under normal condi ons of temperature and pressure but
changes to its gaseous state under specifi c condi ons is called vapour. For example, water vapour is the
gaseous state of water. Similarly, the gaseous state of iodine is iodine vapour.

Change of a Gas to a Liquid

Condensa on: When a gas is cooled, it condenses to form a liquid. This process is called condensa on.
When a gas is cooled, its molecules lose energy. On loosing energy, their movement slows down. A er
some me, a stage is reached when the movement of molecules becomes so slow that they change into
a liquid state.

Condensa on point: This en re conversion process also occurs at a constant temperature. The constant
temperature at which a gas changes into a liquid is called the condensa on point. The numerical value
of boiling point and condensa on point of a substance is the same. The boiling point of water and the
condensa on point of water vapour is 100°C.

Change of a Solid to a Gas

Sublima on: Some solids like iodine, ammonium chloride, camphor, naphthalene, benzoic acid, mercuric
chloride and solid carbon dioxide (dry ice) on hea ng directly change into gaseous state, without changing
into a liquid state. This process is called sublima on. A direct change of state from a gas to a solid is called
deposi on.
The solid obtained on cooling the vapour is called sublimate. The gaseous state of a solid is called sublime.

Naphthalene balls (moth balls) are placed in cupboards between woollen clothes to keep insects away.
This is because naphthalene sublimes into vapour at room temperature, which drive away the insects.
Thus, with passage of me, the size of a naphthalene ball decreases.

Expansion of Matter

All the three states of ma er (solid, liquid and gas) expand on hea ng. On hea ng a solid, the par cles
gain energy and vibrate more vigorously about their respec ve fi xed posi ons, forcing each other further
apart. This results in expansion of a solid. Similarly, the par cles in liquid or gas on gaining energy are
forced further apart causing expansion. The degree of expansion depends on the substance. A liquid on
hea ng will expand more than a solid. Gases expand enormously when heated.

Let’s perform some ac vi es to understand expansion of ma er.

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