Vapour

When a liquid is heated, some of its surface molecules gain enough energy to escape into the atmosphere. This process is known as evaporation. It can happen at any temperature, although the rate of evaporation depends on several factors.

Examples:

1. Alcohol and ether evaporate rapidly.
2. Lubricating oil and mercury evaporate very slowly.

Evaporation results in the formation of vapour.

1. Nature of the Liquid:
   • Volatile liquids (like alcohol) evaporate faster than non-volatile liquids (like oil).
   • Example: Alcohol has a boiling point of 78°C while water has a boiling point of 100°C.
2. Atmospheric Pressure:
   • High atmospheric pressure reduces the rate of evaporation.
   • Lower pressure allows molecules to escape more easily.
3. Surface Energy of the Liquid:
   • Surface energy acts as a skin or boundary at the liquid surface.
   • Molecules must overcome this energy to escape into the atmosphere.
4. Temperature:
   • Higher temperatures give molecules more energy to escape.
   • This increases the rate of evaporation.
5. Surface Area:
   • A larger surface area allows more molecules to escape at once.
6. Air Movement:
   • Blowing air across the surface removes vapor molecules and speeds up evaporation.

• Both are states of matter made of particles in motion.
• A vapour is a gas below its critical temperature (meaning it can be condensed into liquid by pressure).
• A gas cannot be condensed into liquid by pressure alone at that temperature.

When a liquid is enclosed in a sealed container, the molecules continue to escape into the space above the liquid. Over time, the number of molecules escaping equals the number returning. At this point, dynamic equilibrium is reached, and the vapour is said to be saturated.

• This condition is called a saturated vapour state.
• The pressure exerted by the vapour in this state is called the saturated vapour pressure (S.V.P).

S.V.P depends on temperature:

• As temperature increases, the kinetic energy of molecules increases.
• This leads to more molecules escaping into the vapour phase, increasing S.V.P.

1. S.V.P is the pressure exerted by the vapour in a closed container when the rate of evaporation equals the rate of condensation.
2. As temperature increases, the saturated vapour pressure also increases.
3. This relationship can be shown on a graph of S.V.P against temperature.

The graph of saturated vapor pressure (svp) against Temperature

1. The boiling point of a liquid is the temperature at which the saturated vapour pressure equals the external atmospheric pressure.
2. At this point, the liquid boils throughout its volume, not just at the surface.

Examples of boiling points at standard atmospheric pressure (76 cm of mercury):

• Alcohol: 78°C
• Water: 100°C

On a graph of S.V.P vs Temperature, the boiling point is found where the liquid's S.V.P curve intersects the line representing atmospheric pressure.