Vapours

In physics, a vapour is a substance in the gas phase at a temperature lower than its critical temperature, meaning that the vapour can be condensed to a liquid by increasing its pressure without reducing the temperature.

1. Unsaturated and Saturated Vapours

Vapours are classified into two main types based on their state of equilibrium with their parent liquid:

• Unsaturated Vapour: A vapour is unsaturated when it can still hold more of the substance in the gaseous state at a given temperature. It does not exist in equilibrium with its liquid. It obeys the gas laws (Boyle's and Charles's laws) approximately.
• Saturated Vapour: A vapour is saturated when it is in dynamic equilibrium with its liquid in a closed container. At this point, the rate of evaporation equals the rate of condensation. Saturated vapours do not obey the gas laws; their pressure is independent of volume and depends only on temperature.

2. Saturated Vapour Pressure (S.V.P) and Boiling

The pressure exerted by a saturated vapour is called Saturated Vapour Pressure (S.V.P).

• Effect of Temperature: S.V.P increases as temperature increases because more molecules gain kinetic energy to escape the liquid surface.
• Relationship with Boiling: A liquid boils when its Saturated Vapour Pressure equals the external atmospheric pressure. This explains why water boils at a lower temperature on a mountain (where atmospheric pressure is low) than at sea level.

3. Determination of S.V.P by Barometer Tube Method

This method involves two barometer tubes. One serves as a standard (mercury only), and the other has a small amount of liquid introduced into the Torricellian vacuum. The liquid evaporates until the space is saturated. The S.V.P is measured as the difference in the heights of the mercury columns in the two tubes.

Formula: S.V.P = h₁ - h₂ (where h₁ is the height of the standard barometer and h₂ is the height of the barometer with the vapour).

4. Dew Point, Humidity, and Weather Phenomena

• Humidity: This is a measure of the amount of water vapour in the atmosphere.
• Relative Humidity (R.H.): The ratio of the mass of water vapour present in a given volume of air to the mass of water vapour required to saturate the same volume at the same temperature. It is expressed as a percentage:
  R.H. = (Partial Pressure of water vapour / S.V.P at current air temperature) × 100%
• Dew Point: The temperature at which the water vapour present in the air is just sufficient to saturate it. If the temperature falls below this point, condensation occurs.

Weather Phenomena: Dew is formed when water vapour condenses on cold surfaces. Mist and Fog occur when water vapour condenses on dust particles in the air near the ground. Rain occurs when water droplets in clouds become too heavy to remain suspended.

5. Hygrometry

Hygrometry is the measurement of humidity. The most common instrument is the Wet and Dry Bulb Hygrometer. It consists of two thermometers; the bulb of one is kept wet with a muslin wick dipped in water. Evaporation from the wet bulb cools it, causing it to show a lower temperature than the dry bulb. The difference in readings is used with humidity tables to find the Relative Humidity of the air. A large difference indicates dry air (low R.H.), while a small difference indicates moist air (high R.H.).