Vapours, Saturated Vapour Pressure, and Atmospheric Phenomena | Physics for JAMB PDF Download

Unsaturated and Saturated Vapours

• Vapour: A substance in its gaseous phase that is below its boiling point at a given temperature and pressure.
• Unsaturated Vapour: A vapour that has not reached its maximum capacity to hold the gaseous form of the substance at a specific temperature and pressure.
• Saturated Vapour: A vapour that is in equilibrium with its liquid or solid phase at a specific temperature and pressure. It has reached its maximum capacity to hold the gaseous form of the substance.

Key points:

• Unsaturated vapours can further absorb more energy without condensation, while saturated vapours are unable to absorb additional energy without undergoing phase change.
• The transition from unsaturated to saturated vapour occurs when the substance reaches its boiling point at a given pressure.

Relationship between Saturated Vapour Pressure (S.V.P) and Boiling

• Saturated Vapour Pressure (S.V.P): The pressure exerted by the vapour in equilibrium with its liquid or solid phase at a specific temperature.
• Boiling Point: The temperature at which the saturated vapour pressure equals the atmospheric pressure.

Key points:

• As the temperature increases, the saturated vapour pressure of a substance also increases.
• The boiling point of a substance is directly related to the atmospheric pressure. At higher altitudes where atmospheric pressure is lower, the boiling point decreases.

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

• Barometer: An instrument used to measure atmospheric pressure.
• Barometer Tube Method: A technique used to determine the saturated vapour pressure of a liquid.

Procedure:

• Fill a glass tube with the liquid whose saturated vapour pressure is to be measured.
• Close one end of the tube and invert it into a beaker containing the same liquid.
• Gradually raise the open end of the tube, allowing the liquid to flow out until the liquid level stabilizes.
• Measure the height (h) of the liquid column in the tube above the liquid level in the beaker.
• The height (h) corresponds to the pressure exerted by the vapour of the liquid, which is equal to the saturated vapour pressure at that temperature.

Formation of Dew, Mist, Fog, and Rain - Explanation

• Dew: The condensation of water vapour on cool surfaces, such as grass or leaves, during the night when the temperature drops below the dew point.
• Mist: Fine droplets of water suspended in the air, often forming when warm air passes over a cool surface, like in the case of foggy mornings near bodies of water.
• Fog: A cloud that forms near the ground when air becomes saturated with water vapour, usually due to cooling or mixing of warm and cool air masses.
• Rain: The precipitation of liquid water droplets that form from condensed water vapour in the atmosphere, becoming heavy enough to fall under the influence of gravity.

Key points:

• Dew, mist, fog, and rain are all results of condensation, which occurs when air becomes saturated with water vapour.
• The temperature, humidity, and cooling of air masses play significant roles in the formation of these atmospheric phenomena.