The temperature at which the water vapour present in the air is just sufficient to saturate air is

Analysis of the Question

The question asks for the specific temperature at which the amount of water vapour currently in the air is enough to make that air fully saturated (i.e., at 100% relative humidity).

Concept Explanation

To understand this, we need to look at Humidity and Saturation:

• Saturation: Air can hold a certain amount of water vapour depending on its temperature. Warmer air can hold more water vapour than cooler air.
• Dew Point: This is the temperature to which a given sample of air must be cooled for the water vapour in it to become saturated. At this temperature, the air can no longer hold all the water vapour it contains, so the excess begins to condense into liquid water (dew, fog, or frost).
• Boiling Point: This is the temperature at which the saturated vapour pressure of a liquid equals the external atmospheric pressure.
• Ice Point: This is the temperature at which pure ice melts (0°C).

Step-by-Step Solution

1. Identify the state: The question specifies that the vapour present is "just sufficient to saturate the air."
2. Relate to temperature: If we cool the air, its capacity to hold water vapour decreases. The temperature at which the current amount of vapour becomes the maximum amount the air can hold (saturation) is defined scientifically as the Dew Point.
3. Evaluate the options: • A (Boiling point) relates to phase change from liquid to gas at high temperatures. • B (Ice point) relates to the freezing/melting point of water. • C (Saturation point) is a general term, not a specific temperature measurement. • D (Dew point) is the precise meteorological term for the temperature described in the question.

Conclusion

Exam Tip

Think of the Dew Point as the "tipping point." If the temperature drops even slightly below the dew point, the air can no longer hold the moisture, and you get condensation (like dew on grass in the early morning). Remember: Dew Point = Condensation Temperature.