The temperature at which the water vapour present in the air is just enough to saturate the air is the _______ point.

Step 1: Analyze the question The question asks for the name of the temperature at which the water vapour in the air is just enough to saturate it (the air cannot hold any more vapour at that temperature).

Step 2: Explain the concept Air can hold only a limited amount of water vapour, and this amount depends on temperature. Warm air holds more vapour than cold air. If air is cooled, a temperature is eventually reached where the vapour present is exactly enough to saturate it. Cooling any further causes the vapour to condense into liquid droplets (dew, mist, fog or clouds).

This temperature is called the dew point.

Step 3: Check each option

• A. Boiling point: the temperature at which a liquid's saturated vapour pressure equals the external pressure. It has nothing to do with saturation of air.
• B. Critical point: the temperature and pressure above which a gas cannot be liquefied by pressure alone. This is not what the question describes.
• C. Dew point: the temperature at which air becomes saturated with the water vapour it already contains. This matches the question.
• D. Ice point: the temperature at which pure water freezes (0°C), the lower fixed point on a thermometer.
• E. Steam point: the temperature at which water boils at standard pressure (100°C), the upper fixed point.

Step 4: Answer The correct option is C. dew.

Exam tip: Remember "Dew point = Do the vapour condense?" At the dew point, the air is saturated, so the relative humidity is 100%. Dew point is also measured in the lab using a hygrometer (such as Regnault's or Daniell's), where a polished surface is cooled until mist first appears on it. Questions on this often come up in the humidity topic, so know it well.