Question

The temperature at which water boils depends mainly on
A. Air temperature
B. Relative humidity
C. Air pressure
D. Air density

Answer 1

Hint: Boiling is defined as a phenomenon in which a liquid changes into its vapour form, when heated to its boiling point. It is a change in the physical state of the substance while its chemical state is not altered at all.

Complete answer:
Boiling occurs when a liquid (water in this case) is heated so that it changes from its liquid state to gaseous state by spreading out and as a result bubbles are formed. The pressure of the gas present above the liquid determines its boiling point. When the system is open, this pressure is called atmospheric pressure (air pressure).
When the water is heated in an open system air molecules start colliding with the surface of the water that creates a pressure. This pressure (air pressure) is passed to the whole water that makes it difficult for water to create bubbles. As a result boiling takes place. Thus the temperature at which the water boils depends on the air or atmospheric pressure.

Hence, option (C) Air pressure is the correct option.

Additional Information: The boiling point of a liquid is directly proportional to its air pressure. The more the air pressure, the higher is the boiling point of that liquid. This is so because when the atmospheric pressure is high, more energy will be required to boil the liquid (higher temperature), thus making the boiling point higher.

Note: Boiling occurs when a liquid is heated up to its boiling point, which is specific for each liquid. Water has a boiling point at ${100^ \circ }C$ at a pressure of 1 atm. Apart from the atmospheric pressure, boiling also depends upon temperature and vapor pressure of the water.