Question

The triple point of air-free water is defined as 273.15 K. Why is it important that the water be free of air?

Short answer

As a solute, dissolved air will change the vapor pressure and, hence, the solution’s freezing point.

Step-by-step solution

Definition of Triple Point

The triple point may be defined as where all three phases co-exist at a certain temperature and atmospheric pressure.

Explanation

The triple point should be free from the air because the vapor pressure will change due to the presence of air with water vapor. The pressure exerted by air will add to the vapor pressure. Due to changes in vapor pressure, the freezing point of water also changes.