Question

Liquids boil when their vapor pressure function p(T) exceeds the ambient pressure, for example water boils at 100 °C at 1 atm atmospheric pressure because at this pressure, its vapor pressure function p(T) has the value of 1 atm. However, how is this atmospheric pressure defined? Is it just the gas pressure without the water vapor?

Suppose the water is heated up slowly until 100°C. It will form its own vapor, alongside the ambient pressure of 1 atm. So at the boiling point, does the water need to overcome the 1 atm ambient pressure + its own vapor pressure in the surrounding atmosphere to boil (would be over 1 atm tho) or just the pressure of the other gases?

Answer 1

At the boiling point of a liquid, its vapor pressure equals the surrounding atmospheric pressure, not the sum of the atmospheric pressure and the vapor's own pressure. Water boils at 100°C when its vapor pressure reaches 1 atm, matching the atmospheric pressure, allowing boiling to occur without overcoming additional pressure.

Step-by-step explanation:

The atmospheric pressure is defined as the pressure exerted on a liquid by the gases above it, inclusive of water vapor and other gases such as air. When a liquid, like water, heats up slowly to its boiling point, the rising vapor pressure of the liquid ultimately equals this atmospheric pressure.

At the boiling point, the liquid doesn’t need to overcome the sum of atmospheric pressure plus its own vapor pressure. Instead, it simply needs its vapor pressure to match the existing atmospheric pressure. This is because the ambient pressure already includes the partial pressure of the water vapor present in the atmosphere.

Therefore, at 100°C, water boils because its vapor pressure reaches 1 atm, which is the same as the atmospheric pressure. This applies to an open container where water can boil away because the surrounding gas includes air, reducing the number of water molecules needed to condense and allowing evaporation to continue.

The presence of air means that the pressure of the steam is less than the total atmospheric pressure at boiling, so boiling proceeds with the vaporization of the water.