Boiling Point Lowering with Pressure

Boiling occurs when the vapor pressure of a liquid equals the surrounding atmospheric pressure.

Key Concept: A liquid boils at the temperature where its internal vapor pressure matches the external pressure exerted on it.

Effect of Lowering Atmospheric Pressure

• When atmospheric pressure is lowered, the external pressure on the liquid decreases.
• The liquid's vapor pressure needs to reach a lower value to equal the external pressure.
• Since vapor pressure increases with temperature, a lower temperature is sufficient to achieve this.
• Therefore, lowering the atmospheric pressure lowers the boiling point of the liquid.

For example, water boils at 100°C (212°F) at standard sea-level atmospheric pressure. At higher altitudes, where atmospheric pressure is lower, water boils at a temperature below 100°C.