Chemistry · Chapter 10

Vapor Pressure and Boiling

A liquid boils when its vapor pressure catches up with the pressure pushing down on it. Move the temperature, move the outside pressure, and watch the two numbers race each other.

Vapor pressure against temperaturegold line = outside pressure

Curves come from fitted measurements. The fit is tuned from 0 to 100 °C, so readings above 100 °C are a stretch beyond the data.

In the beakerquiet

Controls

Liquid

Temperature: 25 °C

Outside pressure: 760 torr

Or pick a place

The numberssea level

24torr vapor pressure QUIET
Outside pressure760 torr
Vapor pressure here24 torr
Boils at this pressure100.0 °C
Still needed75 °C more

Straighten the curveln P against 1/T

Slope gives ΔH of vaporization–

What you are seeing

Below the boiling point
Quiet liquid