Vapour pressure of water below 100°C - molar heat of vaporisation

Principle

The vapour pressure of water in the range of 40 °C to 85 °C is investigated. It is shown that the Clausius-Clapeyron equation describes the relation between temperature and pressure in an adequate manner. An average value for the heat of vaporisation of water is determined.

Benefits

• Compact, easily transportable setup
• For both demonstration and student experiments
• No gas burner required - Easy to operate and no consumption of consumables
• No open flame in the setup - Reduction of thermal hazard

Tasks

1. About 250 ml of demineralised water are allowed to boil for about 10 minutes to eliminate all traces of dissolved gas. The water is then cooled down to room temperature.
2. The 3-neck round flask is filled about three-quarters full with gas-free water and heated. At 35 °C the space above the water within the round flask is evacuated. Further heating causes an increase in pressure p and temperature T of water within the round flask. p and T are read in steps of 5 °C up to a maximum of T = 85 °C.

Learning objectives

• Pressure
• Temperature
• Volume
• Vaporization
• Vapour pressure
• Clausius-Clapeyron equation