Passage V

In an enclosed space, an aqueous salt solution produces an atmosphere that has a constant relative humidity (RH). Chemists did 3 experiments to study this phenomenon using 5 identical apparatuses. One of the 5 is shown in Figure 1.

A hatch could be opened to place a dish containing a salt mixture (solid salt in an aqueous solution of the salt) in the airtight tank. A heat exchanger could increase or decrease the temperature in the tank. A hygrometer measured the RH in the tank and the aqueous tension (a measure of the tendency of the to evaporate from a solution) of the salt mixture in the tank. The controller regulated the temperature and recorded the hygrometer readings.

Experiment 1

On Day 1, in preparation for the experiment, a drying agent was placed in each tank. Then, 5 salt mixtures were made by placing an excess of a salt in. The mixtures were stirred overnight.

By Day 2, the RH in each tank was 0.0%, so the drying agent was removed from each tank. Then, 5 dishes, each containing 1 of the 5 salt mixtures, were placed in separate tanks. The temperature in each tank was adjusted to a constant 10°C. By Day 3, the RH and the aqueous tension had each reached a constant value in each tank (see Table 1).

Experiment 2

Experiment 1 was repeated except that the temperature in each tank was maintained at 30°C (see Table 2).

Experiment 3

Experiment 1 was repeated except that the temperature in each tank was maintained at 60°C (see Table 3).