Getting Both Back: Evaporation and Distillation

Filtering fails on a solution because the dissolved solid slips through with the liquid. To split a solution you must make the liquid leave as a gas — and the dissolved solid cannot follow it.

Evaporation. Leave the solution in a wide, shallow dish in a warm place. The water slowly turns into water vapour and drifts off into the air. The dissolved solid stays behind as dry crystals.

• You keep the dissolved solid.
• You lose the liquid to the air.

This is exactly how salt is harvested at the salt pans on South Africa's West Coast: sea water is let into shallow ponds, the sun and wind do the work for free, and workers scrape up the salt left behind.

Distillation. Sometimes it is the liquid you want — think of a Karoo town wanting clean drinking water out of brak (salty) borehole water. Distillation gets both back:

1. Heat the solution until the water boils away as vapour, leaving the salt in the flask.
2. Lead the vapour past something cold. It condenses back into liquid water.
3. Collect that pure water in a separate container.

Desalination plants built during the Western Cape drought work on this principle at an enormous scale.

Worked example. A dish holds $500$ g of salt water containing $20$ g of dissolved salt. Left in the sun, all the water evaporates. How much salt is left? The salt cannot evaporate — only the water leaves — so $20$ g of salt remains, and $480$ g of water went into the air.