What Is Evaporation?
Evaporation is a separation technique used to recover a dissolved solid (solute) from a solution. The solution is heated so that the solvent (usually water) turns to vapour and escapes, leaving the solute behind as a solid.
It is used when:
- You want to recover the solid, not the liquid.
- The solid does not decompose when heated.
The Technique in a Laboratory
- Pour the solution into an evaporating basin (a shallow, heat-resistant dish).
- Heat the basin using a Bunsen burner or hot-plate, or leave it to evaporate slowly at room temperature.
- The water evaporates, and as the solution becomes more concentrated, crystals begin to form.
- Stop heating when most of the water has evaporated (leave some liquid to avoid spitting).
- Allow to cool — more crystals form as the solution cools.
- Filter off the crystals and dry them.
The world's largest salt flat — the Salar de Uyuni in Bolivia — formed by evaporation. Millions of years ago, the area was a large saltwater lake. Over time, the water evaporated in the dry climate, leaving behind a 10,000 km² crust of salt up to 11 metres thick. Today it contains some of the world's largest lithium reserves.
Evaporation vs Distillation
| Feature | Evaporation | Distillation |
|---|---|---|
| What you keep | The solid | The liquid (solvent) |
| Solvent | Discarded (evaporates away) | Collected and kept |
| Use | Recover dissolved salts, crystals | Purify water or separate liquids |
Crystallisation
Crystallisation is a more careful form of evaporation. The solution is heated until it becomes saturated (concentrated), then allowed to cool slowly. As it cools, the solubility decreases and the excess solute forms clean, well-shaped crystals.
Slow cooling → larger, purer crystals. Fast cooling → smaller crystals.
To grow large, beautiful crystals of copper sulfate at home, dissolve as much copper sulfate as possible in hot water to make a saturated solution. Tie a small seed crystal to a thread, suspend it in the solution, cover with a cloth, and leave in a cool place for a week or two. The crystal slowly grows as the solution cools and evaporates.
Real-World Applications
- Salt harvesting: Sea water is trapped in shallow pans and allowed to evaporate in the sun and wind to produce sea salt.
- Sugar production: Sugar cane juice is boiled to evaporate water; the concentrated solution is then cooled to crystallise sucrose.
- Pharmaceutical industry: Many medicines are produced as crystals by crystallisation.
Real world: Sea salt harvested from Mediterranean solar pans
Sea salt is produced in coastal regions (France, Italy, Portugal) by flooding shallow pans with seawater and letting the sun and wind evaporate the water over weeks. As the water evaporates, the solution becomes increasingly concentrated until salt crystals form. Workers rake the crystallised salt into piles. This is exactly laboratory evaporation, scaled up to tonnes per year. The different minerals that crystallise at different concentrations (gypsum first, then halite/NaCl) give artisan sea salts their distinct mineral flavours.
Real world: Sugar production from sugar cane
Sugar cane juice is about 15–20% sucrose. To produce table sugar, the juice is filtered (to remove plant matter), then boiled in large vacuum evaporators to evaporate water and concentrate the solution. When it reaches saturation, sucrose crystals form. The crystals are separated by centrifugation, washed, and dried — producing the white crystalline sugar in your kitchen. The remaining brown syrup (molasses) is used for rum production. Sugar production is an industrial evaporation and crystallisation process.
Real world: The Dead Sea — natural evaporation on a geological scale
The Dead Sea (between Israel and Jordan) has a salinity of ~33% — ten times saltier than the ocean. Water flows into the Dead Sea from the Jordan River but has no outlet — it can only escape by evaporation in the hot, dry climate. Over thousands of years, the evaporation has concentrated all the dissolved salts. The Dead Sea chemical works extract potassium, bromine, and magnesium from the brine by controlled evaporation and crystallisation. The sea is actually shrinking visibly because more water evaporates than flows in.
Where do you see this in real life? Sea salt harvested from solar evaporation pans, white sugar crystallised from sugar cane juice, the Dead Sea becoming saltier each year (water evaporates, salts stay), limescale in your kettle (water evaporates from tap water leaving dissolved minerals behind) — evaporation recovers dissolved solids wherever you need the solid, not the water.
Practice Problems
A student has a solution of sodium chloride (salt) in water. She wants to obtain dry salt crystals. Describe the steps she should follow.
Show answer
Why should you stop heating a salt solution before all the water has evaporated?