Evaporation and Crystallization

Evaporation separates a dissolved solid from a solution by heating the liquid until it turns to vapor, and crystallization forms pure solid crystals from a solution by cooling it or evaporating part of the solvent. In brief: sea salt is obtained by evaporating seawater, and large sugar or copper sulfate crystals grow from a hot saturated solution that is left to cool slowly.

Both methods recover a solute from its solvent. Evaporation to dryness is quick but can leave small crystals and heat-sensitive substances may be damaged. Crystallization is slower but gives purer, better-formed crystals. This article explains how each works, compares them, describes a step-by-step procedure, works through a yield calculation and notes where they are used.

How does separation by evaporation work?

When a solution is heated, the solvent turns to vapor and leaves, while the solute does not evaporate because it has a much higher boiling point. As the solvent disappears, the solution becomes more concentrated until only the solid solute remains. This works well when only the solid is wanted, for example recovering salt from salt water.

The vapor can be lost to the air, or collected and condensed if the solvent is wanted as well, which is the principle of distillation. For safety, heat in an evaporating dish on a water bath or on a hot plate, and stop heating when only a small amount of liquid is left, so the remaining liquid evaporates from the warm dish without spitting.

How does crystallization work?

Crystallization uses the fact that most solids are less soluble in cold solvent than in hot solvent. The solid is dissolved in the minimum amount of hot solvent to make a saturated solution. On cooling, the solution cannot hold as much solute, so the excess solid forms crystals. Soluble impurities remain dissolved in the liquid, which is called the mother liquor.

Slow cooling produces large, well-formed crystals with fewer impurities trapped inside. Fast cooling gives many small crystals that may trap liquid and impurities between them.

Evaporation and crystallization compared

How do you grow pure crystals?

1. Heat the solvent and dissolve the solid in small portions until no more dissolves, which gives a saturated hot solution.
2. Filter the hot solution to remove any insoluble impurities.
3. Let the filtrate cool slowly, undisturbed, to room temperature, and then in an ice bath.
4. Collect the crystals by filtration and wash them with a little cold solvent.
5. Dry the crystals on filter paper or in a warm place.

Worked example: yield of crystals

A student has an impure sample of potassium nitrate and wants to recrystallize it, using the approximate solubility values of 110 g per 100 g of water at 60 °C and 32 g per 100 g of water at 20 °C.

1. Hot solution. To dissolve 55 g of potassium nitrate at 60 °C the student needs at least 55 ÷ 110 × 100 = 50 g of water.
2. Cooling. At 20 °C, 50 g of water holds at most 32 × 50 ÷ 100 = 16 g of the salt.
3. Theoretical crystals. 55 − 16 = 39 g of crystals can form.
4. Percentage recovered. 39 ÷ 55 × 100 = 70.9%, so about 71% of the salt is recovered, and the remaining 29% stays in the mother liquor.

The calculation shows a trade-off. Recovering more of the salt requires cooling to a lower temperature or using less solvent, but too little solvent risks trapping impurities with the crystals.

Where are these methods used?

• Salt production: seawater is left to evaporate in shallow ponds, called salt pans, in hot, dry regions.
• Sugar industry: sugar syrup is concentrated and crystallized.
• Pharmaceuticals: crystallization purifies drug substances.
• Mining: minerals such as sodium carbonate and potash are recovered from brines.

How did people use them in the past?

Salt production by evaporating seawater and brine is among the oldest chemical industries, and salt works were in use in many parts of the ancient world. Alchemists and early chemists used crystallization to purify salts, and the shapes of crystals later became a clue to the arrangement of atoms. In the 20th century, X-ray studies of crystals revealed the structures of many compounds, including DNA.

Which mistakes are common?

Mistake 1: Boiling the solution to dryness

Heating to complete dryness can cause spitting and may decompose heat-sensitive solutes such as sugar. Stop when a small amount of liquid remains and allow the dish to dry in the residual heat.

Mistake 2: Cooling the solution too fast

Rapid cooling gives tiny, impure crystals. Let the solution cool slowly.

Mistake 3: Using too much solvent

If the solution is not saturated when hot, few or no crystals form when it is cooled.

Mistake 4: Washing the crystals with warm solvent

Warm solvent dissolves part of the crystals. Use a small volume of cold solvent.