Crystallisation

Introduction

Evaporating a solution to dryness recovers the solute, but the result is often a crust of tiny, impure grains, and some substances are damaged by the heat. Chemists usually prefer crystallisation : they remove only part of the solvent, then let the solution cool so that the solute comes out slowly as well-formed crystals. The crystals that grow are purer, and hydrated salts keep their water of crystallisation. This is how the beautiful blue crystals of copper(II) sulfate are made.

Core explanation

Solubility and temperature. For most solids, solubility rises with temperature : hot water can dissolve more solute than cold water. For example, around 21 g of copper(II) sulfate (as anhydrous salt) dissolves in 100 g of water at 20 °C, but around 75 g dissolves at 100 °C. Crystallisation makes use of this difference.

The key idea. Start with a solution that is saturated at a high temperature . As it cools, the solubility falls. The water can no longer hold all the solute, so the excess comes out of solution as solid crystals. The more the solubility drops, the more crystals form.

The stages. 1. Filter the solution first, if necessary, to remove insoluble impurities. 2. Heat the solution in an evaporating basin to evaporate some of the water and concentrate it. 3. Test for the crystallisation point. Dip a cold glass rod in the solution and lift it out. If small crystals form on the rod as it cools, the solution is saturated enough. 4. Leave to cool slowly. Crystals grow as the temperature falls. 5. Filter off the crystals, rinse them with a little cold distilled water, and dry them on filter paper or in a warm place — not by strong heating.

Why the crystals are purer. A crystal is built from particles fitting together in a regular, repeating pattern. Particles of an impurity have the wrong size or shape and do not fit easily, so they tend to stay dissolved in the leftover solution, called the mother liquor . Because impurities are usually present only in small amounts, they seldom reach their own saturation point and so remain in solution.

Crystal size. Slow cooling gives fewer, larger crystals, because particles have time to add neatly onto existing crystals. Fast cooling gives many small crystals. Large crystals trap less mother liquor and are usually purer.

Crystallisation versus evaporation. Evaporation to dryness keeps everything — including impurities — and can decompose hydrated salts. Crystallisation leaves most impurities in solution and avoids overheating, but some solute is always lost in the mother liquor.

Step-by-step reasoning

To explain why crystals appear on cooling:

1. The hot solution is saturated: it holds the maximum solute at that temperature. 2. As the temperature falls, the solubility decreases. 3. The solution now holds more solute than it can at the lower temperature. 4. The excess solute leaves the solution and forms crystals.

Visual explanation

Picture a basin of deep blue solution cooling on a bench. After an hour, blue, diamond-shaped crystals with flat faces and sharp edges line the bottom, while the liquid above has become paler blue because some copper(II) sulfate has left it.

Real-world analogy

Imagine a crowded bus that can carry more passengers when it is "hot" (big) than when it is "cold" (small). As the bus shrinks, the extra passengers must step off. In a cooling solution, the extra solute particles "step off" by joining crystals.

Real-world example

Sugar refineries dissolve raw sugar, filter and concentrate the syrup, then crystallise white sugar from it; dark molasses is left as the mother liquor. Pharmaceutical companies crystallise medicines to reach very high purity, which matters for safety.

Why?

Why does slow cooling give bigger crystals? When cooling is slow, only a few crystal seeds form, and dissolved particles have time to move to them and fit into place. Rapid cooling creates many seeds at once, so the solute is shared among many tiny crystals.

Common misconception

"Crystallisation means boiling off all the water." That is evaporation to dryness. In crystallisation you remove only some water, then let cooling do the work. Heating to dryness would destroy hydrated crystals such as blue copper(II) sulfate.

Worked example

Question: 100 g of water is saturated with potassium nitrate at 60 °C (about 110 g dissolved) and cooled to 20 °C, where the solubility is about 32 g per 100 g of water. What mass of crystals forms?

Reasoning: Mass crystallising = mass dissolved at 60 °C − mass that can stay dissolved at 20 °C = 110 − 32.

Answer: About 78 g of potassium nitrate crystals.

Quick check

1. How can you tell that a heated solution has reached its crystallisation point? Answer: Crystals form on a cold glass rod dipped into it and removed.

Exam focus

Method questions often ask for the full sequence: heat to evaporate some water, test with a glass rod, leave to cool, filter off crystals, wash with a little cold water, dry. Explain why you do not evaporate to dryness. Solubility-curve questions ask you to calculate the mass of crystals formed on cooling.

Advanced insight

Chemists purify solids by recrystallisation : the impure solid is dissolved in the minimum volume of hot solvent, filtered hot to remove insoluble matter, then cooled. A solvent is chosen in which the substance is very soluble when hot but only slightly soluble when cold, while impurities stay dissolved. Repeating the process increases purity further.

Summary

Crystallisation forms pure crystals by cooling a hot, concentrated, saturated solution: solubility falls and excess solute comes out as crystals. Concentrate the solution, test with a cold glass rod, cool slowly, then filter, wash and dry the crystals. Impurities stay in the mother liquor, and hydrated salts are not damaged, so crystallisation is purer than evaporation to dryness.

Practice questions

1. Why do crystals form when a hot saturated solution cools? Answer: Solubility decreases as the temperature falls, so the excess solute can no longer stay dissolved and forms crystals. 2. Why are crystals dried on filter paper rather than by strong heating? Answer: Strong heating could decompose them or drive off water of crystallisation. 3. How can a student obtain larger crystals? Answer: Let the solution cool slowly and undisturbed. 4. Explain why the crystals are purer than the original solid. Answer: Impurity particles do not fit the regular crystal pattern and, being in small amounts, stay dissolved in the mother liquor. 5. Using solubility values of 60 g at 50 °C and 25 g at 10 °C per 100 g water, find the mass of crystals formed by cooling a saturated solution in 100 g of water from 50 °C to 10 °C. Answer: 60 − 25 = 35 g.