Exploring Mixtures and their Separation · Lesson 5 of 13
Crystallization
“A patient solution cools slowly and builds crystals one orderly particle at a time.”
• Explain crystallization using temperature-dependent solubility. • Describe the complete preparation and purification procedure. • Explain why hot filtration and slow cooling are used. • Relate cooling rate to crystal size and shape. • Describe salt formation from seawater.
A clear solution may seem to contain nothing solid, yet cooling can reveal beautifully shaped crystals. The solid was present all along as dissolved particles. As the solution cools and can hold less solute, particles leave the solution and arrange themselves in a regular pattern.
A crystal is a solid whose particles are arranged in a regular geometric pattern.
Crystallization is the process of forming crystals from a saturated solution, commonly by cooling or by removing some solvent.
Crystallization can separate a pure solid from a homogeneous mixture and can purify a solid contaminated with small quantities of impurities. Its usefulness depends on differences in solubility: the desired substance dissolves well under one condition and separates when the condition changes, while many impurities remain dissolved or are removed by filtration.
Activity: Let Us Prepare
Place about 1 g copper sulfate in a beaker and add about 25 mL water. A teacher may add one drop of dilute sulfuric acid where appropriate. Heat the beaker gently in a water bath while stirring. Add copper sulfate gradually until no more dissolves and a hot saturated solution is obtained.
Filter the hot solution into a clean beaker to remove insoluble impurities. Cover it with a watch glass and allow it to cool slowly without disturbance. When crystals form, separate them by filtration, rinse them with a small quantity of cold water and leave them to dry on a watch glass.
| Step | Purpose |
|---|---|
| Prepare a hot saturated solution | Dissolves a large amount of the desired solid |
| Filter while hot | Removes insoluble impurities before crystals form |
| Cover and cool slowly | Reduces contamination and allows orderly crystal growth |
| Filter formed crystals | Separates the solid crystals from the remaining solution |
| Rinse with cold water | Removes adhering solution while limiting crystal dissolution |
| Dry the crystals | Removes surface moisture |
Copper sulfate is harmful if swallowed and should not be touched with bare hands. Dilute sulfuric acid must be handled by a teacher with great care. Use gentle heating and adult supervision.
Think as a Scientist
Slow cooling allows dissolved particles more time to reach a growing crystal and arrange regularly. Rapid cooling produces many crystal-forming points at once, so the available solute is divided among numerous crystals. The result is usually smaller and less well-formed crystals.
Problem
Design a fair comparison of slow and rapid cooling.
- 1.Prepare one hot saturated copper sulfate solution so both samples begin with the same composition.
- 2.Divide it into two equal portions in similar containers.
- 3.Cool one portion slowly at room temperature and the other rapidly in ice-cold water.
- 4.Keep solution volume, container, and starting temperature as similar as possible.
- 5.Compare the number, size and shape of the dried crystals.
- 6.A difference can then be linked mainly to cooling rate.
Pause and Ponder
Increasing the rate of evaporation generally encourages faster formation of many smaller salt crystals. Slower solvent loss provides more time for orderly growth and can produce larger crystals. The exact shapes depend on the substance and conditions, but growth rate remains an important factor.
Activity: Let Us Describe a Process
Seawater is placed in shallow areas where sunlight and moving air remove water. As the water amount decreases, the salt solution becomes more concentrated and eventually saturated. Further evaporation causes salt to separate as crystals, which can then be collected and dried.
India’s Scientific Contributions
Coastal communities have long produced salt by crystallization. Concentrated sea brines were boiled to obtain panga salt, while seawater was evaporated to obtain karkatch salt. Differences in the conditions produced crystals of different sizes.
Ready to Go Beyond
Crystals also form naturally. Rock salt and sugar candy show familiar crystal shapes. Snowflakes form when water vapour becomes ice in the atmosphere, while frost forms on cold surfaces. Caves and the Earth’s crust can contain striking mineral crystals such as quartz.
Problem
A hot saturated solution contains 120 g solute, but only 85 g can remain dissolved after slow cooling. What mass crystallises?
- 1.Initial dissolved mass = 120 g.
- 2.Mass remaining dissolved = 85 g.
- 3.Mass crystallised = 120 g − 85 g.
- 4.Mass crystallised = 35 g.
Quiz
Which description best matches Crystal?
Which description best matches Crystallization?
Which term matches this description: A crystal is a solid whose particles are arranged in a regular geometric pattern.
Which term matches this description: Crystallization is the process of forming crystals from a saturated solution, commonly by cooling or by removing some solvent.
Which statement is a key takeaway from this lesson?
Practice Problems
- Why is a saturated rather than dilute solution used for crystallization?
- Why is the solution filtered while still hot?
- Explain why crystals are rinsed with cold rather than warm water.
- Compare crystallization with simply evaporating all the solvent.
Key Takeaways
• Crystallization uses changes in solubility to form a pure solid. • Hot filtration removes insoluble impurities. • Slow, undisturbed cooling favours larger, better-formed crystals. • Cold rinsing limits the loss of crystals by dissolving. • Evaporation of seawater eventually produces salt crystals.