Growing Crystals to Explore Ionic Lattices

Diagrams of ionic lattices show neat rows of spheres repeating in every direction, and it’s fair to ask how anyone knows that’s true. One of the oldest clues is something you can see with your own eyes: ionic crystals have flat faces that meet at the same angles every time. A crystal of table salt is a tiny cube; a crystal of alum is an octahedron, like two pyramids joined at the base. Shapes that regular only make sense if the particles inside are stacked in a regular, repeating pattern.

In this practical you’ll grow your own crystals from a saturated solution, first as a crop of small crystals and then as a single larger crystal grown on a seed. It takes a lesson to set up and a week or two to finish, and the results are among the most striking in school chemistry.

Purpose

• To grow crystals of an ionic compound from a saturated solution by slow cooling and slow evaporation.
• To relate the shape of the crystals to the regular arrangement of ions in an ionic lattice.
• Optional investigation: to find out how the rate of cooling affects the size of the crystals formed.

Principle

Solubility and saturation

A saturated solution holds the maximum mass of solute that can dissolve at that temperature. For most ionic solids, including copper(II) sulfate and alum, solubility rises steeply with temperature. So if you dissolve as much as you can in hot water and then let it cool, the solution ends up holding more solute than it can keep dissolved. It is then supersaturated, and the excess comes out of solution as crystals.

Evaporation has a similar effect at constant temperature: as water leaves, the same amount of solute is left in less solvent, and the solution becomes supersaturated again.

How a crystal grows

Crystals start at nucleation sites: a speck of dust, a scratch on the glass or a tiny cluster of ions that happens to form. Once a small crystal exists, ions in the solution attach to its surface in the positions that fit the lattice. Each new layer repeats the arrangement of the one beneath it, so the crystal keeps its shape as it gets bigger.

That’s the link to bonding. In an ionic lattice, each positive ion is surrounded by negative ions and each negative ion by positive ones, in a pattern that repeats in three dimensions (see ionic lattice structure). The flat faces of the crystal are planes of ions, and the angles between faces are set by the geometry of the lattice. Sodium chloride has a cubic arrangement of Na⁺ and Cl⁻ ions, and its crystals grow as cubes.

Why slow growth gives big crystals

If a solution cools quickly, many nuclei form at once and they all compete for the dissolved ions, giving lots of small crystals. If it cools or evaporates slowly, fewer nuclei form and each has time to grow. Adding a seed crystal to a solution that is only just saturated gives the ions one obvious place to go, so a single large crystal can develop.

A note on hydrates

The blue crystals you grow from copper(II) sulfate are copper(II) sulfate pentahydrate, CuSO₄·5H₂O. Water molecules are part of the crystal structure, arranged around the copper ions. Alum, KAl(SO₄)₂·12H₂O, is also a hydrate. So these crystals are ionic lattices with water of crystallisation built in; see hydrates and water of crystallisation. Sodium chloride crystals contain no water of crystallisation, which makes salt the simplest “pure” ionic lattice to compare.

Choosing a compound

Compound	Crystal colour and shape	Notes
Copper(II) sulfate pentahydrate, CuSO₄·5H₂O	bright blue, slanted blocky crystals	the most striking; handle with care (see Safety)
Potassium alum, KAl(SO₄)₂·12H₂O	colourless, octahedral	low hazard; excellent for seed crystals
Sodium chloride, NaCl	colourless, cubic	solubility changes little with temperature, so use slow evaporation rather than cooling

This guide uses copper(II) sulfate as the main example, but the method works for alum without changes. Your teacher will choose based on the equipment and time available.

Equipment

• Copper(II) sulfate pentahydrate (or potassium alum), about 50–60 g per group
• Two clean 250 cm³ beakers
• 100 cm³ measuring cylinder
• Glass stirring rod
• Spatula
• Water bath or hot water from a kettle (about 60 °C); no open flame is needed
• Filter funnel and filter paper
• Petri dish or watch glass
• Nylon thread (fishing line works best; cotton wicks solution up and grows extra crystals)
• Pencil or wooden splint to hang the thread from
• Paper towel or filter paper to cover the beaker
• Tweezers, hand lens, and a balance (optional, for recording mass)
• Eye protection and gloves

Safety

Your teacher’s risk assessment for your school always takes priority. Key points:

• Copper(II) sulfate is harmful if swallowed and irritating to the eyes and skin. Wear eye protection throughout, avoid skin contact with the solid and solution, and never eat or drink in the lab.
• Hot solutions can scald. Use water at around 60 °C rather than boiling, handle warm beakers with care, and let spills be cleared by a teacher.
• Wash your hands after handling any chemicals or crystals, and before leaving the lab.
• Disposal: copper compounds are toxic to aquatic life. Leftover solution and unwanted crystals go into the residue container your teacher provides, not down the sink.
• Labelling: label beakers left to stand with the contents, the hazard and your name, and leave them where they won’t be knocked or mistaken for anything else.
• Crystals you take home should stay in a closed, labelled container, away from young children and food. Alum is a much lower-hazard choice if crystals are to be taken home.

Method

Part A: making seed crystals (lesson 1)

1. Measure 100 cm³ of warm water (about 60 °C) into a clean beaker.
2. Add copper(II) sulfate a spatula at a time, stirring after each addition, until no more dissolves and a little solid stays on the bottom. The solution is now saturated at that temperature.
3. Filter the warm solution into a second clean beaker to remove undissolved solid and dust. Every leftover grain would act as an unwanted nucleus.
4. Pour a shallow layer (a few millimetres deep) into a Petri dish. Cover the rest of the solution with paper and keep it for Part B.
5. Leave the dish undisturbed overnight. Small crystals will form as it cools and evaporates.
6. Next lesson, use tweezers to choose one or two of the best-shaped crystals: clear, regular faces and no smaller crystals stuck to them. These are your seed crystals.

Part B: growing a large crystal (lesson 2 onwards)

7. Warm the stored solution gently and add a little more solid if needed so it’s saturated again, then let it cool to room temperature. Make sure no solid is left at the bottom; decant or filter if there is.
8. Tie the seed crystal to a length of nylon thread with a small knot. Tie the other end to a pencil laid across the top of the beaker, so that the crystal hangs in the middle of the solution without touching the sides or the bottom.
9. Cover the beaker loosely with paper or filter paper. This keeps dust out and slows evaporation.
10. Leave the beaker somewhere with a steady temperature, away from windows and radiators.
11. Every day or two, record the crystal’s appearance, and if possible its mass (lift it out briefly, blot it dry and weigh it). Remove any small crystals that start growing on the thread or the beaker, because they compete with your seed.
12. When the crystal is big enough (typically after one to two weeks), lift it out, pat it dry and store it in a closed container.

Optional investigation: cooling rate

Pour equal volumes of the same warm saturated solution into three Petri dishes. Leave one at room temperature, place one on a bench in a cool spot, and stand one on ice. Next day, compare the number and size of crystals in each. Keep the volume of solution and the depth in each dish the same so that cooling rate is the only thing you change.

Expected results

• Part A gives many small, bright blue crystals in the dish. The best ones have flat, shiny faces and sharp edges, and many look like the same shape at different sizes.
• Part B gives a single larger crystal, often a centimetre or more across after a week or two, with the same angles between its faces as the tiny seed you started with.
• Cooling investigation: the dish on ice gives many small crystals, often as a crust; the dish left at room temperature gives fewer, larger crystals.
• Alum crystals grow as clear octahedra. If you grow them alongside copper(II) sulfate, the difference in shape is obvious, showing that each compound has its own lattice geometry.

Crystals of copper(II) sulfate left in warm, dry air for a long time can slowly lose some of their water of crystallisation and develop a pale, powdery surface. Storing them in a closed container prevents this.

Questions

1. Why must the solution be filtered before it’s left to crystallise?
2. Explain, in terms of solubility, why crystals appear as a hot saturated solution cools.
3. What would happen to the seed crystal if the solution in Part B were not saturated? Why?
4. Why do crystals of the same compound have the same angles between their faces, whatever their size?
5. Use the molar mass calculator to find the molar mass of CuSO₄·5H₂O, then calculate the percentage of its mass that is water. (Answer: about 249.7 g/mol and about 36 % water.)
6. Sodium chloride’s solubility changes very little between 20 °C and 60 °C. Which method, cooling or evaporation, would work better for growing salt crystals, and why?
7. Suggest why quick cooling produces many small crystals.

Sources of error and things that go wrong

• The seed dissolves. The solution wasn’t saturated when the seed went in, often because it was still warm. Let it reach room temperature first.
• Lots of small crystals instead of one big one. Dust, undissolved solid or scratches acted as extra nucleation sites, or the solution was strongly supersaturated. Filter carefully and use clean glassware.
• Crystals growing up the thread or the beaker wall. Solution creeping up the thread evaporates and deposits solid. Nylon line reduces this; remove unwanted growth regularly.
• Uneven or cloudy crystals. Temperature swings cause alternating growth and dissolving. Choose a spot with a steady temperature.
• The crystal is lopsided. A crystal resting against the glass can’t grow on that side. Make sure it hangs freely.
• Mass readings vary. Solution clinging to the surface adds mass. Blot the crystal the same way each time before weighing.

Extension ideas

• Compare crystals of copper(II) sulfate, alum and sodium chloride under a hand lens and sketch the shapes. Read about how different ion arrangements give different shapes in ionic crystal structures.
• Heat a few small copper(II) sulfate crystals gently in an evaporating basin (under supervision) to see the blue hydrate turn white as it loses its water, then add a drop of water to reverse the change.
• Look up copper to see why many of its compounds are coloured.

Key takeaways

• Crystals form when a solution becomes supersaturated, by cooling or by evaporation.
• Slow cooling and a single seed crystal give large crystals; fast cooling gives many small ones.
• Flat faces and constant angles between them are visible evidence of a regular, repeating ionic lattice.
• Copper(II) sulfate and alum crystals are hydrates, with water of crystallisation built into the lattice.
• Wear eye protection, avoid contact with copper(II) sulfate, take care with hot solutions and dispose of copper waste in the residue container, not the sink.