||What are crystals?
In this practical activity, students develop their understanding of crystals and their structures and relate this to the particles in substances. Working in groups, they observe a variety of substances to help them decide criteria for identifying crystalline substances.
Students will be able to describe:
- crystals as substances with regular structures
- the regular structure as due to particles being arranged in an orderly way.
Sequence of activities
Prepare a variety of substances for observation (see Resources).
Use images of different substances to introduce the topic – for example, gemstones, minerals, metals, glass, chalk.
Raise the question, ‘What are crystals?’ pointing out that only some substances are crystals. Record students’ suggestions for reference at the end of the session.
Say that if a substance is a crystal, this tells us something about the way particles of the substance are arranged.
Give each student a copy of the What is a crystal? worksheet.
Organise students into groups of four and give each group a set of substances to look at (see Resources).
Circulate and support with prompts as groups:
- look at the substances provided
- decide if each substance is crystalline or not
- answer the questions on the worksheet
- agree a definition of ‘crystal’
- elect a spokesperson.
Allow at least 25 minutes for the observation exercise, to answer the questions and to agree a definition for ‘crystal’.
In a plenary:
- go through the questions on the worksheet and collect ‘crystal’ definitions from each group
- work towards agreeing a whole class definition of ‘crystal’
- try to include key words regular structure, particles, regular shape
- discuss individual substances, particularly glass and metal
- help test the agreed definition, use an extra set of substances (see Resources).
The discussion may raise questions such as:
- Why do / don’t you think this fits our definition?
- Do we need to change our definition to ‘let in’ this substance?
- What other types of materials are there besides crystals?
- What names do we have for these?
Revisit the suggestions from the start of the session and indicate class progress against these.
- Well-chosen sets of substances will enable non-crystalline, crystalline and mixed / composite substance groups to be identified.
- If students need to modify their crystal definitions, this can be done on the reverse of the worksheet.
- Students can also write down how their views have changed since the start of the lesson. If this is done, give written feedback reinforcing students’ thinking where this is correct and indicating where ideas may need revision.
Assessment for learning commentary
The activity introduces the idea that particles form regular structures called crystals. Students use their own ideas about the nature of different substances, together with those of others, to arrive at a definition for crystal. They evaluate their own and other’s viewpoints in the light of evidence.
Through feedback and the plenary discussion, teachers can modify the views towards a scientific one, using terms appropriate for the level. Written feedback, where given, assesses the extent to which students have understood the points and provides indicators for any further work needed.
For each student
||What is a crystal?
For each student
For each group
- At least two hand-lenses
- A set of substances for observation (two copies of each set should provide sufficient for a class).
- A: paper clip, granite, amethyst, copper(II) sulfate crystals (Harmful), iodine crystals (Harmful and may stain), ceramic, soil
- B: magnesium ribbon (Flammable), basalt, emerald, aluminium potassium sulfate, sugar, graphite, wood
- C: copper sheet, calcite, ruby, sodium chloride, sand, flowers of sulfur (Flammable and Irritant), paper
- D: zinc granules, coal, rose quartz, sapphire, fool’s gold, amber, sugar.
For the teacher demonstration
- Substance set for the plenary discussion: glass, diamond, clay, plastic carrier bag, ice, liquid crystal display
- Jewellery for access to gemstones (optional).
It is the responsibility of the teacher to carry out appropriate risk assessments.
- Copper sulfate
- Magnesium ribbon
Completion of the table depends on the substances available.
4. How did you decide if a substance was a crystal or not?
regular shape (correct); shiny (not always), transparent (not always); coloured (not always), cannot bend it, cannot see the shape.
The ‘regular shape’ answer may be difficult to draw out, but is important. The use of hand‑lenses may help.
Be aware that students may regard glass as a crystal. It is, in fact, an amorphous solid.
Plastics are mixed: they have crystalline regions. For example, the stretch points on the handles of a carrier bag are crystalline ‑ the intermolecular bonds at these points are stronger than in other regions, so the bag can bear greater weight.
V. Kind, Contemporary chemistry for schools and colleges. London: Royal Society of Chemistry, 2004.