How are crystals made? | 11-14 years

In this practical activity, students work in pairs to explore the formation of crystals, and investigate how the rate at which a substance cools influences the size of the crystals produced. The activity is a good introduction to growing crystals.

Learning objectives

Students will be able to describe that:

• Large crystals grow as substances cool slowly.
• Small crystals grow as substances cool rapidly.
• Crystals grow as particles and form regular arrangements during the cooling process.

Sequence of activities

Prepare the equipment for the practical work beforehand.

Introduction

1. Introduce the learning objectives by using the image of a very large diamond, eg the Cullinan diamond, and comparing this large diamond to the average sized ones we see more often.
2. Ask students how they think a large diamond is formed. Record their suggestions for reference later.

Activity: stage 1

1. Organise the students into pairs and give each student a copy of the ‘Growing crystals: big or small?’ worksheet.
2. Support and supervise as the pairs carry out the first practical activity. Allow about 20 minutes for the practical activity.

Activity: stage 2

When the practical work is complete, arrange for pairs of students to join into groups of four. Circulate and support as the groups:

1. Share results from the practical.
2. Agree on answers to the questions for group discussion.
3. Select a spokesperson.

Allow about 20 minutes for the group discussion.

Plenary

In a plenary:

1. Invite spokespersons to describe the outcomes of the practical work.
2. Prompt students to explain their answers to the questions.
3. Review and give feedback on the responses.
4. Compare student thinking with their original views about how crystals form.
5. Ensure that the students realise larger crystals grow with slower cooling as more bonds between particles can form under these conditions.
6. Introduce particle ideas into the discussion using key words:
   • ‘Regular arrangement’
   • ’Lattice structure’
   • ‘Atoms’
   • ‘Molecules’
   • ‘Ions’
   • ‘Bonds between particles’

Prompt questions may include:

• How do we know the particles are there?
• How powerful a microscope would we need to see particles?
• How can particles form non-crystals as well?

Feedback

Collect in the worksheets. Review students’ thinking about crystal formation using written feedback. Highlight where students still need to make the connection between particles and regular arrangements in crystals.

Commentary

Working in pairs to carry out the practical activity means that there is enough work for each student and that everyone has a chance to see what is happening. Joining pairs to make new groups enables students to share their experiences and hear a wider range of ideas.

As well as using questions to develop what the students recount in their feedback, during the plenary discussion, teachers can emphasise the importance of the link between particles and structure.

The written feedback on worksheets, should reinforce this.

Practical notes

Equipment

For the class (adjust quantities to suit)

• Eye protection
• Microscope slides, x20, heated to about 60 °C (eg by placing in a beaker of hot water)
• Microscope slides, x20, cooled (eg by placing in a freezer or beaker of ice)
• Cover slips, x40
• Melted salol (phenyl 2-hydroxybenzoate/phenyl salicylate) (minimal hazard), about 50 g – split into samples so pairs of students can access it easily (see ’Health, safety and technical notes’, note 4)

For each pair of students

• Dropping pipette
• Access to a microscope or hand-lens
• Paper towel
• Stopwatch

Health, safety and technical notes

1. Read our standard health and safety guidance.
2. Wear eye protection.
3. It is the responsibility of the teacher to carry out an appropriate risk assessment.
4. Keep the salol in a water bath at about 50 °C.

Principal hazards

• Hot water
• Phenyl 2-hydroxybenzoate/phenyl salicylate (minimal hazard)

Answers

Discussion points:

• The hottest slide will produce the largest crystals.
• The coolest slide will produce crystals first.

This happens because the salol cools most slowly on the hottest slide, so the molecules take longer to crystallise.

The crystals form because the salol molecules line up alongside each other in a regular pattern and form bonds between each other.