States of matter | Johnstone’s triangle

Substances can exist in the solid, liquid or gas state.  Each state has specific properties that determine how it behaves.

Learning objectives

1. Recall the three states of matter.
2. Compare the properties of solids, liquids and gases.
3. Use the particle model to explain why different states of matter have different properties.

How to use Johnstone’s triangle

Use Johnstone’s triangle to develop learners’ thinking about scientific concepts at three different conceptual levels:

• Macroscopic: what we can see. Think about the properties that we can observe, measure and record.
• Sub-microscopic: smaller than we can see. Think about the particle or atomic level.
• Symbolic: representations. Think about how we represent chemical ideas, including symbols and diagrams. 

For learners to gain a deeper awareness of a topic, they need to understand it at all three levels.

When introducing a topic, do not try to introduce all of the levels of thinking at once. This will overload working memory. Instead, complete the triangle over a series of lessons, beginning with the macroscopic level and introducing other levels, in turn, once secure.

All of the levels are interrelated; for example, learners need visual representation of the sub-microscopic in order to develop mental models of the particle or atomic level.

Teacher demonstration

Use this demonstration of the three states of matter to encourage learners to observe and describe the shape and compressibility of substances in different states.

Equipment for teacher

• Beaker of water
• Ice cube
• Boiling kettle
• Three syringes filled with plasticine or sand (solid state), water (liquid state) and air (gas state)

Method

1. Show learners the three examples of water in different states and discuss the shape of each (fixed for the solid state, non-fixed for liquid and gas state).
2. Demonstrate the compressibility of each state of matter using the syringes filled with different substances (only the gas can be compressed).

Scaffolding

It is important to share the structure of the triangle with learners prior to use. Tell them why you want them to use the triangle and how it will help them to develop their understanding. Use an ‘I try, we try, you try’ approach when you are introducing Johnstone’s triangle for the first time.

Next steps

To further develop learners’ thinking in all areas of Johnstone’s triangle, try our Developing understanding of states of matter worksheet. This includes icons in the margin referring to the conceptual level of thinking needed to answer the question.