Dissolving salt | Johnstone’s triangle

When a solid dissolves in water, the particles within the solid break apart and spread out until they are distributed throughout the water.

Learning objectives

1. State that solutions are formed when a solute is dissolved in a solvent.
2. Explain this in terms of the particles present.

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.

Learner activity

Use this learner activity to encourage learners to observe and describe the macroscopic formation of a solution of salt in water.

Equipment per learner pair

• Spatula
• Mixture of sand and salt
• 100 ml beaker
• Water

Method

1. Add water to the beaker.
2. Add a spatula of the sand and salt mixture.
3. Stir gently.
4. Observe.

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 solutions worksheet. This includes icons in the margin referring to the conceptual level of thinking needed to answer the question.