What happens when something dissolves? | 11-14 years

In this activity, students interpret information gained from practical work. They observe some simple practical demonstrations and use a question sheet to help them think about the process of dissolving.

It provides an opportunity for students to:

• Observe some chemical events and describe them fully.
• Think about the process of dissolving and use their knowledge of the particle model to explain their observations.

Depending on when this activity is used, students may be familiar with the terms solvent, solute and solution, in which case they could be used here.

It may be necessary to spend time recapping the particle models for solids, liquids and gases.

Learning objectives

Students will be able to:

• Calculate the mass of solution from the masses of the liquid and the solids.
• Describe that particles of the solid are dispersed throughout the liquid.
• Describe the process of diffusion and say why this is evidence of the movement of particles in a liquid.

Sequence of activities

Introduction and demonstration

1. Demonstrate what happens when crystals of potassium manganate(VII) crystals are added to water using ‘Solubility: teacher demonstration’.
2. Tell the students that:
   • They need to keep an eye on the demonstration.
   • They should make notes about what the mixture looks like and how it changes during the course of the session.
   • At the end of the session they will be asked to explain their observations.

Student activity 1

1. Give each student a copy of ‘Solubility: student demonstration’ and the sheet ’Mass and dissolving’.
2. Supervise the students as they:
   • Work in pairs.
   • Carry out their own investigations.
3. Ask the students to work individually and try to complete the questions.
4. When a student has finished, check their answers.
5. Appoint the first four with correct answers as ‘Experts’. Authorise them to:
   • Help the others arrive at correct answers.
   • Give out appropriate stickers.

Demonstration review

Towards the end of the session, return to the potassium manganate(VII) demonstration. Invite the students to think about what happened drawing upon the work they did on the questions.

Student activity 2

Circulate and support as the students work in pairs to:

1. Write a brief description of the arrangement of particles in a liquid and in a solid (they can make some drawings if they find it helpful).
2. Agree on the descriptions.
3. Write an explanation of their observations in terms of particles.

Feedback

Give verbal and written feedback.

Commentary

As soon as the session starts, students are actively engaged in learning.

A semi-formal assessment process, in which students confirm each other’s success, coupled with less formalised appraisal, while working in pairs and reaching agreement, gives students incentives to get their thinking right.

Verbal feedback from the teacher is particularly useful in ensuring that students arrive at the correct explanation if given when circulating during the final part of the session.

Practical notes

Equipment

For the teacher demonstration

• Tall beaker, 1 dm³, or gas jar
• Water
• Several crystals of potassium manganate(VII) (HARMFUL and OXIDISING)
• White card to provide a background

For the student demonstration

• Three test tubes
• Test tube rack
• Water
• A pea-size quantity of each of the following:
  • Sugar
  • Salt
  • Copper(II) sulfate crystals
• White card to provide a background

Other equipment

• Stickers, eg ‘Expert’, ‘See the teacher’

Health, safety and technical notes

• Read our standard health and safety guidance.
• It is the responsibility of the teacher to carry out an appropriate risk assessment.

Principal hazards

• Potassium manganate(VII) (HARMFUL and OXIDISING)
• Copper (II) sulphate (HARMFUL)