Explore what happens during precipitation reactions and when substances dissolve using this lesson plan with downloadable activities for 11–14 year olds

This simple practical activity challenges students’ thinking about mass conservation when substances dissolve or when chemical reactions produce precipitates. Working in small groups they carry out a simple experiment and agree answers to questions.

Learning objectives

Students will be able to explain that:

  • Mass is conserved during dissolving.
  • Mass is conserved during a precipitation reaction.
  • Whatever change occurs, the total mass of the substances involved does not change.

Sequence of activities

Introduction

Introduce the topic, and the learning objectives, by explaining to the students that they are going to look at two events which:

  • People often explain using different ideas.
  • They will come across a lot in chemistry, so they need to understand them correctly.

Activity: stage 1

Give each student the worksheet, ‘Dissolve and precipitate’.

Organise the students into groups of three. Give each group one flask labelled ‘Dissolve’.

Circulate and support with prompts as groups:

  1. Work on the dissolving task.
  2. Discuss the results and agree on an explanation.
  3. Elect a spokesperson to explain their group’s reasoning to the rest of the class.

Allow about 15 minutes for the groups to complete the task.

Plenary 1

In a plenary:

  • Draw out, in feedback from each group, their understanding that mass is conserved when substances dissolve.

Explain that in the next task they are going to test another event.

Activity: stage 2

Give each group one flask labelled ‘Precipitate’.

Circulate and support with prompts as groups:

  1. Work on the precipitation task.
  2. Discuss the results and agree on an explanation.
  3. Elect a spokesperson to explain their group’s reasoning to the rest of the class.

Allow about 15 minutes for the groups to complete the task.

Plenary 2

In a plenary:

  1. Draw out, in feedback from each group, their understanding that mass is conserved when a precipitate forms.
  2. Ask students to reflect how their thinking changed while doing the experiments and to write this on the reverse of the worksheets.
  3. Collect in the worksheets.

Feedback

In giving written feedback:

  1. Point out where students’ individual ideas are incorrect.
  2. Support and encourage students in changing their thinking to a scientifically correct viewpoint.

Commentary

The idea of mass conservation is central to developing good understanding of chemical reactions. By sharing their ideas in ‘safe’ groups, students can progress towards scientific understanding in a supported way.

Feedback to the class enables the teacher to assess whether groups have understood the key concept.

Written feedback can pinpoint students who still need to change their thinking and encourage those who have developed the correct view.

Practical notes

Equipment

For the prepared flasks labelled ‘Dissolve’ (see ‘Health, safety and technical notes’, notes 3 and 4)

  • Conical flask, 500 cm3
  • A small tube to fit comfortably inside the flask
  • Stoppers to fit the flask tightly
  • Water, about 150 cm3 in the flask
  • About 5–10 g of one of these solids in the small tubes: sodium chloride, sugar, copper(II) sulfate (HARMFUL)

For the prepared flasks labelled ‘Precipitate’ (see ‘Health, safety and technical notes’, notes 3 and 5)

  • Conical flask, 500 cm3
  • A small tube to fit comfortably inside the flask
  • Stoppers to fit the flask tightly
  • Pairs of solutions at 1 mol dm-3 that form a precipitate on mixing (for example, sodium sulfate / barium nitrate (HARMFUL and OXIDISING), potassium iodide / lead(II) nitrate (TOXIC), ammonium phosphate / calcium chloride (IRRITANT))

Other equipment

  • Access to a balance weighing to 0.01 g

Health, safety and technical notes

  1. Read our standard health and safety guidance.
  2. It is the responsibility of the teacher to carry out appropriate risk assessments.
  3. To prepare the flasks, tie the thread around the necks of the small tubes. Ensure that the length of the thread supports the tube in an upright position in the flask, but allows the contents to mix when the flask is tilted without being opened.
  4. Put the chosen solid into the tube. Put about 150 cm3 of water in the flask. Use the thread to arrange the prepared tube in the flask so that the contents mix when the flask is tilted. Label the flask ‘Dissolve’.
  5. Put the chosen solution (10–20 cm3) into the tube. Put the second solution in the flask. Use the thread to arrange the prepared tubes in the flask so that the contents mix when the flask is tilted. Label the flask ‘Precipitate’.

Principal hazard

  • Stoppers insecure in flask, leading to spillage.

Alternative strategy

If time and/or resources are short, the teacher can demonstrate the chemical events. Groups can discuss the results and make their predictions as suggested.

Answers

The mass values should remain unchanged during dissolving and precipitation. Responses should reflect this.

Primary teaching notes

If you teach primary science, see the guidance below to find out how to use this resource.

Skill development

Children will develop their working scientifically skills by:

  • Asking their own questions about scientific phenomena.
  • Using a range of scientific equipment to take accurate and precise measurements or readings.
  • Using appropriate scientific language and ideas to explain, evaluate and communicate their findings.

Learning outcomes

Children will:

  • Observe that some materials will dissolve in liquid to form a solution, and describe how to recover a substance from a solution.
  • Demonstrate that dissolving, mixing and changes of state are reversible changes.

Concepts supported

Children will learn:

  • That some materials dissolve to form a solution.
  • That materials are still present when they have dissolved and that they haven’t disappeared.
  • That mass is conserved when dissolving and precipitating.

Suggested activity use

This activity can be used as a whole class investigation into the dissolving and precipitation processes, with children working in small groups to observe and answer the questions given. Alternatively, the activities could be demonstrated by an adult to stimulate discussion and questioning.

Practical considerations

It is important that the key vocabulary ‘dissolve’ and ‘precipitate’ are understood correctly by children in the introduction of this activity.

The ‘Dissolve’ task is more relevant to the primary science curriculum and could be more heavily focused on.

Conical flasks, stoppers and tubes may not be required for this activity if alternatives can be sourced, such as mini pop bottles and clean fromage frais pots.

Downloads

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Additional information

This lesson plan was originally part of the Assessment for Learning website, published in 2008.

Assessment for Learning is an effective way of actively involving students in their learning.  Each session plan comes with suggestions about how to organise activities and worksheets that may be used with students.

Acknowledgements

V. Barker, Beyond Appearances: Student’s misconceptions about basic chemical ideas: A report prepared for The Royal Society of Chemistry, London. London: Royal Society of Chemistry, 2000.