15 What happens when substances dissolve? What happens when a precipitate forms? 11-14 Working in groupsSelf assessmentPeer assessmentSharing objectives and criteriaQuestioningUsing feedbackUsing tests

This simple practical activity challenges studentsí thinking about mass conservation during dissolving and precipitation. 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

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.
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:

  • work on the dissolving task
  • discuss the results and agree on an explanation
  • elect a spokesperson to explain their groupís reasoning to the rest of the class.

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

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.

Give each group one flask labelled Precipitate.

Circulate and support with prompts as groups:

  • work on the precipitation task
  • discuss the results and agree on an explanation
  • elect a spokesperson to explain their groupís reasoning to the rest of the class.

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

In a plenary:
  • draw out, in feedback from each group, their understanding that mass is conserved when a precipitate forms
  • ask students to reflect how their thinking changed while doing the experiments and to write this on the reverse of the worksheets
  • collect in the worksheets.
In giving written feedback:
  • point out where studentsí individual ideas are incorrect
  • support and encourage students in changing their thinking to a scientifically correct viewpoint.

Assessment for learning commentary

The idea of mass conservation is central to developing good understanding of chemical reactions. By sharing their ideas in Ďsafeí 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.

Resources

Props

For each student

Download Word Download PDF Dissolve and precipitate

Equipment

For each group of students

  • A prepared flask labelled Dissolve
  • A prepared flask labelled Precipitate
  • Access to a balance weighing to 0.01 g.

Notes

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.

Each prepared flask for Dissolve

  • 500 cm3 conical flask
  • A small tube to fit comfortably inside the flask
  • Stoppers to fit the flask tightly
  • About 150 cm3 water, in the flask
  • About 5‑10 g of one of these solids in the small tubes: sodium chloride, sugar, copper(II) sulfate (Harmful).

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.

Each prepared flask for Precipitate

  • 500 cm3 conical flask
  • 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)).

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.

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.

Safety notes

It is the responsibility of the teacher to carry out appropriate risk assessments.

Principal hazard

  • Stoppers insecure in flask, leading to spillage.

Answers

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

Acknowledgements

V. Barker, Beyond Appearances: Studentís misconceptions about basic chemical ideas: A report prepared for The Royal Society of Chemistry, London. London: Royal Society of Chemistry, 2000, available at www.chemsoc.org/networks/learnnet/miscon.htm (accessed October 2005).