Investigating the composition of limestone | 11-14 years

In this practical activity, students develop their ability to make accurate observations of rock samples. They work in small teams to plan, implement and evaluate a short investigation.

Learning objectives

Students will be able to:

• Plan, implement and evaluate a short practical investigation.

Sequence of activities

Introduction

1. Using images of different carbonate rocks such as the white cliffs at Dover, stalactites, fossils, Portland stone buildings, open with questions about the differences.
2. Introduce the learning objective to the students and explain that they are going to devise a way of finding out if all limestones are the same.

Making observations

Place students in groups of four. Give each student four cards and a ‘Worksheet’, and each group four different samples of limestone labelled A, B, C and D. Circulate and support as students:

1. Write a description of each sample on four different cards.
2. Exchange cards with the other students in their group and try to match each other’s descriptions with the appropriate sample.
3. As a group, identify what was written on the cards that they found helpful in matching a card to a rock sample and what could have been added to make it even more useful.
4. Summarise the feedback on their work on their ’Worksheet’.

Investigation: introduction

1. Explain the next task: to devise a way of finding out if all the samples of limestone have a different chemical composition.
2. Ask questions to remind students of their previous work on acids and carbonates and how they might use this for finding out the chemical composition of the rock samples. For example:
   • What do you know about the chemical make-up of limestone?
   • What kind of substances react with carbonates?
   • If different samples of limestone contain different amounts of carbonate, how will their reactions with acids differ?
   • How might you measure this?
   • How will you be able to use the data that you measure?

Investigation: planning

Support students as they:

1. Individually devise an experimental plan to find out if the carbonate composition of samples of limestone are different.
2. Write the plan on their ’Worksheet’.
3. Consider, as a group, all four plans and agree on a composite plan using the best ideas from the individual plans.
4. Send a spokesperson to you with the plan.

Either give the go-ahead or ask questions to help the group reconsider and modify their plan. Plans may include:

• Weighing the rock sample before and after adding and reacting with acid.
• Measuring the volume of acid needed to react with the rock sample.

Investigation

Allow 15 minutes for the practical work. Supervise as students:

1. Try out their experimental plan (groups may not have time to complete their experiment).
2. Identify some of the difficulties involved (for example, rock will take a long time to react and it is difficult to know when the reaction has stopped).
3. Consider, as a group, any problems and ways of overcoming them.
4. Record these points on their ’Worksheet’.

Plenary

Organise a plenary. Draw out from students the practical difficulties that they encountered and their solutions. Collect in the ‘Worksheets’.

Commentary

Although ostensibly about limestones, this activity is really about planning, carrying out and evaluating an experiment. This purpose is made clear at the outset so that students understand the importance of process.

Discussing each others’ descriptors and experiment plans brings in a wider range of ideas using the students’ own vocabulary. This enhances credibility. It also promotes coherent thinking. There is a strong element of evaluation as well, that is carried out in a group context.

Teacher feedback is essential in relation to the experimental plans, both during formulation and at approval stage. Having been so closely involved with planning and reviewing the investigation, students will understand the relevance of any further help given in the written feedback.

Practical notes

Equipment

For observations:

• Four different samples of limestone labelled A, B, C, D
• Hand lenses
• Cards or paper on which students write down rock descriptions

For the experimental investigation:

• Pieces of four different limestones
• Supply of 1 mol dm^-3 hydrochloric acid (HARMFUL)
• Beakers, 100 cm³
• Filter funnels
• Filter papers
• Teat pipettes
• Measuring cylinders
• Access to a balance

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection.
• It is the responsibility of the teacher to carry out an appropriate risk assessment.
• Student plans should be checked for health and safety issues before they start practical work.

Principal hazard

• Hydrochloric acid

Answers

To find out if all limestones are the same I will:

Method A

1. Weigh out limestone samples of similar mass.
2. Place samples in separate small beakers.
3. Add the same volume of acid to each sample.
4. At the end of the same time period, take each sample out of the acid, wash with water, dry and reweigh.
5. Compare the mass loss of each sample.

Method B

1. Weigh out limestone samples of similar mass.
2. Place samples in separate shallow dish.
3. Measure out a fixed volume of acid using a measuring cylinder.
4. Add a little acid to one of the samples using a dropping pipette.
5. Observe whether there is ‘fizzing’ on the surface of the limestone sample.
6. If there is fizzing, add more acid and observe the sample again.
7. Keep adding the acid until there is no longer any fizzing when acid is added to the sample.
8. Record how much has been added to the limestone sample.
9. Repeat the process with other limestone samples.
10. Compare the volume of acid added to each sample.