Accuracy and precision in practical investigations | 14-16 years

In this activity, students think about an investigation to measure the boiling point of water. They decide what equipment to use, giving their reasons. They then examine the results of four unknown groups and comment on the accuracy and precision of the data.

Learning objectives

Students will be able to:

• Select equipment most suited for practical work.
• Explain the difference between accuracy and precision.

Sequence of activities

Introduction

Boil some water in an electric kettle and ask the question, ‘How could we measure the temperature at which water boils?’

If possible, open some cupboards in the laboratory to show the range of laboratory apparatus available – eg different sized beakers, conical flasks, measuring cylinders, pipettes, burettes and so on.

Activity 1: choosing equipment

1. Ask students to work in pairs. Explain that they are to list the equipment needed to boil 30 cm³ of water and measure its temperature. Tell them to think carefully about:
   • How to measure out the 30 cm³ of water.
   • What container to boil the water in.
   • What thermometer to use to measure the temperature.
2. Hand out copies of the ‘Apparatus’ sheet. Taken in order, ask students to write, on a mini whiteboard, the letter corresponding to the apparatus they would use to:
   • Measure the 30 cm³ of water.
   • Boil the water in.
   • Measure the temperature of the boiling water.
3. Starting with the volume measuring apparatus, ask students to show their answer. Invite students with different answers to explain their choice. Repeat with boiling vessel and temperature measurement.

Activity 2: accuracy and precision

1. Hand out copies of the ’Results’ sheet.
2. Ask students why each group carried out three tests.
3. Guide the discussion towards an understanding of the terms ‘accuracy’ and ’precision’. Use the analogy of a dartboard to help.
4. Ask each student, working individually, to answer the questions.
5. Arrange students into small groups to compare their answers and to arrive at an agreed answer for each question.

Plenary

Organise a plenary. For a given practical, ask students to explain how they would decide:

• The best way to measure a volume of liquid.
• The best way to measure temperature.
• About the accuracy and precision of data collected.

Commentary

Students are more likely to question their own ideas and look more openly at the quality of their answers if they work in pairs, rather than individually.

By inspecting anonymous results, students have more freedom to be critical and thus take an impartial approach to the questions, which are designed to develop their understanding.

Targeted questions, during the plenary, ensure that every student is clear about the way accuracy and precision affect practical procedures.

Equipment

• Kettles to boil water
• Selection of glassware and thermometers
• Mini whiteboards, one for each student