How does burning magnesium affect its mass? | 11-14 years

Students explore, in a practical activity, what happens to the mass of magnesium when it burns. This helps them consolidate their ideas about the increase in mass on burning due to combination with oxygen. Students also evaluate and develop their ability to draw and interpret simple graphs.

Learning objectives

Students will be able to:

• Describe and explain what happens to the mass of magnesium when it burns.
• Draw and interpret simple graphs.

Sequence of activities

Demonstration and introduction

1. Issue a blue safety glass to each student.
2. Ignite a piece of magnesium ribbon over a cement mat. Ensure that students use the safety glass to observe this.
3. Ask students to use ’traffic light’ cards to indicate whether they think that the mass of magnesium when burnt in air will:
   • Increase – green
   • Stay the same – yellow
   • Decrease – red
4. Invite:
   • A student to explain their answer.
   • Other students to add to this if appropriate.
5. Explain to the students:
   • The learning objectives.
   • That they are going to investigate what happens to the mass of magnesium when it is burnt.

Practical: set-up

Describe the practical details, with students to work in pairs and follow safe procedures to:

1. Weigh a crucible and lid.
2. Add a prepared piece of magnesium ribbon to the crucible and weigh it again.
3. Place the crucible on a pipeclay triangle supported on a tripod and heat it with a Bunsen burner flame, lifting the lid of the crucible from time to time using a pair of tongs.
4. Let the crucible cool and weigh it again.
5. Place the crucible on the pipeclay triangle again and re-heat for a short time, let it cool and reweigh.

Practical

Circulate and support while pairs:

1. Carry out the practical.
2. Add their results to a class set written on a board or projected.
3. Write them down on their copy of the ’Results table’ sheet.
4. Complete all parts of their ’Results table’.

A set of specimen results is provided that may be used if student data is not available (see downloads below).

Graphs exercise

Discuss with the whole class What makes a good graph? Refer to the criteria on their ‘Results table’ sheet.

Support individuals as they:

1. Draw the graph of mass of magnesium against mass of magnesium oxide formed.
2. Check that their graph meets the expected standard, and modify it if necessary.
3. Work with the other member of the pair to discuss whether they think that the graph meets the required standard.
4. Offer help to their partner only in the form of questions.
5. Confirm with you that the graph is up to standard.
6. Answer a question that you pose, if it is not to standard.
7. Use their graphs to predict how much magnesium oxide would be produced from a specified mass of magnesium.
8. Write in words on their ’Results table’ sheet how they can use their graph to predict the mass of magnesium oxide which will be formed from a particular mass of magnesium when it is burnt.

Feedback

Take in the graphs and written sheets and comment on any features that the student still needs to develop.

Commentary

Sharing objectives and criteria gives purpose to the activity and a framework for self criticism. Through the discussion about their own and their partner’s graphs, students will see how to improve. When interpreting their graph, students will see why certain standards need to be met.

Teacher feedback is used to affirm standards or to prompt improvement.

Practical notes

Equipment

• Blue safety glass
• Set of traffic light cards
• Graph paper

For the practical (see note 4 in ‘Health, safety and technical notes’ below):

• Crucible and lid
• Pipeclay triangle
• Tripod
• Bunsen burner
• 10–20 cm lengths of magnesium ribbon (FLAMMABLE) ranging from 0.1 g to 0.5 g
• Access to a balance measuring to 0.01 g

Health, safety and technical notes

1. Read our standard health and safety guidance.
2. Wear eye protection.
3. It is the responsibility of the teacher to carry out an appropriate risk assessment.
4. The ‘Set of specimen results’ handout can be used in place of carrying out the practical, or if student data is not available (see downloads below).

Principal hazards

• Crucibles remaining hot for some time (leave to cool on the tripods).
• Magnesium ribbon flammability.