22 Combustion of iron 11-14 Working in groupsSelf assessmentPeer assessmentSharing objectives and criteriaQuestioningUsing feedbackUsing tests

In this activity, students observe and interpret a practical demonstration and then use the particle model to explain their observations.

The demonstration leads to the conclusion that iron increases in mass when heated in air. Students are asked to think about why and may need to be prompted through a series of questions to the class:

  1. How do you picture the particles in iron?
  2. Is air a mixture or a pure substance?
  3. How do you picture the particles in air?
  4. Which part of air is reactive?

Learning objectives

Students will be able to:

  1. make and interpret observations
  2. explain that new substances, usually oxides, are formed when materials burn or are heated strongly in air (oxygen)
  3. use a particle model to explain how particles rearrange to form a new substance.

Sequence of activities

Demonstrate The spontaneous combustion of iron before outlining the learning objectives.

Show the experimental set up for The combustion of iron wool and explain what is going to happen – that the iron wool will be heated with a strong Bunsen flame.

Hand out a set of traffic light cards and a mini whiteboard to each student. Ask them to:

  1. hold up an appropriate coloured card
    • red if they think the iron will get heavier
    • yellow if they think the iron will get lighter
    • green if they think the iron will stay the same
  2. draw on the mini whiteboard what they think the apparatus will look like after the iron has been heated for one minute
  3. hold up their boards.

Pose questions about their responses and ideas.

Carry out the demonstration, ensuring all students have a clear view.
Ask the students to:

  1. decide whether the iron wool was heavier, lighter or the same after heating (use traffic light cards again)
  2. modify, if necessary, their drawing of the apparatus after the iron has been heated for 1 minute.

At this point make sure all learners have a record of the correct drawing and the statement: the iron wool was heavier after it had been heated in air.

Organise students to work in pairs. Give out Prompt cards. Circulate and support as the students:

  1. discuss why the iron became heavier
  2. identify the particles representing iron, oxygen and nitrogen
  3. use a mini-whiteboard to sketch a particle diagram that explains the increase in mass
  4. display their drawings.

In a plenary:

  1. select a few students to explain their drawings
  2. draw together ideas and summarise the change using a word equation
  3. make a general statement that chemical change involves atoms in reactants rearranging to form products.

Assessment for learning commentary

The initial stimulus is an important trigger for student inquiry.

Although there is a small element of self and peer assessment, as students collaborate on the particle diagram, the principal technique here is questioning by the teacher. Questions are used to develop and confirm ideas and concepts.

Resources

Stimuli

  Download PDF The spontaneous combustion of iron
  Download PDF The combustion of iron wool

Props

For each student

  1. A set of traffic light cards
  2. Mini whiteboard

For each pair of students

Download Word Download PDF Prompt cards

Safety note

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

Answers

Drawings should show that particles of oxygen from the air have combined with particles of iron. This is why the mass increases.

iron + oxygen iron oxide

When chemical change takes place, atoms in reactants rearrange to form products.

Acknowledgement

T. Lister, Classic chemistry demonstrations. London: Royal Society of Chemistry, 1996.