Use this lesson plan for 11–14 year olds to explore what happens when substances warm, cool, boil or freeze, tackling some misconceptions about changes of state

In this activity, students watch a demonstration and manipulate molecular models. They answer questions to probe their misconceptions and to develop scientific understanding about what happens when a substance changes state.

Learning objectives

Students will be able to explain that:

  • Molecules do not break up and reform when a substance boils and cools.
  • Particles stay the same size and shape during state changes.
  • Particles move around and are not static.
  • Freezing means ‘liquid changing to solid’ and does not rely on cold temperatures.

Sequence of activities

Eliciting ideas

Collect student ideas about the meaning of ‘freezing’, ‘melting’ and ‘boiling’. Save these for later reference.

Demonstration and activity: stage 1

  1. Demonstrate what happens when water boils.
  2. Give each student a copy of the worksheet ‘What happens when?’ and remind them of the formula for water.
  3. Invite students to work individually on completing the first part of the worksheet.

Activity: stage 2

Organise students into groups of four. Circulate and support as groups:

  1. Discuss their individual responses, working towards consensus.
  2. Prepare a response to feedback to the class.
  3. Elect a spokesperson.

Plenary 1

In a plenary:

  1. Invite reports from each group.
  2. Lead students towards a scientifically correct viewpoint using molecular models of water.
  3. Encourage students to see with ‘molecular spectacles’ what is happening to the water.
  4. Ask them to correct their worksheet, if they need to.

Demonstration and activity: stage 3

  1. Introduce the next task, which is to look at the liquid–solid state change.
  2. Place a beaker of ice cubes in front of the class. This can be warmed gently, but may also be left at room temperature.
  3. Invite students to work individually on completing the second page of the worksheet.

Activity: stage 4

Reform the groups, or create different groups of four students. Circulate and support as groups:

  1. Discuss their individual responses, working towards consensus.
  2. Prepare a response to feedback to the class.
  3. Elect a spokesperson.

Plenary 2

In a plenary:

  1. Invite reports from each group.
  2. Use a model of ice to support the development of the concept that bonds exist between water molecules in ice.
  3. Introduce the terms ‘melting’ and ‘freezing’ or ‘solidifying’ to label the change from solid to liquid, depending on whether the substance is being heated up or cooled down.
  4. Reinforce that these terms apply to all substances.

Revisit the introductory discussion and invite students to write down (on their worksheets) how their views have changed. Collect the worksheets.


Provide written feedback commenting on the extent to which students have demonstrated learning of the scientific viewpoints about state changes.


Discussing personal viewpoints with others allows students to review each other and feed back. Reaching consensus in groups stimulates this process.

The teacher-led plenary discussions help students whose thinking has not moved forward and gives the teacher an opportunity to assess the extent of any misconceptions in the class. Checking the written feedback on worksheets also gives an opportunity to deal with misconceptions as well as confirming correct thinking.

Practical notes


For the first demonstration

  • Beakers, 250 cm3, x2 (see ‘Health, safety and technical notes’, note 4)
  • Bunsen burner
  • Tripod
  • Gauze
  • Heatproof mat
  • Eye protection
  • Water
  • Molecular models of water molecules

For the second demonstration

  • Beaker, 250 cm3
  • Ice cubes
  • A molecular model of ice
  • Optional: Bunsen burner, tripod, gauze, heatproof mat, eye protection

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 appropriate risk assessments for the demonstrations.
  4. Boil the water in one beaker. Place the water molecule models in the second beaker, to use in explaining the state change.


What happens when water boils?

  • ‘The bubbles contain a mixture of hydrogen and oxygen.’
    • False
  • ’The bubbles contain carbon dioxide.’
    • False
  • ‘The bubbles contain steam (water vapour).’
    • True
  • ‘The bubbles are empty (vacuum).’
    • False
  • ‘The bubbles contain air.’
    • False
  • ‘The bubbles contain oxygen only.’
    • False
    • Note: The initial small bubbles, seen as water heats up, contain oxygen.

What happens when ice melts?

  • ‘The molecules in ice get smaller because water takes up less space than ice.’
    • False
  • ’The molecules in ice get warmer because the water is hotter than ice.’
    • False
  • ’The molecules move around more as water than they did in the ice.’
    • True
  • ’Ice molecules change to water molecules.’
    • False
  • ’Ice changes to water at 0 °C.’
    • True (if temperature is increasing)
  • ‘Ice only melts above its melting temperature.’
    • False

Primary teaching notes

If you teach primary science, see the guidance below to find out how to use this resource.

Skill development

Children will develop their working scientifically skills by:

  • Drawing conclusions and raising further questions that could be investigated, based on their data and observations.
  • Using appropriate scientific language and ideas to explain, evaluate and communicate their methods and findings.

Learning outcomes

Children will:

  • Compare and group materials together according to whether they are solids, liquids or gases.
  • Observe that some materials change state when they are heated or cooled.

Concepts supported

Children will learn:

  • What is meant by the terms boiling, melting, solidifying and freezing, and that cold temperatures are not a requirement for freezing.
  • What does and doesn’t happen when substances change state, including:
    • That molecules do not break up and reform when a substance boils and/or cools.
    • That particles stay the same size and shape during changes of state.
  • That particles are always moving when they are at any temperature other than absolute zero.

Suggested activity use

The experiments in the activity can be used as a demonstration to stimulate discussion and questions about what is actually happening when substances change state. Alternatively, the mini experiments could be adapted and the children could carry these out in small groups, noting down their observations and ideas.

Practical considerations

Access to and use of Bunsen burners in primary schools can be difficult, so viable alternatives may need to be sourced.

Note that primary children may not need to go into as much detail as looking at molecular models of water.