16 What happens when a substance changes state? 11-14 Working in groupsSelf assessmentPeer assessmentSharing objectives and criteriaQuestioningUsing feedbackUsing tests

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

Collect student ideas about the meaning of ‘freezing’, ‘melting’ and ‘boiling’. Save these for later reference.
Demonstrate what happens when water boils.

Give each student a copy of the worksheet What happens when? and remind them of the formula for water.

Invite students to work individually on completing the first part of the worksheet.

Organise students into groups of four. Circulate and support as groups:
  • discuss their individual responses, working towards consensus
  • prepare a response to feedback to the class
  • elect a spokesperson.
In a plenary:
  • invite reports from each group
  • lead students towards a scientifically correct viewpoint using molecular models of water
  • encourage students to see with ‘molecular spectacles’ what is happening to the water
  • ask them to correct their worksheet, if they need to.
Introduce the next task, which is to look at the liquid ‑ solid state change.

Place a beaker of ice cubes in front of the class. This can be warmed gently, but may also be left at room temperature.

Invite students to work individually on completing the second page of the worksheet.

Reform the groups, or create different groups of four students.
Circulate and support as groups:
  • discuss their individual responses, working towards consensus
  • prepare a response to feedback to the class
  • elect a spokesperson.
In a plenary:
  • invite reports from each group
  • use a model of ice to support the development of the concept that bonds exist between water molecules in ice
  • 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
  • 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.


Assessment for learning commentary

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.



For each student

Download Word Download PDF What happens when?


For the first demonstration

  • Two 250 cm3 beakers
  • Bunsen burner
  • Tripod
  • Gauze
  • Heatproof mat
  • Eye protection
  • Water
  • Molecular models of water molecules.

For the second demonstration

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

Note on the first demonstration

Boil the water in one beaker. Place the water molecule models in the second beaker, to use in explaining the state change.

Safety note

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


What is in the bubbles when water boils?

The bubbles contain a mixture of hydrogen and oxygen.

The bubbles contain carbon dioxide.

The bubbles contain steam (water vapour).

The bubbles are empty (vacuum).

The bubbles contain air.

The bubbles contain oxygen only.

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.

The molecules in ice get warmer because the water is hotter than ice.

The molecules move around more as water than they did in the ice.

Ice molecules change to water molecules.

Ice changes to water at 0 °C.
True (if temperature is increasing)

Ice only melts above its melting temperature.


V. Kind, Changing matter, Ed. Chem. July 2001, 92.