Science · Year 5 · Properties and Changes of Materials · Autumn Term

Changing States: Melting and Freezing

Investigating the processes of melting and freezing, focusing on temperature changes and reversibility.

• National Curriculum Attainment Targets
• NC-KS2-Science-Y5-PCM-3

About This Topic

This topic focuses on the critical distinction between reversible and irreversible changes in materials. Students investigate how some changes, such as melting, freezing, and dissolving, can be undone to recover the original materials, while others, like burning or reacting acid with bicarbonate of soda, result in the formation of new materials. This aligns with the KS2 requirement to demonstrate that dissolving, mixing, and changes of state are reversible changes.

Understanding these processes is essential for students to make sense of the world around them, from cooking food to industrial manufacturing. It encourages them to look for evidence of chemical reactions, such as gas production or color changes. This topic comes alive when students can physically model the patterns of change through experimentation and observe the results of their own chemical reactions.

Key Questions

1. Explain what happens to the particles of a substance when it melts or freezes.
2. Analyze how temperature affects the rate at which ice melts.
3. Predict whether a substance will melt or freeze at a given temperature.

Learning Objectives

• Explain the particle behavior during melting and freezing.
• Analyze the relationship between temperature and the rate of ice melting.
• Compare the melting and freezing points of different substances.
• Predict the state of a substance at a given temperature based on its melting and freezing points.

Before You Start

States of Matter

Why: Students need to identify solids and liquids before they can investigate how they change between these states.

Temperature and Measurement

Why: Understanding how to read a thermometer is essential for observing and recording temperature changes during melting and freezing experiments.

Key Vocabulary

Melting	The process where a solid changes into a liquid due to an increase in temperature.
Freezing	The process where a liquid changes into a solid due to a decrease in temperature.
Reversible Change	A change where the original substance can be recovered, such as melting and freezing.
Melting Point	The specific temperature at which a solid substance turns into a liquid.
Freezing Point	The specific temperature at which a liquid substance turns into a solid.

Watch Out for These Misconceptions

Common Misconception

All changes involving heat are irreversible.

What to Teach Instead

Students often think that if you use a stove or a flame, the change cannot be undone. By melting and then refreezing wax or chocolate, students can see that heat can cause reversible physical changes, whereas burning wood causes an irreversible chemical change.

Common Misconception

When a gas is produced, the material has disappeared.

What to Teach Instead

In reactions like vinegar and bicarbonate of soda, students may think the 'fizz' means matter is lost. Using a balloon over a bottle to capture the gas during the reaction helps students visualize that the new material (gas) still exists and has mass, which is best explored through collaborative observation.

Active Learning Ideas

Predict-Observe-Explain: The Mystery Reactions

Provide students with several scenarios, such as mixing vinegar and milk or melting chocolate. Students predict if the change is reversible, observe the reaction in small groups, and then write an explanation using evidence like 'no new material was formed' or 'a gas was produced'.

45 min·Small Groups

Stations Rotation: Recovery Lab

Students visit stations where a change has already occurred, such as salt dissolved in water or a burnt piece of paper. Their task is to brainstorm and, where possible, attempt a method to reverse the change, such as using evaporation or filtration, to see which materials can be recovered.

60 min·Small Groups

Think-Pair-Share: The Kitchen Scientist

Students think of three changes that happen during cooking, such as frying an egg, boiling water, or making toast. They pair up to categorize these as reversible or irreversible and then share their reasoning with the class, focusing on whether a new substance was created.

20 min·Pairs

Real-World Connections

• Ice cream makers use controlled freezing processes to create smooth textures by managing the rate of ice crystal formation.
• Chefs use their understanding of melting and freezing points when making confectionery, like tempering chocolate, to achieve desired textures and prevent blooming.
• Meteorologists track temperature changes to predict when rain might turn to snow or when puddles will freeze overnight, impacting travel conditions.

Assessment Ideas

Exit Ticket

Provide students with a diagram showing a solid turning into a liquid. Ask them to label the process (melting) and write one sentence describing what is happening to the particles. Then, ask them to name one common substance that melts.

Quick Check

Show students three beakers, one with ice cubes, one with water, and one with steam. Ask: 'Which beaker shows a substance that has frozen? Which shows a substance that has melted? Which shows a substance that is neither melting nor freezing?'

Discussion Prompt

Pose the question: 'Imagine you leave a glass of water outside on a very cold day. What will happen to the water? How do you know? What if you bring it back inside?' Encourage students to use the terms melting, freezing, and reversible.

Frequently Asked Questions

How do I identify an irreversible change?

An irreversible change usually results in the formation of a new material. Key signs include a permanent color change, the release of a gas (fizzing), the formation of a solid from two liquids, or a significant change in temperature. Unlike melting ice, you cannot simply cool or heat the substance to get the original ingredients back.

What are some common reversible changes for Year 5?

Common reversible changes include melting (solid to liquid), freezing (liquid to solid), evaporating (liquid to gas), condensing (gas to liquid), and dissolving. Students should practice reversing these, such as by using a sieve to separate sand from water or evaporating water to find salt.

How can active learning help students understand reversible changes?

Active learning, such as 'Predict-Observe-Explain' cycles, forces students to confront their initial assumptions. When they physically try to reverse a change and fail (or succeed), the scientific principle becomes much more memorable. Collaborative problem-solving around how to recover a dissolved substance encourages them to apply their knowledge of states of matter in a practical way.

Why is burning considered an irreversible change?

Burning is a chemical reaction where a fuel reacts with oxygen to create new substances like ash, carbon dioxide, and water vapor. Because these new substances have different properties and the original fuel has been chemically altered, the process cannot be undone to recover the original wood or paper.