Changes of State: Melting & Freezing

Active learning works particularly well for changes of state because students need to see, touch, and feel the science in action. When students physically observe melting ice or freezing water, the abstract concept becomes concrete, and misconceptions are easier to address. Hands-on activities also encourage careful observation, which is critical when distinguishing between physical and chemical changes.

• 6th Class
• Scientific Inquiry and the Natural World
• 3 activities
• 20 min–40 min

Learning Objectives

1. Explain why a substance's temperature remains constant during a phase change from solid to liquid or liquid to solid.
2. Analyze the role of heat energy in causing melting and freezing.
3. Design an experiment to compare the melting points of at least three different solid substances.
4. Classify observed changes as melting or freezing based on temperature and energy transfer.

Think-Pair-Share

⏱ 20 min·Pairs

Think-Pair-Share: The Kitchen Chemist

The teacher lists five changes: frying an egg, melting ice, burning toast, dissolving sugar, and rusting a nail. Students work in pairs to decide which are reversible and which are not, providing one piece of evidence for each choice.

Prepare & details

Explain why a substance's temperature remains constant during a phase change.

Facilitation Tip: During The Kitchen Chemist, remind students to focus on observable clues like texture or temperature rather than just the disappearance of a substance.

Setup: Standard classroom seating; students turn to a neighbor

Materials: Discussion prompt (projected or printed), Optional: recording sheet for pairs

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Inquiry Circle

⏱ 40 min·Small Groups

Inquiry Circle: The Mystery Reaction

Groups are given vinegar and baking soda. They must observe the reaction, identify the signs of a chemical change (bubbles/gas), and then try to figure out if they can get the original materials back, leading to a discussion on irreversibility.

Prepare & details

Analyze the role of heat energy in melting and freezing.

Facilitation Tip: In The Mystery Reaction, circulate with a small cup of vinegar and baking soda to prompt quiet groups who may be stuck on how to test their predictions.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Gallery Walk

⏱ 30 min·Whole Class

Gallery Walk: Change in Action

Students set up 'before and after' stations for different changes (e.g., a crushed can, a burnt piece of paper, a dissolved salt solution). Classmates walk around and use a checklist to identify if the change was physical or chemical.

Prepare & details

Design an experiment to compare the melting points of different solids.

Facilitation Tip: For the Gallery Walk, assign a 'recorder' in each group to ensure all observations are captured before rotating to the next station.

Setup: Wall space or tables arranged around room perimeter

Materials: Large paper/poster boards, Markers, Sticky notes for feedback

• Understand
• Apply
• Analyze
• Create
• Relationship Skills
• Social Awareness

Teaching This Topic

Teachers should emphasize observation first, labeling second, before introducing the terms 'physical' or 'chemical.' Avoid starting with definitions; instead, let students experience the change and describe it in their own words. Research shows that students retain concepts better when they connect new knowledge to familiar examples, so begin with everyday changes like ice melting or butter softening before moving to lab-based examples.

What to Expect

Successful learning looks like students confidently using terms such as 'melting point,' 'freezing point,' and 'physical change' while explaining their observations. They should recognize that dissolving salt in water is a physical change by retrieving the salt through evaporation. Students should also identify the 'clues' of a chemical reaction in everyday contexts, like the kitchen or garden.

Watch Out for These Misconceptions

Common Misconception

During The Kitchen Chemist, watch for students who assume dissolving salt in water is a chemical change because the salt 'disappears.'

What to Teach Instead

During The Kitchen Chemist, redirect students by having them set aside a sample of the saltwater and observe as the water evaporates over a few days, recovering the salt crystals to demonstrate it is still the same substance.

Common Misconception

During The Gallery Walk, listen for students who claim all reversible changes are physical and all irreversible changes are chemical.

What to Teach Instead

During The Gallery Walk, challenge students to find an exception by showing them a simple chemical reaction, like baking soda and vinegar, which produces a gas and is irreversible but not a change of state. Use this to reinforce the 'new material' rule.

Assessment Ideas

Exit Ticket

After The Mystery Reaction, provide students with a graph showing temperature over time for a substance being heated and then cooled. Ask them to: 1. Identify the plateau where melting occurs. 2. Explain in one sentence why the temperature does not change during this plateau.

Quick Check

After The Kitchen Chemist, show students two identical containers, one with ice cubes and one with water at room temperature. Ask: 'If we add the same amount of heat energy to both, which will reach its melting/freezing point first, and why?'

Discussion Prompt

During The Gallery Walk, pose the question: 'Imagine you have a block of ice and a block of butter, both at 0°C. Which will melt faster if you place them in a warm room? Use the terms 'melting point' and 'heat energy' in your explanation.' Observe whether students can apply these terms correctly in context.

Extensions & Scaffolding

• Challenge: Ask students to design an experiment to determine whether dissolving sugar in water is a physical or chemical change, using only household items. They should present their method and evidence to the class.
• Scaffolding: Provide students with a simple sentence frame: 'When _____ melts, it changes from a _____ to a _____. This is a _____ change because...' to structure their explanations.
• Deeper: Introduce the concept of latent heat by showing a time-lapse video of ice melting in a warm room and asking students to explain why the ice stays at 0°C even as heat energy is added.

Key Vocabulary

Melting Point	The specific temperature at which a solid substance changes into a liquid. For pure substances, this temperature is constant.
Freezing Point	The specific temperature at which a liquid substance changes into a solid. For pure substances, this is the same as the melting point.
Phase Change	The physical process where a substance transitions from one state of matter (solid, liquid, gas) to another, such as melting or freezing.
Heat Energy	Energy transferred from one object or system to another due to a temperature difference. It is absorbed during melting and released during freezing.