Phase Changes: Melting and Freezing

Active learning helps students grasp phase changes because the invisible processes of energy transfer become visible through hands-on observation. Timing melting ice or watching wax solidify turns abstract concepts into concrete, memorable experiences that build lasting understanding.

• 5th Class
• Scientific Inquiry and the Natural World
• 4 activities
• 25 min–45 min

Learning Objectives

1. Explain the energy transfer that occurs when water melts or freezes.
2. Compare the melting and freezing points of pure water with those of salt water.
3. Predict the state of water (solid, liquid, or gas) at given temperatures between -10°C and 50°C.
4. Analyze how adding a solute, like salt, affects the freezing point of water.

Experiential Learning

⏱ 30 min·Pairs

Melting Race: Ice Cubes Under Conditions

Provide pairs with identical ice cubes on plates. Place one in sunlight, one shaded, one salted, and one with black paper underneath. Students measure melting time and temperature every 2 minutes, then graph results to compare conditions.

Prepare & details

Explain the energy changes that occur during melting and freezing.

Facilitation Tip: During the Melting Race, have students record start times and weights to standardize conditions, then compare melting rates in groups to identify variables like surface area or insulation.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Experiential Learning

⏱ 45 min·Small Groups

Freezing Point Investigation: Salt Solutions

Mix varying salt concentrations in water samples. Students place them in a freezer and check freezing times hourly, recording temperatures. Discuss how salt affects the freezing point using class data.

Prepare & details

Analyze how impurities can affect the melting and freezing points of substances.

Facilitation Tip: For the Freezing Point Investigation, ensure students use identical containers and stir salt solutions gently but consistently to maintain fair temperature readings.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Experiential Learning

⏱ 35 min·Pairs

Temperature Logging: Paraffin Wax Cycle

Heat paraffin wax to melt, log temperature every minute until liquid, then cool and log freezing. Pairs plot graphs to identify plateaus where phase change occurs without temperature rise.

Prepare & details

Predict the state of water at different temperatures.

Facilitation Tip: During the Temperature Logging activity, assign small groups to track wax temperatures every 30 seconds so the class can collaboratively graph the entire freezing cycle.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Experiential Learning

⏱ 25 min·Small Groups

Prediction Stations: Water States

Set up stations with thermometers showing -10°C, 0°C, 10°C, 100°C. Students predict water's state, test with small samples if possible, and justify using particle models.

Prepare & details

Explain the energy changes that occur during melting and freezing.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Teaching This Topic

Teach this topic by anchoring lessons in everyday examples students can relate to, like ice melting or chocolate softening. Avoid introducing the term latent heat too early; instead, let students articulate energy changes through observations before formalizing the concept. Research shows that guided inquiry with structured observations reduces misconceptions about instantaneous phase changes.

What to Expect

Successful learning looks like students accurately predicting phase changes, measuring temperature plateaus during transitions, and explaining how impurities or energy affect melting and freezing points. They should use evidence from activities to challenge initial misconceptions and articulate energy’s role in state changes.

Watch Out for These Misconceptions

Common Misconception

During the Melting Race, watch for students assuming ice melts at the exact moment it reaches 0°C.

What to Teach Instead

Use the Melting Race to demonstrate that melting takes time and temperature remains constant during the phase change; have students graph time versus temperature to see the plateau.

Common Misconception

During the Freezing Point Investigation, watch for students believing impurities do not affect freezing points.

What to Teach Instead

In the Freezing Point Investigation, guide students to compare salted and pure water freezing times, then discuss how salt disrupts ice crystal formation to lower the freezing point.

Common Misconception

During the Temperature Logging activity, watch for students thinking no energy changes occur during melting or freezing.

What to Teach Instead

In the Temperature Logging activity, have students note the steady temperature during wax’s phase change and connect it to energy being used to break or form bonds, not raise temperature.

Assessment Ideas

Exit Ticket

After the Prediction Stations activity, provide students with three scenarios to explain states and energy changes, then collect responses to assess their ability to apply knowledge of phase changes to new contexts.

Quick Check

During the Melting Race, circulate and ask students to hold up a card indicating ‘Melting,’ ‘Freezing,’ or ‘Neither’ when the ice temperature reaches 0°C, then briefly discuss their reasoning to assess understanding of energy transfer.

Discussion Prompt

After the Freezing Point Investigation, pose the question: ‘Why does salt melt ice faster than just waiting for warm air?’ Facilitate a class discussion where students use evidence from their salt solution tests to explain freezing point depression.

Extensions & Scaffolding

• Challenge students who finish early to design a method to slow the Melting Race using household materials, then test and present their results to the class.
• For students who struggle, provide a pre-labeled diagram of the wax temperature graph with blanks for key terms like ‘plateau’ or ‘energy transfer’ to fill in during the Temperature Logging activity.
• Deeper exploration: Ask students to research how antifreeze works in car engines and present a short explanation connecting freezing point depression to real-world applications.

Key Vocabulary

Melting point	The specific temperature at which a solid substance changes into a liquid. For pure water, this is 0°C.
Freezing point	The specific temperature at which a liquid substance changes into a solid. For pure water, this is 0°C.
Phase change	The process where matter changes from one state (solid, liquid, or gas) to another, such as melting or freezing.
Solute	A substance that dissolves in another substance (the solvent) to form a solution. Salt is a solute when dissolved in water.