Phase Changes: Melting and Freezing

Active learning works well for melting and freezing because students can see energy transfer firsthand. Watching ice turn to water or wax solidify makes abstract particle movement visible, which builds stronger mental models than lectures alone. The hands-on nature of these activities helps students connect temperature changes to real phase shifts in a way that stickers or videos cannot.

• 4th Class
• Exploring Our World: Scientific Inquiry and Discovery
• 4 activities
• 25 min–45 min

Learning Objectives

1. Identify the melting point and freezing point of water by observing temperature changes during phase transitions.
2. Compare the melting points of different solid materials, such as chocolate, butter, and wax, using experimental data.
3. Explain the process of melting and freezing at a particle level, describing the arrangement and movement of particles in solid and liquid states.
4. Predict the effect of adding impurities, like salt, on the freezing point of water based on experimental observations.

Stations Rotation

⏱ 45 min·Small Groups

Stations Rotation: Melting Points Comparison

Prepare stations with ice, butter, and chocolate in beakers over warm water. Groups measure and record temperature every minute until melting completes, noting plateaus. Rotate stations, then graph class data on shared chart paper.

Prepare & details

Explain the process of melting and freezing at a particle level.

Facilitation Tip: During Station Rotation: Melting Points Comparison, circulate with a timer and ensure each group records the exact moment their substance begins to melt or freeze.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Experiential Learning

⏱ 30 min·Pairs

Pairs Experiment: Salt on Ice Freezing

Pairs place ice cubes in bowls, measure initial temperature, sprinkle salt, and record changes every 2 minutes for 20 minutes. Discuss why temperature drops below 0°C and predict effects on roads in winter.

Prepare & details

Compare the melting points of different solid materials.

Facilitation Tip: For Pairs Experiment: Salt on Ice Freezing, ask pairs to switch roles halfway so both students practice measuring and observing carefully.

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·Whole Class

Whole Class: Cooling Curve Graphing

Melt paraffin wax in a shared water bath, then cool while class logs temperature every 30 seconds on a large graph. Identify freezing plateau through teacher-led questions and student annotations.

Prepare & details

Predict how impurities might affect a substance's freezing point.

Facilitation Tip: When Whole Class: Cooling Curve Graphing, provide graph paper with pre-labeled axes to save time and focus on data interpretation.

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·Individual

Individual Prediction: Impurity Effects

Students predict and sketch temperature curves for pure water versus saltwater freezing. Test predictions in pairs using ice-salt mixtures, then share graphs in plenary.

Prepare & details

Explain the process of melting and freezing at a particle level.

Facilitation Tip: For Individual Prediction: Impurity Effects, have students sketch their predictions first before moving to the lab station to reduce impulsive guesses.

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

Start by demonstrating a simple melting or freezing activity yourself so students see what to watch for. Use guided questions to prompt observations rather than giving answers, which encourages critical thinking. Avoid rushing through the plateau phase, as this is where most misconceptions take root. Research shows that students need multiple exposures to phase change graphs before the flat line makes sense, so revisit these visuals over several lessons.

What to Expect

By the end of these activities, students will correctly identify melting and freezing points on graphs and explain why temperature plateaus during phase changes. They will use evidence from experiments to challenge misconceptions and predict outcomes when variables like impurities are introduced. Clear discussions and written reflections will show their understanding of particle behavior during these transitions.

Watch Out for These Misconceptions

Common Misconception

During Station Rotation: Melting Points Comparison, watch for students who assume the temperature keeps rising even after the substance starts melting.

What to Teach Instead

Have students trace the flat section of their graphs with a highlighter and annotate it with an arrow labeled 'Energy used to break bonds, not raise temperature.' Ask them to explain this to their group before moving on.

Common Misconception

During Pairs Experiment: Salt on Ice Freezing, watch for students who believe the freezing point is higher when salt is added.

What to Teach Instead

Prompt pairs to compare their thermometer readings to the control ice before adding salt, then ask them to predict the new freezing point based on their data. Discuss why the lowest temperature reached is below 0 degrees Celsius.

Common Misconception

During Individual Prediction: Impurity Effects, watch for students who think impurities raise freezing points.

What to Teach Instead

Ask students to refer to their salt-ice data and write a one-sentence explanation comparing pure and impure freezing points. Circulate to correct misconceptions immediately with their own evidence.

Assessment Ideas

Exit Ticket

After Station Rotation: Melting Points Comparison, hand each student a blank graph template and ask them to plot temperature data for a substance, label the melting point, and write a sentence explaining what happens to the particles at that temperature.

Quick Check

During Whole Class: Cooling Curve Graphing, pause when most groups reach the freezing plateau and ask students to hold up their thermometers. Pose the question, 'Is the substance melting, freezing, or neither? Justify your answer using the graph and temperature.'

Discussion Prompt

After Pairs Experiment: Salt on Ice Freezing, gather the class and ask, 'Why did the salted ice reach a lower temperature than the plain ice? How does this relate to making homemade ice cream?' Have students discuss in small groups before sharing with the class.

Extensions & Scaffolding

• Challenge early finishers to design a new experiment testing how varying amounts of salt affect freezing time, then share results with the class.
• For students who struggle, provide a partially completed graph with some data points filled in to help them see the plateau pattern.
• Deeper exploration: Have students research why antifreeze works in car engines by connecting it to the concept of freezing point depression demonstrated in the salt-ice activity.

Key Vocabulary

Melting Point	The specific temperature at which a solid substance changes into a liquid when heated.
Freezing Point	The specific temperature at which a liquid substance changes into a solid when cooled. For pure substances, this is the same as the melting point.
Phase Change	The process where a substance changes from one state (solid, liquid, or gas) to another, such as melting or freezing.
Thermal Energy	The energy associated with the random motion of atoms and molecules in a substance, often referred to as heat.