Chemistry · 9th Grade

Active learning ideas

Heating Curves and Phase Diagrams

Active learning works well for heating curves and phase diagrams because these topics rely on students interpreting visual information and connecting macroscopic observations to particle-level behavior. Students need repeated practice reading graphs and diagrams to build confidence and precision in their explanations.

• Common Core State Standards
• HS-PS1-3
• HS-PS3-2

• 20–40 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Graffiti Wall
• 25 min · Pairs

Annotating: Heating Curve Walk-Through

Give students a blank heating curve for water and a set of labeled events (melting, vaporization, temperature rise in liquid phase, etc.). They place the labels on the correct segments and write a two-sentence explanation for each region. Partners check each other's placements before a class review.

Explain why the temperature remains constant during a phase change on a heating curve.

Facilitation Tip

For the Heating Curve Walk-Through, have students label each segment with the phase present and the type of energy change occurring at both the macro and particle levels.

What to look for

Provide students with a heating curve for an unknown substance. Ask them to: 1. Identify the melting point and boiling point. 2. Calculate the energy required to raise the temperature of 10g of the substance from 20°C to 120°C, given specific heat values for each phase and the enthalpy of vaporization.

• Remember
• Understand
• Create
• Self-Awareness
• Relationship Skills

Activity 02

• Think-Pair-Share
• 20 min · Pairs

Think-Pair-Share: Pressure and Phase Behavior

Display the phase diagram for CO2 and ask students why CO2 cannot exist as a liquid at normal atmospheric pressure. Students reason independently, share with a partner, and then the class discusses why dry ice sublimes rather than melts under ambient conditions.

Interpret a phase diagram to identify triple points, critical points, and phase boundaries.

Facilitation Tip

During the Pressure and Phase Behavior Think-Pair-Share, provide real-world scenarios such as cooking at high altitude to ground the discussion in students' experiences.

What to look for

Present students with a phase diagram for water. Ask them to discuss in small groups: 'Imagine you have a sample of ice at -10°C and 1 atm. Describe the changes that occur as you slowly increase the temperature to 110°C at constant pressure. What happens at the triple point and critical point for water?'

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

Activity 03

• Gallery Walk
• 40 min · Small Groups

Gallery Walk: Comparing Phase Diagrams

Post phase diagrams for water, CO2, and one additional substance (e.g., sulfur). Students rotate with recording sheets, identifying the triple point, critical point, and slope of the solid-liquid line for each. Groups discuss the physical significance of the differences they find.

Predict the state of matter of a substance at different temperatures and pressures using a phase diagram.

Facilitation Tip

In the Gallery Walk: Comparing Phase Diagrams, assign each group a different substance so students see how triple points and critical points vary across materials.

What to look for

Give each student a phase diagram for CO2. Ask them to: 1. Locate the triple point and critical point on the diagram. 2. Predict the state of CO2 at 1 atm and -80°C, and explain their reasoning using the diagram.

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

Activity 04

• Collaborative Problem-Solving
• 30 min · Small Groups

Collaborative Problem-Solving: Phase Diagram Navigation

Give student groups a scenario such as starting at high pressure and low temperature and slowly decreasing pressure at constant temperature. Groups trace the path on a phase diagram, predict what phase changes occur, and present their reasoning to another group for peer review.

Explain why the temperature remains constant during a phase change on a heating curve.

Facilitation Tip

For Phase Diagram Navigation, require students to draw particle diagrams for each region to reinforce the connection between phase and particle arrangement.

What to look for

Provide students with a heating curve for an unknown substance. Ask them to: 1. Identify the melting point and boiling point. 2. Calculate the energy required to raise the temperature of 10g of the substance from 20°C to 120°C, given specific heat values for each phase and the enthalpy of vaporization.

• Apply
• Analyze
• Evaluate
• Create
• Relationship Skills
• Decision-Making
• Self-Management

A few notes on teaching this unit

Teachers often find that students grasp heating curves more quickly when they start with a familiar substance like water, then generalize to other materials. Avoid rushing past the flat regions on heating curves, as these are where students' misconceptions about energy and phase changes typically emerge. Research shows that having students annotate graphs themselves, rather than passively observing, deepens understanding of the underlying concepts.

Successful learning looks like students accurately interpreting flat regions on heating curves as energy used for phase changes, not temperature changes. They should also correctly identify phase boundaries and explain how pressure and temperature interact using phase diagrams.

---

Watch Out for These Misconceptions

• During the Annotating: Heating Curve Walk-Through activity, watch for students labeling flat regions as 'no energy added' or 'heating stopped'.

  Use the walk-through to explicitly connect flat regions to particle-level explanations. Ask students to describe what is happening to intermolecular forces and potential energy during these segments, referencing the energy input from the heating source.

• During the Think-Pair-Share: Pressure and Phase Behavior activity, watch for students describing the triple point as a 'mixture' of solid, liquid, and gas.

  Use the scenario cards in this activity to clarify that the triple point is a state of dynamic equilibrium, not a mixture. Have students draw particle diagrams to show the balance between phase transitions at the triple point.

• During the Problem-Solving: Phase Diagram Navigation activity, watch for students assuming that temperature alone determines the phase.

  Use the phase diagram navigation worksheet to reinforce that both temperature and pressure must be considered. Ask students to trace paths on the diagram that show how changes in both variables lead to different phases.

---

Methods used in this brief

• Graffiti Wall
• Think-Pair-Share
• Gallery Walk
• Collaborative Problem-Solving