Melting and Freezing: Process and Heating/Cooling Curves

TL;DR

Melting is when a solid turns into a liquid, and freezing is the opposite, both happening at a specific temperature called the melting/freezing point. During these phase changes, the temperature stays constant even if you're adding or removing heat because the energy is used to break or form intermolecular bonds. Heating and cooling curves visually show these temperature plateaus, illustrating the energy absorption or release during phase transitions.

1. The Mental Model

Think of melting and freezing like changing gears in a car: you need to add energy to shift up (melt) or remove it to shift down (freeze). During the actual gear change, the car's speed (temperature) doesn't increase, even though the engine (heat) is working.

2. The Core Material

When you heat a solid, its particles vibrate faster. Eventually, they gain enough kinetic energy to overcome the forces holding them in a fixed structure, and the solid starts to melt into a liquid. This happens at a specific temperature called the melting point. While the substance is melting, all the added heat energy is used to break these intermolecular bonds, not to increase the kinetic energy of the particles. That's why the temperature doesn't rise during melting, creating a plateau on a heating curve.

Conversely, freezing is the reverse process: a liquid turns into a solid. As you cool a liquid, its particles lose kinetic energy, move slower, and eventually settle into an ordered solid structure. This occurs at the freezing point, which is the same temperature as the melting point for a pure substance. During freezing, the substance releases energy (called the latent heat of fusion) as new bonds form, and again, the temperature stays constant until all the liquid has solidified. This also creates a plateau, but on a cooling curve.

Heating and Cooling Curves

Photo by Mesayu Elida Irawati on Pexels

These curves are graphs that show how temperature changes over time as a substance is heated or cooled at a constant rate. They clearly illustrate the phase changes.

• Heating Curve: Starts with a solid, temperature rises, then plateaus at the melting point (solid + liquid coexisting), then temperature rises again as it becomes a liquid. If heating continues, it will eventually plateau again at the boiling point (liquid + gas).
• Cooling Curve: Starts with a liquid, temperature drops, then plateaus at the freezing point (liquid + solid coexisting), then temperature drops again as it becomes a solid.

The length of the plateau on these curves indicates the amount of energy required for the phase change (latent heat). A longer plateau means more energy is needed.

graph TD
    A["Solid (Low Temp)"] --> B["Solid Heating (Temp Rises)"]
    B --> C["Melting Point Plateau (Solid + Liquid)"]
    C --> D["Liquid Heating (Temp Rises)"]
    D --> E["Boiling Point Plateau (Liquid + Gas)"]
    E --> F["Gas Heating (Temp Rises)"]

3. Worked Example

Let's consider a heating curve for ice starting at -10°C, melting into water, and then heating the water to 20°C.

1. -10°C to 0°C (Ice): As you add heat, the temperature of the ice increases steadily from -10°C to 0°C. The added energy increases the kinetic energy of the water molecules in the solid state.
2. 0°C (Melting Plateau): At 0°C, the ice starts to melt. Even though you're still adding heat, the temperature stays at 0°C. This energy is being used to break the hydrogen bonds holding the water molecules in their rigid crystal structure, allowing them to move more freely as a liquid. You'll see both ice and liquid water present. This part of the curve will be flat.
3. 0°C to 20°C (Water): Once all the ice has melted, the temperature of the liquid water will start to rise again from 0°C to 20°C as more heat is added. The added energy now increases the kinetic energy of the water molecules in the liquid state.

If you were to graph this, you'd see an upward slope, then a flat line at 0°C, then another upward slope.

4. Key Takeaways

• Melting and freezing are opposite phase changes that occur at the same temperature for a pure substance.
• During a phase change, the temperature of the substance remains constant despite continuous heating or cooling.
• The energy added or removed during a phase change is called latent heat (e.g., latent heat of fusion for melting/freezing).
• This latent heat is used to break or form intermolecular bonds, not to increase particle kinetic energy.
• Heating and cooling curves visually represent these phase changes and temperature plateaus.
• The length of a plateau on a heating/cooling curve is related to the amount of latent heat involved.
• Pure substances have distinct, sharp melting/freezing points, unlike mixtures.

5. Now Try It

Draw a cooling curve for steam starting at 110°C, condensing to liquid water, and then freezing to ice at -5°C. Label the axes (Temperature on y-axis, Heat Removed/Time on x-axis), each distinct region (e.g., "Liquid cooling," "Freezing Plateau"), and the relevant temperatures (100°C and 0°C). What would happen to the shape of the curve if the steam had impurities?