Why does ice stay at 0 degrees Celsius while it melts, even when you keep heating it?

All the thermal energy you add is breaking the bonds holding ice particles in a solid structure. Those particles are not speeding up, so temperature—which measures how fast particles move—does not increase. Only after all the ice has melted and become liquid water will additional thermal energy start making particles move faster, raising the temperature.

Is the melting point the same as the freezing point?

Yes. For any pure substance, the melting point and freezing point are the same temperature. At that temperature, a solid becomes a liquid when thermal energy is added, or a liquid becomes a solid when thermal energy is removed. For water, this is 0 degrees Celsius at sea level.

What is the difference between evaporation and boiling?

Evaporation happens when particles at the surface of a liquid gain thermal energy and escape into the gas phase. It can occur at any temperature and is relatively slow. Boiling happens at a specific temperature (the boiling point) when thermal energy is supplied to the entire liquid and particles throughout escape rapidly in bubbles. For water, boiling occurs at 100 degrees Celsius at sea level.

Why does ice melt faster than frozen milk melts?

Different pure substances have different melting points. Ice melts at 0 degrees Celsius, but frozen milk has a lower melting point and begins to melt at a warmer temperature than ice. However, if both are placed in the same warm environment, the time each takes to melt also depends on how much thermal energy it receives, its mass, and how its particles are arranged—not just the melting point itself.

M8SCI-6.2

Changes of State & Thermal Energy

Learn how thermal energy causes changes of state—melting, freezing, evaporation, boiling, and condensation—and why temperature stays constant during these transitions.

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A voice-first session with the Crimsora tutor on Changes of State & Thermal Energy, then targeted practice and FRQs — with the tutor adapting to where you get stuck.

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What this lesson covers

When you heat ice, it melts into water. When you boil water, it turns into steam. These changes happen because thermal energy affects how particles move and interact with each other. In this lesson, you'll discover why temperature stays the same during a change of state, even though energy is being added. Understanding these processes helps explain everyday observations—from frost forming on windows to puddles disappearing on a sunny day.

What Happens to Particles During a Change of State

Every substance made of particles that attract each other. When thermal energy is added or removed, those particles gain or lose the ability to move around freely. In a solid, particles vibrate in fixed positions. In a liquid, particles move around but stay close together. In a gas, particles move fast and spread far apart. During a change of state, energy goes into breaking apart the attractions holding particles in their current arrangement—not into making them move faster. This is why temperature stays constant while melting, freezing, boiling, condensation, and sublimation occur. Once a change of state finishes, any additional thermal energy will speed up the particles again, raising the temperature. Think of it like unlocking a door: the energy you use to turn the key (break attractions) doesn't make people walk faster once they're through the door; it just allows them to move freely.

Melting and Freezing

Melting happens when a solid gains thermal energy and its particles vibrate so vigorously that they break free from their fixed positions and can slide past each other—the solid becomes a liquid. The temperature at which this occurs is called the melting point. During melting, temperature stays constant even though thermal energy is being added, because that energy is breaking particle bonds, not speeding up particle motion. Freezing is the reverse: a liquid loses thermal energy, particles slow down and lock into fixed positions, and the liquid becomes a solid. The freezing point is the temperature at which this happens. For pure water, melting and freezing both occur at 0 degrees Celsius at sea level. An important fact: the melting point and freezing point of a pure substance are the same temperature. If you add thermal energy to ice at 0 degrees Celsius, it melts while staying at 0 degrees Celsius. If you remove thermal energy from water at 0 degrees Celsius, it freezes while staying at 0 degrees Celsius. Different substances have different melting and freezing points, which is one way scientists identify unknown materials.

Evaporation, Boiling, and Condensation

Evaporation occurs when a liquid gains thermal energy and some particles at the surface escape into the gas phase. This can happen at any temperature below the boiling point. It feels slow because only surface particles are leaving. Boiling is much faster: it happens when a liquid has gained enough thermal energy that particles throughout the entire liquid escape at once, forming bubbles of gas that rise to the surface. The boiling point is the specific temperature at which boiling occurs. For pure water, the boiling point is 100 degrees Celsius at sea level. Like melting, the temperature stays constant during boiling because the thermal energy is breaking the attractions holding particles in the liquid state, not speeding them up. Condensation is the reverse of evaporation and boiling: a gas loses thermal energy, particles slow down, and the gas becomes a liquid. Water vapor in warm air condenses into liquid water droplets when air cools, which is why dew forms on grass at night and mirrors fog up in a warm bathroom. The condensation point of a pure substance equals its boiling point—just as the freezing point equals the melting point. Understanding these processes explains why clothes dry faster on a hot, dry day (more evaporation) and why steam burns you worse than hot water (condensation releases large amounts of thermal energy).

Why Temperature Stays Constant During State Changes

This is the trickiest idea, so it's worth understanding deeply. Temperature measures how fast particles are moving. During a change of state, particles are not speeding up—instead, the bonds and attractions holding them in one arrangement are breaking apart so they can move into a different arrangement. All the thermal energy added during melting goes into breaking solid bonds. All the thermal energy added during boiling goes into breaking liquid bonds and allowing particles to escape into the gas phase. Until those bonds are completely broken and all particles have entered the new state, particle speed—and therefore temperature—does not increase. Once the change of state is complete, further thermal energy input speeds up the particles, and temperature rises again. You can see this in a heating curve: the graph stays flat (horizontal) during melting, boiling, and other state changes, then rises steeply once the change finishes. The amount of thermal energy required to complete a change of state without changing temperature is called latent heat. It varies by substance and by which change of state is occurring. Water requires a lot of latent heat for boiling, which is why steam is such an effective heat source and why boiling water stays at 100 degrees Celsius for a long time.

Identifying State Changes in Real Situations

You encounter changes of state constantly. Frost forms on winter windows through deposition or condensation—water vapor in air loses thermal energy and becomes ice directly, or becomes liquid water that then freezes. A wet sidewalk dries because liquid water evaporates into water vapor. Ice cream melts on a warm day because it gains thermal energy. These processes are reversible: the same substance can melt, then freeze; evaporate, then condense. The key skill is recognizing which change of state is occurring and predicting what happens to particle motion, temperature, and state. A substance undergoing a change of state will show constant temperature even though thermal energy is being added or removed—that's your signal that bonds are breaking or forming, not that particles are speeding up. If you know the identity of a pure substance, you can predict its melting point, boiling point, and the direction of state change based on whether thermal energy is being added or removed.

Key terms

Melting.: The change of state from solid to liquid that occurs when a substance gains thermal energy and particles break free from fixed positions.
Boiling.: The rapid change of state from liquid to gas that occurs at a specific temperature when thermal energy is applied throughout the liquid.
Condensation.: The change of state from gas to liquid that occurs when a substance loses thermal energy and particles slow down enough to form a liquid.
Freezing.: The change of state from liquid to solid that occurs when a substance loses thermal energy and particles lock into fixed positions.
Latent heat.: The thermal energy required to cause a complete change of state without changing the temperature of the substance.
Melting point.: The specific temperature at which a pure solid begins to melt into a liquid; the same as the freezing point for that substance.
Boiling point.: The specific temperature at which a pure liquid boils into a gas; the same as the condensation point for that substance.
Thermal energy.: The total energy of moving particles in a substance; heat is the transfer of thermal energy from one object to another.

Worked example

An ice cube at negative 5 degrees Celsius is placed in a beaker and heated steadily. Sketch or describe what happens to the temperature and state of the ice as thermal energy is added over time. Explain why the temperature stops changing at 0 degrees Celsius even though heat is still being applied.

Start by identifying what state the ice begins in: solid, at negative 5 degrees Celsius. As thermal energy is added, the solid ice particles vibrate faster and the temperature rises toward 0 degrees Celsius. Once the temperature reaches 0 degrees Celsius—the melting point of ice—something interesting happens: even though you keep adding thermal energy, the temperature stops rising and stays at 0 degrees Celsius. This is the melting process. Why? All the thermal energy being added is now going into breaking the bonds that hold ice particles in a rigid crystal structure. The particles are not speeding up; they are breaking free from their fixed positions. This continues until all the ice has melted and become liquid water. Only after all the ice has melted—when every particle has made the transition to liquid—does the temperature begin to rise again. The liquid water then warms from 0 degrees Celsius toward higher temperatures. If you continued heating, the temperature would eventually reach 100 degrees Celsius, where the same flat-line behavior would happen again during boiling: thermal energy would break the bonds holding water particles in the liquid state, the temperature would stay at 100 degrees Celsius, and this would continue until all the liquid had evaporated into steam. The key insight: during a change of state, thermal energy goes into rearranging particles, not into speeding them up, so temperature remains constant.

Practice questions

A student observes water boiling at 100 degrees Celsius. She keeps the burner on high heat and the water continues to boil. What happens to the temperature of the boiling water as more thermal energy is added?

The temperature increases above 100 degrees Celsius.
The temperature stays at 100 degrees Celsius.
The temperature decreases below 100 degrees Celsius.
The temperature fluctuates between 100 and 110 degrees Celsius.

Answer: The temperature stays at 100 degrees Celsius.

While a pure substance is boiling—changing from liquid to gas—its temperature remains constant at the boiling point, even though thermal energy is being added. The thermal energy goes into breaking the bonds holding particles in the liquid state and allowing them to escape as gas, not into speeding up the particles. Once all the liquid has boiled away and the substance is entirely gas, further thermal energy will increase the temperature of the gas.

Explain why ice at 0 degrees Celsius can absorb thermal energy and melt without its temperature changing. Use the particle model in your answer.

Answer: The thermal energy being added is used to break the bonds that hold ice particles in fixed positions. As long as ice is melting, all the energy goes into separating particles from their rigid solid structure and allowing them to move freely like liquid particles. The particles are not speeding up, so their average kinetic energy—which determines temperature—does not increase. Temperature only rises again once all the ice has melted and the liquid water particles can move more freely.

This question tests whether students understand that during a change of state, thermal energy does two different jobs depending on whether particles are changing state or just moving faster. If they answer that the ice absorbs energy to break bonds while remaining at 0 degrees Celsius, they show they understand latent heat and the difference between thermal energy and temperature. A common wrong answer is to say the temperature should increase, which shows confusion between 'adding heat' and 'increasing temperature.' During a state change, they are not the same thing.

A puddle of water on the sidewalk disappears on a sunny day, but the air temperature is only 20 degrees Celsius—well below water's boiling point of 100 degrees Celsius. Explain how the liquid water becomes water vapor without boiling.

Answer: The water evaporates. Evaporation is a change of state in which particles at the surface of a liquid gain enough thermal energy from the sun to escape into the gas phase, even though the bulk liquid is below the boiling point. Only surface particles need enough energy to leave; they do not have to overcome the attractions holding all particles in the liquid state. Over time, enough surface particles escape that the entire puddle disappears, turning into water vapor in the air.

This question connects the particle model to a real-world observation and distinguishes evaporation from boiling. Evaporation is slower and occurs at the surface; boiling is rapid and occurs throughout the liquid. Students should recognize that thermal energy from sunlight gives some surface particles enough energy to escape without raising the temperature of the remaining liquid to the boiling point. This also previews why evaporation is a cooling process for the remaining liquid—the fastest particles escape, leaving slower ones behind.

FAQ

Why does ice stay at 0 degrees Celsius while it melts, even when you keep heating it?: All the thermal energy you add is breaking the bonds holding ice particles in a solid structure. Those particles are not speeding up, so temperature—which measures how fast particles move—does not increase. Only after all the ice has melted and become liquid water will additional thermal energy start making particles move faster, raising the temperature.
Is the melting point the same as the freezing point?: Yes. For any pure substance, the melting point and freezing point are the same temperature. At that temperature, a solid becomes a liquid when thermal energy is added, or a liquid becomes a solid when thermal energy is removed. For water, this is 0 degrees Celsius at sea level.
What is the difference between evaporation and boiling?: Evaporation happens when particles at the surface of a liquid gain thermal energy and escape into the gas phase. It can occur at any temperature and is relatively slow. Boiling happens at a specific temperature (the boiling point) when thermal energy is supplied to the entire liquid and particles throughout escape rapidly in bubbles. For water, boiling occurs at 100 degrees Celsius at sea level.
Why does ice melt faster than frozen milk melts?: Different pure substances have different melting points. Ice melts at 0 degrees Celsius, but frozen milk has a lower melting point and begins to melt at a warmer temperature than ice. However, if both are placed in the same warm environment, the time each takes to melt also depends on how much thermal energy it receives, its mass, and how its particles are arranged—not just the melting point itself.

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