Why Temperature Stays Constant While Melting and Boiling

Introduction

Heat a beaker of crushed ice steadily and watch the thermometer. It sits at 0 °C for minutes while the ice melts, even though the flame is transferring energy the whole time. Later, the boiling water stays at 100 °C no matter how long you heat it. Where is all that energy going? Answering this question takes us to the heart of the particle model: the difference between making particles move faster and pulling them apart.

Core explanation

What temperature measures. Temperature is a measure of the average kinetic energy of the particles — how fast, on average, they are moving or vibrating. The temperature only rises if the particles speed up.

Two jobs for energy. Energy supplied to a substance can do one of two jobs:

1. Increase kinetic energy — the particles move faster, so the temperature rises. This happens in the sloping sections of a heating curve. 2. Overcome attractive forces — the particles are pulled further apart, weakening or breaking the attractions between them. The particles do not speed up, so the temperature does not rise. This happens during a change of state.

During melting. At the melting point, the energy supplied is used to loosen particles from their fixed positions in the solid, overcoming some of the attractions. Until every particle has been freed, the energy continues to go into this job, so the temperature stays at the melting point. Solid and liquid are present together.

During boiling. At the boiling point, the energy is used to separate particles completely, overcoming almost all the remaining attractions so they can escape as a gas. The temperature stays at the boiling point until all the liquid has turned to gas.

Stored energy. The energy used to separate particles is not lost. It is stored in the new arrangement, rather like energy stored by stretching a spring. It is released again when the gas condenses or the liquid freezes.

Strength of attractions. Substances with stronger attractions between particles need more energy to change state. They have longer plateaus on a heating curve at the same heating rate, and usually higher melting and boiling points.

Step-by-step reasoning

To explain a flat section of a heating curve:

1. Energy is still being supplied at a steady rate. 2. The substance is changing state. 3. The energy is used to overcome attractive forces between particles. 4. The average kinetic energy of the particles does not increase. 5. So the temperature stays constant until the change is complete.

Visual explanation

Picture two energy meters for the particles, one labelled "speed" and one labelled "separation". On a sloping section, only the speed meter rises. On a flat section, the speed meter stays still while the separation meter fills up. When the separation meter is full, the change of state is complete and the speed meter starts rising again.

Real-world analogy

Imagine saving money for a holiday. Your spending money (temperature) goes up each week. Then you decide to pay off a debt first: your income keeps arriving, but it all goes on the debt, so your spending money stays the same. Once the debt is cleared, your spending money starts rising again. The debt is like the attractions that must be overcome.

Real-world example

A drink with plenty of ice stays close to 0 °C on a hot day for as long as ice remains. Energy flowing in from the warm air is used to melt the ice rather than warm the drink. Only when all the ice has melted does the drink's temperature begin to rise.

Why?

Why does separating particles not raise the temperature? Temperature depends on how fast particles move, not on how far apart they are. During a change of state, the energy changes the particles' positions relative to one another, increasing their separation, but their average speed stays the same.

Common misconception

"When the temperature stops rising, the heating has stopped working." The heater is still transferring energy. The energy has simply switched from speeding up particles to separating them.

Worked example

Question: Substance X and substance Y are heated at the same rate. X's melting plateau lasts 2 minutes and Y's lasts 5 minutes for the same mass. Which has stronger attractions between its particles?

Reasoning: A longer plateau at the same heating rate means more energy is needed to overcome the attractions during melting.

Answer: Substance Y has stronger attractions between its particles.

Quick check

1. During boiling, is the energy supplied used to speed up particles or to separate them? Answer: To separate them, by overcoming the attractive forces between them.

Exam focus

A strong answer has three parts: energy is still supplied; it is used to overcome forces of attraction between particles; so the kinetic energy of the particles, and hence the temperature, does not increase. Avoid saying that bonds inside molecules break — in water, the molecules stay intact during boiling.

Advanced insight

In a molecular substance such as water, boiling overcomes the relatively weak forces between molecules, not the strong covalent bonds within each molecule. That is why steam is still made of H₂O molecules. Breaking the bonds inside water molecules would need far higher temperatures, above 2000 °C, and would be a chemical change rather than a change of state.

Summary

Temperature measures the average kinetic energy of particles. During melting and boiling, energy supplied is used to overcome attractive forces between particles rather than to make them move faster, so the temperature stays constant until the change of state is complete. This energy is stored in the new arrangement and released when the change is reversed. Stronger attractions mean more energy is needed.

Practice questions

1. What does temperature measure in terms of particles? Answer: The average kinetic energy of the particles. 2. Explain why the temperature of melting ice stays at 0 °C while it is being heated. Answer: The energy supplied is used to overcome the attractions holding the particles in the solid, not to increase their kinetic energy, so the temperature does not rise. 3. What happens to the energy used to separate particles during boiling? Answer: It is stored in the gas and released again when the gas condenses. 4. Does boiling water break the bonds within water molecules? Explain. Answer: No; it only overcomes the weaker attractions between molecules, so the molecules stay as H₂O.