Freezing and Solidifying

Introduction

Water in an ice-cube tray becomes solid in the freezer, molten metal hardens inside a mould and hot candle wax sets as it drips. Each is an example of freezing , also called solidifying : a liquid turning into a solid. Freezing is the reverse of melting. As a liquid cools, its particles lose energy until the forces of attraction pull them back into fixed positions.

Core explanation

Particles in a liquid. In a liquid, particles are close together but randomly arranged. They move around and slide past each other. Forces of attraction act between them, but the particles have enough energy to keep moving from place to place.

Cooling removes energy. When a liquid is cooled, energy is transferred from its particles to the colder surroundings. The particles slow down and the temperature falls.

Settling into place. At the freezing point, the particles have so little energy that the forces of attraction can hold them in fixed positions. They settle into a regular arrangement and can now only vibrate on the spot. The liquid has become a solid, and it keeps its own shape.

Freezing gives out energy. As particles are pulled into fixed positions by attractions, energy is released to the surroundings. Freezing is therefore an exothermic change. The amount of energy released when a substance freezes equals the amount taken in when it melts.

The temperature stays constant during freezing. While a pure liquid is freezing, the energy released by particles settling into place balances the energy lost to the surroundings. So the temperature stays at the freezing point until all the liquid has become solid. Only then does the temperature fall further.

Freezing point equals melting point. For a pure substance, the freezing point and melting point are the same temperature. Water freezes at 0 °C; molten lead solidifies at about 327 °C.

"Freezing" does not have to mean cold. In everyday speech, freezing suggests low temperatures, but scientists use the word for any liquid turning solid. Molten iron freezes at about 1538 °C — a very hot "freezing" temperature. That is why the word solidifying is often clearer.

Water is unusual. Most substances contract when they freeze, because particles pack more closely. Water expands by about 9% when it freezes, because its molecules form an open, regular arrangement in ice. This is why ice floats and why frozen water pipes can burst.

Step-by-step reasoning

To describe freezing using the particle model:

1. Start with liquid particles close together, moving past each other. 2. Energy is transferred to the surroundings, so the particles slow down. 3. At the freezing point, attractions pull the particles into fixed positions in a regular pattern. 4. The particles now only vibrate: the substance is a solid, and energy has been released.

Visual explanation

Draw a jumbled group of touching circles with curved arrows (liquid). Draw an arrow labelled "energy out — freezing" to a picture of the same circles lined up in neat rows with small wiggle marks (solid). SIM-STATE-001 shows particles gradually locking into a lattice as the temperature slider is lowered.

Real-world analogy

At the end of a busy school break, pupils are milling around the playground. When the bell rings, they slow down and line up in neat rows, staying in their places and only shuffling on the spot. The moving crowd has "frozen" into an ordered arrangement.

Real-world example

Citrus farmers sometimes spray their trees with water on nights when frost is forecast. As the water freezes on the fruit, it releases energy, which helps keep the fruit itself at around 0 °C instead of dropping lower and being damaged. Freezing, surprisingly, is being used to protect crops from cold.

Why?

Why does freezing release energy? Particles are attracted to one another. When they move closer together and settle into fixed positions under these attractions, energy is given out, just as a ball releases energy when it falls to the ground. This energy passes to the surroundings.

Common misconception

"Things freeze only when they are below 0 °C." Only water freezes at 0 °C. Every substance has its own freezing point: ethanol freezes at −114 °C, while molten lead solidifies at about 327 °C.

Worked example

Question: Liquid stearic acid, a pure waxy solid, is cooled and its temperature is recorded every minute. The readings are 80, 75, 72, 69, 69, 69, 69, 64, 60 °C. What is the freezing point, and why does the temperature stay the same for several minutes?

Reasoning: The temperature stops falling at 69 °C and stays there for several readings. This is where the stearic acid is changing from liquid to solid. The energy released as particles settle into fixed positions balances the energy lost to the surroundings.

Answer: 69 °C; the temperature stays constant because energy is released as the stearic acid solidifies.

Quick check

1. Is freezing an endothermic or exothermic change? Answer: Exothermic, because energy is given out to the surroundings.

Exam focus

Describe freezing as particles losing energy, slowing down and being held in fixed positions by forces of attraction. Remember that the freezing point equals the melting point and that the temperature stays constant while freezing. Cooling-curve questions often ask you to read the freezing point from a flat section.

Advanced insight

Very pure water can be cooled several degrees below 0 °C without freezing, because the first tiny crystal needs somewhere to start forming. This is called supercooling. A small disturbance, a speck of dust or a seed crystal can trigger sudden freezing, and the released energy then warms the water back up to 0 °C.

Summary

Freezing, or solidifying, is the change from liquid to solid. As a liquid cools, its particles lose energy and slow down until forces of attraction hold them in fixed positions in a regular arrangement. Freezing releases energy, happens at the freezing point (equal to the melting point) and keeps the temperature constant until all the liquid has become solid.

Practice questions

1. Describe what happens to the arrangement and movement of particles when a liquid freezes. Answer: They go from a random arrangement, sliding past each other, to a regular arrangement, vibrating about fixed positions. 2. The melting point of lead is 327 °C. What is its freezing point? Answer: 327 °C, because the freezing point of a pure substance equals its melting point. 3. Explain why water pipes can burst in very cold weather. Answer: Water expands when it freezes, so the ice takes up more space than the water and pushes on the pipe until it cracks. 4. Explain why the temperature of a pure liquid stays constant while it is freezing. Answer: Energy is released as particles settle into fixed positions, which balances the energy lost to the surroundings, so the temperature does not fall until freezing is complete.