Changes of State: An Overview

Introduction

An ice cube melts in a drink, a puddle dries up in the sun, a bathroom mirror mists over and frost appears on a car windscreen on a cold morning. Each of these is a change of state : a substance changing between solid, liquid and gas. There are six changes in all, and each one can be understood with the particle model. This page gives an overview; later pages look at each change in detail.

Core explanation

The three states and six changes. Matter commonly exists as a solid, a liquid or a gas. Each pair of states is linked by two changes, one in each direction:

Change From To Energy --- --- --- --- Melting solid liquid taken in Freezing liquid solid given out Boiling or evaporating liquid gas taken in Condensing gas liquid given out Sublimation solid gas taken in Deposition gas solid given out

Changes that take in energy. Melting, boiling, evaporating and subliming all need energy to be supplied, usually by heating. The energy is used to overcome the forces of attraction between particles, so the particles can move more freely and further apart.

Changes that give out energy. Freezing, condensing and deposition release energy to the surroundings. As the particles come closer together, forces of attraction pull them into a more ordered arrangement and energy is transferred away, usually as heat.

Particle picture for each change.

- Melting: particles vibrating in fixed positions gain enough energy to break out of the regular arrangement and slide past each other. - Freezing: liquid particles lose energy and settle back into fixed positions. - Boiling and evaporating: liquid particles gain enough energy to escape from their neighbours and move far apart as a gas. Evaporation happens only at the surface and at any temperature; boiling happens throughout the liquid at the boiling point. - Condensing: gas particles lose energy, slow down and are attracted back together into a liquid. - Sublimation: particles leave the solid directly as a gas, without first forming a liquid. - Deposition: gas particles attach directly to a cold surface as a solid.

Examples with water. Ice melts at 0 °C; water freezes at 0 °C; water boils at 100 °C at normal air pressure; steam condenses on cool surfaces; frost forms by deposition; snow can slowly disappear by sublimation on cold, dry, sunny days.

Reversible changes. Every change of state can be reversed by adding or removing energy. The substance itself does not change into a new substance.

Step-by-step reasoning

To name a change of state:

1. Identify the starting state. 2. Identify the final state. 3. Choose the change that connects them from the table. 4. Decide whether energy is taken in (particles moving apart) or given out (particles coming together).

Visual explanation

Draw a triangle with "solid", "liquid" and "gas" at the corners. Draw two arrows along each side, one in each direction, and label them: melting and freezing between solid and liquid, boiling and condensing between liquid and gas, sublimation and deposition between solid and gas. Colour the arrows that go "up" in energy red and those that go "down" blue. SIM-STATE-001 lets you move between the states by changing temperature.

Real-world analogy

Think of a dance floor. At the start of the evening everyone stands still in neat rows (solid). As the music gets livelier, people start moving around and weaving between one another (liquid). At full energy, some dancers leave the hall entirely and scatter outside (gas). When the music calms down, the reverse happens.

Real-world example

A cold drink taken from the fridge on a warm day shows three changes at once. The ice inside melts, water vapour in the air condenses into droplets on the outside of the glass, and later, some of those droplets evaporate back into the air as the glass warms up.

Why?

Why do some changes need heating while others release heat? Particles in a solid are held tightly by attractions; in a gas they are almost free. Moving particles apart against these attractions needs energy. Letting particles come together releases energy, just as a stretched spring releases energy when it snaps back.

Common misconception

"When ice melts, the water particles themselves melt." Particles do not melt, freeze or boil. They stay the same; only their arrangement, spacing and movement change.

Worked example

Question: Name the change of state and say whether energy is taken in or given out: (a) wax dripping from a candle hardens on the table; (b) solid air fresheners slowly shrink without leaving a puddle.

Reasoning: (a) Liquid wax becomes solid, so this is freezing (solidifying), and energy is given out to the table and air. (b) The solid turns directly into a gas, so this is sublimation, and energy is taken in from the surroundings.

Answer: (a) Freezing, energy given out; (b) sublimation, energy taken in.

Quick check

1. What is the name for the change from gas to liquid? Answer: Condensing (condensation).

Exam focus

Learn all six names and the direction of each. Be ready to say which changes need energy and which release it. Distinguish evaporation from boiling, and remember that sublimation goes straight from solid to gas.

Advanced insight

Whether a substance melts or sublimes depends on pressure. Carbon dioxide cannot exist as a liquid at normal air pressure, so solid carbon dioxide (dry ice) sublimes at about −78 °C. At more than about five times normal atmospheric pressure, liquid carbon dioxide can form, which is how it is stored in fire extinguishers.

Summary

There are six changes of state: melting, freezing, boiling or evaporating, condensing, sublimation and deposition. Changes that separate particles take in energy; changes that bring particles together give out energy. Particles themselves do not change — only their arrangement and movement. All changes of state are reversible.

Practice questions

1. Name the change of state when frost forms on a car windscreen. Answer: Deposition, because water vapour turns directly into solid ice. 2. Which of these take in energy: melting, condensing, boiling, freezing? Answer: Melting and boiling. 3. Describe what happens to the particles when a liquid freezes. Answer: They lose energy, slow down and settle into fixed positions in a regular arrangement, vibrating on the spot. 4. Give one difference between evaporation and boiling. Answer: Evaporation happens only at the surface and at any temperature; boiling happens throughout the liquid at the boiling point.