Changes of State as Physical Changes

Introduction

On a cold morning, water may exist as ice on a windscreen, liquid in a puddle and vapour in the air, all at the same time. Changing between these states is one of the most common kinds of change in nature. Every one of them is a physical change: the particles stay the same and only their arrangement and movement change. The particle model explains exactly why.

Core explanation

The six changes of state.

From To Name --- --- --- Solid Liquid Melting Liquid Solid Freezing (solidifying) Liquid Gas Boiling or evaporating Gas Liquid Condensing Solid Gas Sublimation Gas Solid Deposition

Particles in each state. In a solid , particles are packed closely in a fixed, regular pattern and only vibrate. In a liquid , particles are still close together but can slide past one another. In a gas , particles are far apart and move quickly in all directions.

Melting. When a solid is heated, its particles vibrate more strongly. At the melting point they have enough energy to overcome some of the forces holding them in place, and they begin to slide past each other. For water, this happens at 0 °C.

Boiling and evaporating. Heating a liquid makes its particles move faster. At the boiling point , particles throughout the liquid have enough energy to escape into the gas, forming bubbles of vapour. Pure water boils at 100 °C at normal atmospheric pressure. Evaporation is slower and happens only at the surface, at temperatures below the boiling point, when the fastest-moving particles escape.

Condensing and freezing. These are the reverse processes. As a gas cools, its particles slow down and are pulled together into a liquid. As a liquid cools further, particles settle into fixed positions and form a solid.

Sublimation. A few substances, such as solid carbon dioxide ("dry ice") at normal pressure, change straight from solid to gas without melting.

Why it is physical. In all these changes, the particles themselves are unchanged. In water, each particle is an H₂O molecule whether it is in ice, liquid water or steam. Only the spacing, arrangement and speed of the molecules change, so no new substance is formed.

Energy. Melting, boiling and sublimation take in energy from the surroundings. Freezing, condensing and deposition give energy out. During a change of state of a pure substance, the temperature stays constant until the change is complete, because the energy goes into separating or joining particles rather than into speeding them up.

Step-by-step reasoning

To describe melting in particle terms:

1. The solid is heated and energy is transferred to the particles. 2. The particles vibrate faster about their fixed positions. 3. At the melting point, they overcome some of the forces between them. 4. They leave the fixed pattern and slide past each other: the solid has become a liquid of the same substance.

Visual explanation

Picture three boxes. The first shows circles in neat rows, touching. The second shows the same circles touching but jumbled. The third shows a few circles spread widely with arrows showing fast movement. Arrows between the boxes are labelled melting, boiling, condensing and freezing. The circles are identical in every box.

Real-world analogy

A crowd at a concert is like a change of state. Seated in rows, the audience is like a solid. When the music starts and people move around the dance floor, they are like a liquid. When the concert ends and everyone spreads out across the city, they are like a gas. The people never change — only how they are arranged and how they move.

Real-world example

The water cycle is a giant series of changes of state. Sunlight evaporates water from oceans, the vapour condenses into cloud droplets as it cools higher up, and it falls as rain or snow. Throughout, the water remains water, which is why the cycle can repeat endlessly.

Why?

Why does the temperature of melting ice stay at 0 °C until all of it has melted? The energy being supplied is used to overcome the forces between molecules, not to make them move faster, so the temperature does not rise until the solid is gone.

Common misconception

"The bubbles in boiling water are air or hydrogen and oxygen." They are water vapour. The water molecules have not split into hydrogen and oxygen; they have only moved far apart. Splitting water needs a chemical process such as electrolysis.

Worked example

Question: Describe what happens to the particles when steam condenses on a cold mirror, and state whether a new substance forms.

Reasoning: Water molecules in the steam hit the cold surface and lose energy. They slow down and are attracted close together, forming liquid droplets.

Answer: The molecules slow down and come close together to form liquid water; no new substance forms, so it is a physical change.

Quick check

1. What is the name for the change from gas directly to solid? Answer: Deposition.

Exam focus

Learn the names and directions of all the changes of state, and be able to describe each in terms of particle arrangement, movement and energy. Examiners reward the idea that the particles themselves do not change.

Advanced insight

The boiling point depends on pressure. At the top of a high mountain, where air pressure is lower, water boils below 100 °C, which is why food takes longer to cook there. In a pressure cooker the pressure is higher, so water boils above 100 °C and food cooks faster. The substance is the same; only the conditions differ.

Summary

Melting, freezing, boiling, evaporating, condensing, sublimation and deposition are changes of state. They change the arrangement, spacing and movement of particles, but the particles themselves stay the same, so they are physical changes. Melting and boiling take in energy; freezing and condensing release it.

Practice questions

1. Name the change of state from liquid to gas at the surface only, below the boiling point. Answer: Evaporation. 2. Describe the arrangement and movement of particles in a solid and in a liquid. Answer: In a solid, particles are close together in a fixed regular pattern and vibrate; in a liquid, they are close together but random and can slide past each other. 3. Explain why melting chocolate is a physical change. Answer: The particles only move apart slightly and slide past each other; they are still the same particles, so no new substance forms. 4. Which changes of state release energy to the surroundings? Answer: Freezing, condensing and deposition.