Changes of State Are Physical Changes

Introduction

Melt some ice and you get water. Freeze the water and you get ice again. Boil it and it becomes steam, which condenses back into water on a cold window. Through all these changes the substance is still water, H₂O. A change of state alters how particles are arranged and how they move, but it never changes what the particles are. That is why changes of state are called physical changes .

Core explanation

What makes a change physical? In a physical change, no new substance is formed. The particles before and after are exactly the same particles. Physical changes are usually easy to reverse by simple means such as heating or cooling.

Changes of state fit this description. When ice melts, the water molecules stay whole. Each H₂O molecule still contains two hydrogen atoms bonded to one oxygen atom. What changes is the arrangement:

State Arrangement Movement --- --- --- Ice (solid) regular, fixed positions vibrate on the spot Water (liquid) close together, random slide past each other Steam (gas) far apart, random move quickly in all directions

The forces between separate molecules are overcome or formed; the bonds inside each molecule are not broken.

Mass is conserved. Because the particles are neither created nor destroyed, the mass stays the same. If 50 g of ice melts in a sealed container, the container holds 50 g of water afterwards. If some mass seems to be lost when water is boiled in an open pan, that is because the steam has escaped into the air, not because anything has been destroyed.

How physical changes differ from chemical changes. In a chemical change, atoms rearrange to form new substances with different properties. Burning wood, rusting iron and cooking an egg are chemical changes. Signs of a chemical change include a colour change that cannot be undone simply, a gas being given off that is a new substance, or a permanent change in properties.

A useful comparison. Heating a candle shows both kinds of change. The wax near the flame melts — a physical change, since the liquid wax sets again when it cools. The wax vapour in the flame burns, reacting with oxygen to form carbon dioxide and water — a chemical change, since new substances are made.

Properties that stay the same. After a change of state is reversed, the substance has its original properties: the same melting point, boiling point and chemical behaviour.

Step-by-step reasoning

To decide whether a change is physical:

1. Ask whether a new substance has been formed. 2. Check whether the change is easily reversed by heating or cooling. 3. Consider whether the particles themselves change or only their arrangement. 4. If only the arrangement changes and no new substance forms, it is a physical change.

Visual explanation

Draw the same five water molecules three times: packed in neat rows for ice, clustered but jumbled for liquid water, and widely scattered for steam. Each molecule looks identical in all three pictures — one O with two H atoms attached. Only the spacing and the arrows showing movement differ.

Real-world analogy

A box of building bricks can be stacked into a neat wall, piled loosely in a heap or scattered across the floor. The bricks are the same in each case, and you can rebuild the wall at any time. A change of state is like rearranging the bricks. A chemical change would be like melting the bricks down and moulding them into something new.

Real-world example

Chocolate makers melt chocolate, pour it into moulds and let it set. The chocolate can be melted and set many times and it is still chocolate. Melting and setting are physical changes. If chocolate is overheated and burns, however, it turns into a bitter, dark substance — a chemical change that cannot be reversed by cooling.

Why?

Why is mass conserved when a substance changes state? Mass comes from the particles. During a change of state no particles are created or destroyed; they only move closer together or further apart. The total number of particles, and so the total mass, stays the same.

Common misconception

"Boiling water breaks the water into hydrogen and oxygen." Boiling only separates whole water molecules from one another. The bubbles in boiling water are steam — water vapour — not hydrogen and oxygen gases. Splitting water into its elements needs a chemical change such as electrolysis.

Worked example

Question: A sealed plastic bag contains 20 g of ice. The ice melts completely. What is the mass of the bag's contents now, and is this a physical or chemical change?

Reasoning: No particles can enter or leave the sealed bag. The water molecules are the same molecules, just rearranged. So the mass is unchanged. No new substance has formed, and refreezing would give ice again.

Answer: 20 g; it is a physical change.

Quick check

1. Is condensation of steam on a window a physical or a chemical change? Answer: A physical change, because the water vapour becomes liquid water and no new substance is formed.

Exam focus

Explain your answer with "no new substance is formed" and "the change can be reversed". When asked about mass, mention a closed system and that particles are not created or destroyed. Be careful not to confuse bonds within molecules with forces between molecules.

Advanced insight

Not every easily reversed change is physical, and not every physical change is easily reversed. Dissolving salt can be reversed by evaporation, yet mixing some substances is hard to undo. Chemists therefore rely mainly on the key test: has a new substance, with a different chemical identity, been produced?

Summary

Changes of state are physical changes. The particles stay the same; only their arrangement, spacing and movement change. No new substance forms, the change is reversible by heating or cooling, and mass is conserved in a closed system. This contrasts with chemical changes, in which atoms rearrange to form new substances.

Practice questions

1. Give two reasons why melting is described as a physical change. Answer: No new substance is formed, and the change is easily reversed by cooling. 2. When water boils in an open pan, the mass of the pan and water decreases. Explain why this does not contradict conservation of mass. Answer: The steam escapes into the air; the particles still exist, but they are no longer in the pan. 3. A candle burns. State one physical change and one chemical change that take place. Answer: The wax melting is a physical change; the wax vapour burning to form carbon dioxide and water is a chemical change. 4. What stays the same about water molecules when ice turns to steam? Answer: Each molecule still contains two hydrogen atoms bonded to one oxygen atom; only the spacing and movement of the molecules change.