What Is a Change in Matter?

Introduction

Ice melts in a drink, a puddle dries up in the sun, toast turns brown and a bicycle chain slowly goes rusty. Everything around us is made of matter, and matter is constantly changing. Chemists sort these changes into two big families. In some, a substance only changes its form — its shape, size or state. In others, the substance changes its identity and becomes something completely new. Learning to tell these apart is the starting point for this whole unit.

Core explanation

What counts as a change? A change in matter is any process in which the appearance, arrangement or make-up of a material is different afterwards. Cutting paper, freezing water, frying an egg and burning wood are all changes.

Change of form: physical changes. In a physical change the substance is still the same substance afterwards. Water is still water whether it is ice, liquid or steam. A sheet of paper cut into strips is still paper. Sugar stirred into tea is still sugar, even though you can no longer see it. The particles of the substance are unchanged; they have only moved closer together, further apart or into a new arrangement.

Change of identity: chemical changes. In a chemical change, also called a chemical reaction , one or more new substances are made. The new substances have different properties from the starting materials. When wood burns, it becomes ash, carbon dioxide and water vapour, none of which is wood. When iron rusts, the grey, strong metal becomes a flaky orange-brown solid called rust. The atoms have been rearranged and joined together in new ways.

Starting materials and products. In a chemical change the substances you start with are called reactants and the new substances formed are called products . In a physical change there are no products in this sense — the same substance is present before and after.

Two questions to ask. To classify any change, ask:

1. Is there a new substance with different properties afterwards? 2. Has only the form, shape, size or state changed?

If the answer to the first question is yes, the change is chemical. If only the second is true, the change is physical.

Why it matters. Cooks, builders, doctors and engineers all need to know whether a change can be undone easily, whether it will produce something new, and whether it will release energy. That is why chemistry begins by sorting changes carefully.

Step-by-step reasoning

To decide what kind of change is happening:

1. Name the substance you start with. 2. Observe what you have at the end. 3. Ask whether the end material has the same properties (colour, melting point, smell, behaviour) as the start. 4. Same substance in a new form means physical; a new substance means chemical.

Visual explanation

Imagine a model built from coloured building bricks. Moving the whole model to a new shelf, or pulling the bricks apart and heaping them in a box, is like a physical change: the bricks are still joined in the same small groups. Taking the bricks apart and rebuilding them into a totally different model is like a chemical change: the same bricks are now joined in new ways.

Real-world analogy

Think of a person changing clothes compared with a person being replaced. Put on a coat and a hat and you look different, but you are still the same person — that is a physical change. A chemical change is more like a completely different person walking out of the room.

Real-world example

Making a cake shows both kinds of change. Melting the butter and mixing in the flour are physical changes. Baking is chemical: heat causes reactions that produce gases, set the egg proteins and brown the surface, giving a cake that cannot be turned back into butter, eggs and flour.

Why?

Why do chemists care about identity rather than appearance? Because appearance can change a great deal without anything new being made — crushed ice looks nothing like an ice cube. Identity depends on the particles themselves, and only a chemical change alters them.

Common misconception

"A big, dramatic change must be chemical." Boiling water bubbles violently and makes clouds of steam, yet it is only a physical change. Some chemical changes, such as slow rusting, are quiet and gradual. Size of effect is not the test; a new substance is.

Worked example

Question: Classify (a) folding a sheet of aluminium foil and (b) a match burning.

Reasoning: (a) The foil is still aluminium; only its shape changes. (b) The match head and wood turn into ash, smoke and gases that were not there before.

Answer: (a) physical change; (b) chemical change.

Quick check

1. Is grating cheese a physical or a chemical change? Answer: Physical, because the cheese is still cheese; only its size and shape have changed.

Exam focus

Examiners want the phrase "a new substance is formed" for a chemical change and "no new substance is formed" for a physical change. Always give a reason in terms of the substance, not just the appearance.

Advanced insight

Not every change sits neatly in one box. Dissolving some substances, such as salt in water, involves strong interactions between particles, and a few processes are debated at advanced level. At this stage the new-substance test works very well, and later units show how bonds and energy explain the boundary more exactly.

Summary

Matter can change its form or its identity. A physical change alters shape, size or state but leaves the substance the same. A chemical change, or reaction, turns reactants into new products with different properties. To classify a change, ask whether a new substance has been formed.

Practice questions

1. What is the difference between a physical change and a chemical change? Answer: A physical change produces no new substance; a chemical change produces one or more new substances with different properties. 2. Classify: (a) chopping wood, (b) burning wood. Answer: (a) physical, because it is still wood; (b) chemical, because ash and gases are formed. 3. What are reactants and products? Answer: Reactants are the starting substances in a chemical change; products are the new substances formed. 4. A student says boiling water is chemical because it bubbles so much. Explain why this is wrong. Answer: The steam formed is still water, so no new substance is made; boiling is a physical change whatever it looks like.