Chemical Reactions and Signs of Change

When new substances are born

Strike a match, mix vinegar with baking soda, or leave an iron nail out in the rain, and something dramatic happens: the starting materials turn into completely new substances. This is a chemical reaction — one of the most important ideas in all of science, because it explains everything from cooking and digestion to rusting and fireworks.

In a chemical reaction, the substances you start with are called reactants, and the new substances that form are called products. The reactants are not simply mixed or moved around — their atoms break apart and join together in new ways, creating products with different properties from anything you began with.

The five signs of a chemical change

How can you tell when a real chemical reaction has happened, rather than just a physical change? Watch for these five common signs of change:

1. A colour change. A shiny iron nail turning orange with rust, or a sliced apple going brown.
2. Gas bubbles forming. Vinegar fizzing on baking soda releases carbon dioxide gas (this is different from boiling, which is just a physical change).
3. A new smell. Toast smells different from bread; sour milk smells different from fresh.
4. Heat or light given off (or taken in). A burning candle releases heat and light; some reactions get cold instead.
5. A solid appearing from liquids. When two clear solutions are mixed and a cloudy solid forms, that solid is called a precipitate.

Spotting one or more of these is a strong clue that the atoms have rearranged into something new.

Chemical change versus physical change

Not every change is a chemical reaction. It is vital to tell the two apart.

	Physical change	Chemical change
New substance made?	No	Yes
Easy to reverse?	Usually	Usually not
Examples	Melting, boiling, dissolving, bending	Burning, rusting, cooking, fizzing

Melting ice or dissolving sugar are physical changes — the water is still water and the sugar is still sugar, just in a new state. This idea is explored in Reversible and Irreversible Changes. Burning wood or cooking an egg are chemical changes, because brand new substances are created and you cannot undo them.

Atoms are never lost

Here is one of the most beautiful rules in chemistry: in a reaction, atoms are never created or destroyed — they are only rearranged. This is called the law of conservation of mass, and it means the total mass of the reactants equals the total mass of the products.

When wood burns, it seems to vanish into a tiny pile of ash, but the missing mass has simply floated away as invisible gases like carbon dioxide and water vapour. If you could weigh every gas, the totals would match perfectly. To picture this clearly, it helps to know what atoms are, which you can read about in Atoms and Molecules.

Speeding up and slowing down reactions

Reactions do not all happen at the same speed. You can change the rate by:

• Heat — most reactions go faster when warmer (food cooks faster on high heat, and spoils faster in a warm room).
• Concentration — stronger solutions react faster.
• Surface area — a powder reacts faster than a single lump because more of it is exposed.

This is why a fire catches more easily on small twigs than on a thick log, and why your body keeps food cold in the fridge to slow the reactions that make it go off.

Why chemical reactions matter

Chemical reactions run the living world and the made world alike. Your body digests food and breathes through reactions. Plants make sugar through the reaction of photosynthesis. Factories make medicines, fertilisers and plastics. Cars burn fuel. Even the batteries in your devices store and release energy through controlled reactions. Once you understand reactants, products and the signs of change, chemistry stops being mysterious and starts making sense.

Try this — a safe fizzing reaction

With a grown-up, place a tablespoon of baking soda in a tall glass, then pour in a little vinegar. Watch closely: the mixture bubbles and fizzes, the glass may feel slightly cooler, and the foam climbs — all signs of a chemical reaction releasing carbon dioxide gas. For extra fun, do it over a tray and add a drop of washing-up liquid to make more foam. This is completely safe, but do not seal the container, since the gas needs room to escape. You have just watched reactants turn into new products with your own eyes.

Learn to read the signs of change, and you can spot a chemical reaction happening anywhere — in a kitchen, a forest, or a laboratory.