# What Is a Chemical Reaction?

> From rusting iron to baking a cake, chemical reactions are happening all around us. This guide explains what a chemical reaction is, how to spot one, and the main types, all in plain English.

*Section: Science — By Priya Anand (Lifestyle & Travel Editor) — Published September 22, 2023 — 6 min read*

Canonical URL: https://dailyjunction.co.uk/science/what-is-a-chemical-reaction
Tags: chemical reaction, chemistry, atoms, molecules, reactions

## Key takeaways

- A chemical reaction is a process that rearranges atoms to turn one or more substances into new, different substances.
- The starting substances are called reactants and the new ones are called products.
- Atoms are never created or destroyed, only rearranged, so mass is conserved.
- Signs of a reaction include colour change, bubbles, heat, light or a new smell.
- Reactions can release energy, like burning, or absorb it, and many can be sped up by heat or catalysts.

Strike a match and it bursts into flame. Leave a bicycle out in the rain and it slowly turns orange with rust. Mix vinegar and baking soda and the mixture froths over the side of the bowl. These look like very different events, but they share a hidden similarity: in each case, the substances you started with have turned into something genuinely new. That transformation is a chemical reaction, and reactions like these are happening constantly, inside your body, in your kitchen and across the natural world. This guide explains what a chemical reaction is.

## What it is

**A chemical reaction is a process in which one or more substances are changed into one or more different substances, by rearranging the atoms they are made from.** The crucial word is *different*: a chemical reaction does not just move a substance around or change its shape, it produces something new, with its own distinct properties.

To picture this, it helps to remember what everything is made of. All matter is built from tiny particles called **atoms**, which join together in various combinations to form different substances. In a chemical reaction, the bonds holding atoms together are broken and reformed in new arrangements. The atoms themselves are not destroyed or created; they are simply reshuffled into new combinations, like rearranging the same set of building bricks into a different model.

The substances you begin with are called the **reactants**, and the new substances produced are called the **products**. A reaction is essentially the journey from reactants to products.

## Reactants, products and rearranging atoms

A simple, familiar example brings this to life: burning. When natural gas burns on a cooker hob, the gas (a reactant) combines with oxygen from the air (another reactant). The flame provides the conditions for the atoms to rearrange, and the products are carbon dioxide and water vapour, completely different substances from the gas you started with.

Chemists often write reactions as word equations to show this clearly:

> methane + oxygen → carbon dioxide + water

The arrow means "turns into", and it always points from reactants to products. Notice that nothing is lost along the way. Every atom present in the reactants ends up somewhere in the products. This leads to one of the most important rules in chemistry, the **conservation of mass**: in any chemical reaction, the total mass of the products equals the total mass of the reactants. Atoms are merely rearranged, never created or destroyed. This same atomic shuffling powers the chemistry inside every living [cell](/science/what-is-a-cell), where countless reactions keep an organism alive.

## How to spot a chemical reaction

Because reactions create new substances, they usually come with visible or detectable clues. No single clue is guaranteed, but spotting one or more is a strong hint that a chemical reaction is taking place.

- **Colour change.** Iron turning orange as it rusts, or a sliced apple browning in the air.
- **Bubbles or gas.** Fizzing when vinegar meets baking soda, as carbon dioxide gas is produced.
- **Heat or light.** Burning gives off both; some reactions also feel warm or even cold to the touch.
- **A new smell.** The distinctive smell of toast or of a struck match.
- **A solid forming.** When two clear liquids are mixed and a cloudy solid appears, called a precipitate.

If you see a brand-new substance with properties different from what you began with, and especially if it cannot easily be turned back, a chemical reaction has almost certainly happened.

## Chemical change versus physical change

It is easy to confuse chemical reactions with **physical changes**, but the distinction is important. A physical change alters the form or appearance of a substance without changing what it actually is, and it can often be reversed.

Melting ice is a classic physical change: solid ice becomes liquid water, but it is still water, and you can freeze it back again. The same goes for [condensation](/science/what-is-condensation), where water vapour turns into liquid water without becoming a different substance, and for dissolving sugar in tea. In each case no new substance is created.

A chemical change, by contrast, produces new substances and is usually much harder to undo. Burn a sheet of paper and you get ash, smoke and gases; you cannot reassemble the paper. Fry an egg and the runny clear white turns solid and white, a change you cannot reverse. The rule of thumb is straightforward: if a new substance with different properties has been created, it is a chemical change; if only the form has altered, it is a physical change.

## Common types of reaction

Chemists group reactions into families based on what happens to the atoms. A few of the most common are worth knowing.

| Type | What happens | Everyday example |
| --- | --- | --- |
| Combustion | A substance reacts with oxygen, releasing energy | Burning fuel or wood |
| Oxidation | A substance combines slowly with oxygen | Iron rusting |
| Neutralisation | An acid reacts with a base to cancel each other out | Indigestion tablet easing stomach acid |
| Decomposition | One substance breaks down into simpler ones | Hydrogen peroxide breaking into water and oxygen |

Acids and bases feature in many of these, and how acidic or alkaline a substance is can be measured on the [pH scale](/science/what-is-ph), which is central to understanding neutralisation reactions in particular.

## Energy and the speed of reactions

Every chemical reaction involves a change in energy. Some reactions **release** energy, usually as heat or light. Burning is the obvious example, giving out warmth and a flame. These are called *exothermic* reactions. Other reactions **absorb** energy from their surroundings and can feel cold, known as *endothermic* reactions, such as certain sports cold packs that turn icy when activated.

The speed of a reaction also varies enormously, from an explosion in a fraction of a second to iron rusting over years. Several factors can speed reactions up:

- **Temperature.** Heating things up generally makes reactions go faster, which is why food cooks in the oven but spoils slowly in the fridge.
- **Concentration.** More concentrated reactants tend to react more quickly.
- **Surface area.** A powder reacts faster than a single lump, because more of it is exposed.
- **Catalysts.** Special substances called catalysts speed up a reaction without being used up themselves; the enzymes in your body are biological catalysts that make life's reactions fast enough to sustain you.

## Reactions all around us

Chemical reactions are not confined to the laboratory. Your body runs on them: digesting food, breathing and powering your muscles all depend on chemistry. Cooking is full of them, from bread rising to meat browning. The natural world relies on them too, from plants making food through photosynthesis to the slow weathering of rock. Even the energy that flows through the [food chain](/science/what-is-the-food-chain) begins with chemical reactions in plants capturing sunlight.

## The bottom line

A chemical reaction is a process that turns one or more substances, the reactants, into new and different substances, the products, by rearranging their atoms. Because atoms are only reshuffled and never created or destroyed, the total mass stays the same, a principle known as the conservation of mass. You can often spot a reaction by clues such as a colour change, bubbles, heat, light or a new smell, and these new substances set chemical changes apart from reversible physical changes like melting. Reactions come in many types, can release or absorb energy, and can be sped up by heat, concentration, surface area or catalysts. From the flame of a match to the rust on a gate and the meal on your plate, chemical reactions are the constant, transforming chemistry of everyday life.

## Frequently asked questions

### What is a chemical reaction in simple terms?

A chemical reaction is a change that turns one or more substances into completely new ones by rearranging their atoms. The new substances have different properties from what you started with. Burning wood, rusting iron and digesting food are all chemical reactions, because in each case new substances are produced.

### How can you tell a chemical reaction has happened?

Look for tell-tale signs: a colour change, bubbles of gas, heat or light being given off, a new smell, or a solid forming in a liquid. Not every sign appears every time, but if a brand-new substance with different properties has formed, a chemical reaction has taken place.

### What is the difference between a chemical and a physical change?

A physical change alters the form of a substance but not what it is, such as ice melting into water, and it can often be reversed. A chemical change creates new substances with different properties and is usually much harder to reverse, such as burning paper into ash and smoke.

### Is cooking a chemical reaction?

Often, yes. Baking a cake, frying an egg and toasting bread all involve chemical reactions that create new substances, which is why the food cannot simply be turned back into its raw ingredients. Some cooking steps, like melting butter, are physical changes, but much of cooking relies on chemistry.

## Sources

- [Royal Society of Chemistry](https://www.rsc.org/)
- [Encyclopaedia Britannica: chemical reaction](https://www.britannica.com/science/chemical-reaction)
- [BBC Bitesize: chemical reactions](https://www.bbc.co.uk/bitesize)

---
Daily Junction — https://dailyjunction.co.uk/science/what-is-a-chemical-reaction