A candle burns. An old bike chain turns rusty. A cake rises in the oven, and a leaf grows in the sunshine.

All of these are chemical reactions. Let’s find out what is really happening inside them.

What happens in a chemical reaction?

Everything around you is made of atoms, and many of them are joined into molecules. In a chemical reaction, some chemical bonds break, and the atoms join up again in new ways. The new arrangements are new substances, with new properties.

Take the gas that burns on many kitchen stoves: methane, CH₄. When it burns, each methane molecule meets two oxygen molecules from the air. Their bonds break, and the atoms regroup. They make one molecule of carbon dioxide and two molecules of water.

Methane burning: the same atoms, new partners Before: one methane molecule, a carbon atom with four hydrogen atoms, and two oxygen molecules, each two oxygen atoms. An arrow labelled burning points down. After: one carbon dioxide molecule, a carbon atom between two oxygen atoms, and two water molecules, each an oxygen atom with two hydrogen atoms. Under both rows, the count is the same: 1 carbon, 4 hydrogen and 4 oxygen atoms. before after CHHHHOOOOOCOOHHOHHmethane oxygen oxygen carbon dioxide water water 1 C · 4 H · 4 O 1 C · 4 H · 4 O burning
Before and after burning, there are the same atoms: 1 carbon, 4 hydrogen and 4 oxygen. Only their partners have changed.

Where do the atoms go?

No atom is ever destroyed in a chemical reaction, and no new atom appears. So the total weight stays the same.

About 250 years ago, the French chemist Antoine Lavoisier showed this with very careful weighing. He weighed everything that went into a reaction and everything that came out, even the gases. The totals always matched. This rule is called the law of conservation of mass.

So why does a burning log leave only a small pile of ash? Most of its atoms have floated away into the air, as carbon dioxide gas and water vapour. Imagine catching all those gases and weighing them with the ash. Together they would weigh as much as the log and the oxygen it used.

It works the other way round, too. A rusty nail weighs more than it did when it was shiny, because it has taken oxygen and water from the air.

How can you tell that a reaction is happening?

You can’t see atoms swapping partners. But you can often spot the signs:

  • bubbles of gas, when a new gas forms
  • a change of colour, like a cut apple turning brown
  • heat or light, like a flame or a glow stick
  • a new smell, like bread turning into toast
  • a solid appearing in a liquid

Be careful, though: signs can fool you. Boiling water makes bubbles, yet it is still water. Melting, freezing and boiling are not chemical reactions, because the molecules stay the same. In a real reaction, a new substance appears.

Where can you see reactions?

Everywhere! Here are a few:

  • Rusting. Iron slowly joins with oxygen and water from the air and turns into flaky, reddish-brown rust. Salt speeds it up, so cars rust faster where roads are salted in winter.
  • Baking soda and vinegar. Mix them, and the fizz you see is a new gas: carbon dioxide.
  • Photosynthesis. Plants take in carbon dioxide from the air and water from the soil. With the energy of sunlight, they turn them into sugar and give out oxygen.
  • Your body. Your cells join sugar from your food with oxygen from your breath. This gives you the energy to move, think and grow, and it makes the carbon dioxide you breathe out.

Do reactions give out energy or take it in?

Many reactions give out energy. Burning gives out heat and light. Some hand warmers get hot because iron powder inside them rusts very quickly.

Other reactions take energy in. When baking soda and vinegar fizz, the mixture gets a little colder. Plants take in the energy of sunlight to make sugar.

Where does this energy come from? Breaking bonds takes energy, and making new bonds gives energy out. If the new bonds are stronger than the old ones, energy is left over, and it comes out as heat or light. If they are weaker, the reaction has to take energy in.

Either way, no energy is made or destroyed: it only changes form. That is the rule of conservation of energy.

Why doesn’t everything burn at once?

Paper, wood and candle wax can all burn. Yet they sit around in the air without catching fire. Many reactions, like burning, need a push to get started: a spark, a flame or some heat. Once burning has started, it makes its own heat, and that keeps it going.

Burning needs a push over a bump An energy picture of burning, from left to right. On the left, a ball sits on a high ledge: the gas and the oxygen. Before it can roll down, it must be pushed over a bump, and a spark gives that push. Then the path drops to a much lower ledge: the carbon dioxide and water. The drop is the energy that comes out as heat and light. A dashed path with a lower bump shows how a catalyst makes the start easier. energy gas + oxygen a spark gives the push heat and light come out carbon dioxide + water with a catalyst: a lower bump
A fire needs a push over the bump, like a spark, before it can give out energy. A catalyst makes the bump lower.

Some substances make it easier for a reaction to start, and they are not used up themselves. They are called catalysts.

Your body is full of catalysts called enzymes. Chew a piece of bread for a long time, and it starts to taste sweet. An enzyme in your spit is breaking the starch in the bread into sugar.

Go deeper: Writing reactions down: chemical equations for grown-ups and the extra curious

Chemists write reactions as equations. Burning methane looks like this:

CH4+2 O2→CO2+2 H2O\mathrm{CH_4 + 2\,O_2 \rightarrow CO_2 + 2\,H_2O}

The substances on the left are the reactants, and the ones on the right are the products. The big numbers in front (2 O₂, 2 H₂O) are called coefficients. They tell how many molecules take part.

An equation is balanced when each kind of atom appears the same number of times on both sides. Here there are 1 carbon, 4 hydrogen and 4 oxygen atoms on each side. To balance an equation, you may change only the numbers in front. Never change the small numbers inside a formula: that would turn it into a different substance.

Go deeper: Where the heat of a flame comes from for grown-ups and the extra curious

You can estimate it with the average bond energies from how atoms join into molecules. Burning one mole of methane (16 grams) breaks four C–H bonds (4 × 415 kJ) and two O=O bonds (2 × 498 kJ): 2,656 kJ in all. It makes two C=O bonds in carbon dioxide (2 × 799 kJ) and four O–H bonds (4 × 464 kJ): 3,454 kJ in all.

The difference, about 800 kJ, comes out as heat. Careful measurements give 802 kJ when the water leaves as vapour, and 890 kJ when it condenses into a liquid. Reactions that give out heat like this are called exothermic; reactions that take heat in are endothermic.

Baking soda and vinegar are less simple. Taken as a whole, the fizzing takes in heat. Most of that comes from the solid baking soda dissolving and the carbon dioxide escaping as a gas; the acid and the baking soda reacting in the water give out a little heat of their own.

Go deeper: Activation energy and catalysts for grown-ups and the extra curious

The push a reaction needs is called its activation energy. Old bonds have to start breaking before new ones can form, and that costs energy first. At room temperature, only a tiny share of the collisions between molecules are hard enough. That is why most fuels can sit in the air for years. Other reactions, such as baking soda with vinegar, have such a low bump that the warmth of the room is push enough: they start as soon as the substances meet.

Heat makes molecules move faster, so more of their collisions get over the bump. As a rough rule, many reactions near room temperature go about twice as fast for every 10 °C warmer.

A catalyst gives the reaction a different path with a lower bump. It takes part in the reaction but comes out unchanged at the end. The catalytic converter in a car’s exhaust uses platinum, palladium and rhodium to turn poisonous carbon monoxide into carbon dioxide. Enzymes can speed up reactions in your body by millions of times or more.

Go deeper: Lavoisier, and a tiny correction from Einstein for grown-ups and the extra curious

Lavoisier set out the rule in his 1789 textbook, the Traité élémentaire de chimie: in every operation, an equal quantity of matter exists before and after. The Russian scientist Mikhail Lomonosov had written down a similar idea in 1756, so in some countries the law carries both names. Lavoisier also named oxygen and hydrogen and showed that water is made of the two. His wife, Marie-Anne Paulze Lavoisier, worked alongside him: she drew his apparatus and kept his laboratory records.

Strictly, the products of a reaction that gives out energy weigh a tiny bit less, because energy has mass (E=mc2E = mc^2). Burning 16 grams of methane gives out 890 kJ, which is about 10 billionths of a gram. No scale in a chemistry lab can notice that, so for chemistry the law holds.

Check yourself

A log burns down to a small pile of ash. Where did most of its atoms go?

Show the answer

Into the air, as carbon dioxide and water vapour — Atoms are never destroyed in a reaction. The carbon and hydrogen of the wood joined with oxygen and floated away as carbon dioxide and water vapour.

Check yourself

Which of these is a chemical reaction?

Show the answer

A nail rusting — When ice melts or water boils, the molecules stay H₂O. When iron rusts, its atoms join with oxygen and water to make a new substance: rust.