What is a chemical reaction?

One methane molecule and two oxygen molecules become one carbon dioxide and two water molecules, and not a single atom is lost on the way.

Light a gas stove and a blue flame appears. Nothing seems to go in except gas and air, and nothing seems to come out except heat. Yet something has changed completely. The methane in the gas and the oxygen in the air are gone, and in their place there is carbon dioxide and water vapour. That is a chemical reaction: substances turn into different substances because their atoms let go of old partners and bond to new ones.

Chemical change and physical change

Melt an ice cube and you still have water. Dissolve sugar in tea and the sugar molecules are still sugar, only spread out. These are physical changes: the substance looks different, but its molecules stay the same. A chemical change always produces matter that differs from what was there before. Rust is a different substance from the iron, oxygen and water it came from. Burning, cooking, digesting and rotting food are all chemical changes, because new substances form each time.

You usually cannot see atoms rearrange, so chemists look for clues. A gas appears where there was none, as in the bubbles from baking soda and vinegar. The colour changes, as when a copper wire left in silver nitrate solution turns the liquid blue and grows grey silver crystals. A solid forms when two clear solutions are mixed: lead nitrate and potassium iodide give a bright yellow solid called a precipitate. Heat or light is given out, as in a flame, or taken in. None of these clues proves a reaction on its own, since boiling water also makes bubbles, but together they are a good guide.

Reading a chemical equation

Chemists write reactions as equations. The substances you start with, the reactants, go on the left. The substances that form, the products, go on the right. An arrow points from one side to the other and plus signs separate the substances on each side. For methane burning the equation is CH₄ + 2O₂ → CO₂ + 2H₂O. The small numbers inside a formula are subscripts and tell you what the substance is: H₂O has two hydrogen atoms and one oxygen atom. The large numbers in front are coefficients and tell you how many units of each substance take part. A coefficient of 1 is left out.

Coefficients are ratios, not exact counts. CH₄ + 2O₂ means one methane for every two oxygen molecules, so 3 methane molecules need 6 oxygen molecules, and one mole of methane needs two moles of oxygen. Letters in brackets are often added to show the state of each substance: (s) for solid, (l) for liquid, (g) for gas and (aq) for dissolved in water. With those, the methane equation becomes CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) when the water is cooled to a liquid.

Use the lab above to compare a few reactions. Each one keeps every atom it started with. Iron and oxygen become iron(III) oxide, the main part of rust. Calcium carbonate, the mineral in limestone, breaks apart into calcium oxide and carbon dioxide when it is heated strongly. In every case the bars for reactant atoms and product atoms match, which is the rule the next unit is about.