One simple rule keeps track of matter. In any mixing or reacting, the total amount of matter does not change, so nothing is lost. A chemical change rearranges atoms into new substances without creating or destroying any, and a balanced chemical equation shows this by listing equal atom counts for each element, reactants against products, matching the law that says matter is conserved. A physical change, such as wax melting or sugar dissolving in coffee, produces no new substance, but a chemical change does. Salt dissolves in water to make a solution so evenly mixed that no part differs from any other, and when oil and water are shaken together they stay mixed only briefly before separating again. Oil floats above the water, which is why the two form layers. Rust is no longer iron: it is a different kind of matter built from iron, oxygen and water.

There are clues that something new has formed. A color change can be one: red cabbage juice, for example, changes color when another substance is added, and plants such as berries and beetroot can also make indicators, as can many others. Bubbles of gas are one, as in baking soda and vinegar, where the bubbles are carbon dioxide. When baking soda meets an acid, carbonic acid forms, and the gas that this acid releases as it breaks down makes the bubbles. Citrus fruits like lemons owe their sourness to acids. A solid is another clue. When a solid appears suddenly after two clear liquids are blended, it is known as a precipitate. A precipitation reaction is one where dissolved substances react and form a solid product. Iron kept in air and water becomes rust. Covering iron with paint, grease or plastic blocks rust, because air and water can no longer reach the metal.

According to phlogiston theory, burnable materials such as charcoal and metals contained a substance called phlogiston that escaped into the air during burning, and a burned metal became a calx. Joseph Priestley heated mercury calx with a large burning lens and collected the air it gave off. He named it dephlogisticated air, and Lavoisier later called the gas oxygen. Together with Marie-Anne Paulze Lavoisier, his wife, Lavoisier overturned the phlogiston theory, showing that both burning and breathing require a kind of air that had only recently been isolated. He also proved water to be made up of hydrogen and oxygen, going against the old notion that water was a basic element in the company of earth, air and fire.

The next steps came from careful measuring. Antoine Lavoisier, a French chemist (1743-1794), measured carefully and found that the products of a reaction have the same mass as what went into it. Metals heated in closed vessels were weighed along with everything else, and the metal gained as much mass as the air lost. The modern idea of the atom began with John Dalton, an English schoolteacher, who said atoms could explain how matter behaves. That replaced Aristotle's view, which was 2,000 years old. Another idea described acids in terms of ions: in 1884 Svante Arrhenius said an acid gives hydrogen ions in water and a base gives hydroxide ions, and in 1923 a broader view said an acid donates a proton and a base accepts one.

There are several ways to sort reactions and to read what they do. Many fall into three common types: precipitation, acid-base and oxidation-reduction reactions. In an acid-base reaction called neutralization, an acid meets a base, and its products are frequently a salt along with water. A change in temperature can hint at a reaction, since some reactions give off heat and others take it in, which is why heating or cooling is a clue worth noting. When a reaction is exothermic, meaning heat goes out, chemists give its enthalpy change a negative sign, and when it is endothermic, meaning heat goes in, the sign is positive. Some reactions are able to run in both directions, and once the forward and reverse rates match, the amounts of each substance hold steady.

A higher temperature typically speeds chemical reactions up. A solid in fine pieces reacts faster than one big lump of the same material, because more of its surface touches the other substance. Reaction rates typically climb as a starting substance becomes more concentrated. Cooking changes food, and heat drives reactions in the kitchen too. Cakes and bread rise thanks to gas bubbles inside them. In an egg, heat makes the proteins unwind and bond with each other, forming a network that holds water in, and an egg white kept hot for too long turns rubbery. The Maillard reaction gives the brown crust of bread its smell and taste. Sugar that is heated goes first through caramelization as it browns, which is behind the sweet, nutty taste of caramel apples and caramelized onions.

In general, a catalyst is a helper for chemical reactions, and it accelerates a reaction while staying unconsumed itself. It works by offering an alternative mechanism, a different step-by-step path with a lower activation energy, while the overall reaction, with the same start and finish, is unchanged. Enzymes lower a reaction's activation energy, yet the reactants and the products keep the same free energy. Living things use enzymes as their catalysts, and an enzyme can make a reaction run between 100 million and 10 billion times quicker than it would on its own. An enzyme binds its starting materials, each one a substrate, at its active site, so each enzyme fits only certain starting materials. Amylase is one enzyme, and it helps our bodies digest carbohydrates.

Words to know

• phlogiston: supposed burning substance
• calx: burned metal
• catalyst: speeds up reactions
• substrate: bound reactant
• exothermic: gives off heat