Chemical reactions

Not every change is a chemical reaction. Ice melting into water looks dramatic, but it is still water. Its molecules have only loosened their hold on one another. A change like this, in which no new substance forms, is called a physical change. Melting, freezing, boiling, dissolving, crushing and mixing are all physical changes, and most can be undone. Stir iron filings into sand and a magnet will pull the iron straight back out.

Frying an egg is different. Heat permanently changes the egg’s proteins into new substances, and no amount of cooling brings the raw egg back. That is a chemical change. The deciding test is always the same: whether new substances form. An antacid tablet fizzing in water passes it, because the gas it gives off was not there before. Ice melting in a drink does not.

You cannot watch atoms rearrange, but a reaction usually gives itself away. One sign is a gas appearing where there was none. Pour vinegar onto baking soda and the mixture froths with bubbles of carbon dioxide, a gas that neither ingredient contained. It was made on the spot.

Another sign is a solid appearing out of clear liquids. Mix two transparent solutions of the right kind and the liquid turns cloudy as a new, insoluble substance forms and sinks. Chemists call such a solid a precipitate.

A lasting change of colour, as when a cut apple browns, is a third sign, and a change of temperature with no outside heating or cooling is a fourth. None of these is proof on its own, but each is a hint that new substances have formed.

Burn a heavy log and you are left with a handful of light ash. For centuries that seemed to show that burning destroys matter. In the 1770s and 1780s the French chemist Antoine Lavoisier showed that it does not. He ran reactions in sealed glass vessels, so that nothing could get in or out, and weighed everything before and after on the most accurate balances of his day. The total never changed.

This is the law of conservation of mass: in a chemical reaction, the mass of the products equals the mass of the reactants. It follows from what a reaction is. The atoms are only rearranged, and atoms do not lose their mass by changing partners.

So the log has not vanished. Most of its mass has gone into the air as gas, as carbon dioxide and water vapour that an open fire lets drift away unweighed.

Chemists write a reaction as an equation, with the reactants on the left, an arrow, and the products on the right. Burning methane is written CH₄ + 2 O₂ → CO₂ + 2 H₂O. One molecule of methane and two of oxygen become one of carbon dioxide and two of water.

Count the atoms on each side: one carbon, four hydrogens and four oxygens before the arrow, and exactly the same after it. An equation in which every atom is accounted for like this is said to be balanced, and since atoms are only rearranged, every correct equation must be.

The masses agree as well. Burn 16 grams of methane and it uses 64 grams of oxygen, 80 grams in all. Out come 44 grams of carbon dioxide and 36 grams of water: 80 grams again.

What is fire doing to the thing that burns? For about a century chemists believed that anything that could burn contained a substance called phlogiston, and that burning was phlogiston escaping into the air. The idea had an awkward flaw. Metals heated in air turn to powder, and the powder weighs more than the metal did. How can something gain weight by losing an ingredient?

In 1774 the English chemist Joseph Priestley isolated a new gas in which candles burned with unusual brightness. Lavoisier repeated the work, named the gas oxygen, and drew the right conclusion, which he set out in a textbook in 1789. A burning substance is not giving anything off. It is combining with oxygen from the air, and the extra weight of a heated metal is the weight of the oxygen it has taken up.

Reactions run at very different speeds. Iron takes years to rust through, milk sours in days, and a firework is over in a second. How quickly a reaction turns reactants into products is called its rate.

What sets the rate is collisions. In a gas or a liquid the particles are always on the move, and two of them can react only when they collide. Even then most collisions achieve nothing: the particles bounce apart unchanged. A reaction happens only when they collide hard enough to break the bonds that hold them together.

So anything that makes collisions more frequent, or more violent, speeds a reaction up, and anything that makes them rarer or gentler slows it down. Every way of controlling a reaction, in a kitchen or a factory, works through one of those two.

Temperature is the easiest control. In a warmer substance the particles move faster, so they collide more often and, more importantly, they collide harder, and a far larger share of the collisions is violent enough to break bonds. The effect is steep. As a rough rule, a rise of 10 °C doubles the rate of many reactions.

The rule runs in reverse too, and every kitchen relies on it. Food spoils through chemical reactions, many of them carried out by the bacteria and moulds that feed on it. A refrigerator does not stop those reactions. It slows them down. At 4 °C milk keeps for a week or more, where at room temperature it sours in about a day. A freezer slows the same reactions further still, which is why frozen food lasts for months.

Two other controls work by making collisions more frequent. The first is surface area. A solid can react only where it touches the other reactant, at its surface. Break it into smaller pieces and more of it is exposed. A sack of flour will barely smoulder, but the same flour blown into the air as fine dust has an enormous surface, and a spark can make it explode. A hand warmer uses the idea gently: its iron is a fine powder, so it rusts, and gives out its heat, in hours instead of years.

The second is concentration, the amount of a substance packed into a given volume. More particles in the same space means more collisions every second. Stronger vinegar poured onto baking soda fizzes faster, and for many reactions doubling the concentration of a reactant roughly doubles the rate.

Some substances speed up a reaction without being used up by it. Such a substance is called a catalyst. It works by giving the reactants an easier route, one that needs less violent collisions, and at the end it is left unchanged, ready to do the same again. A small amount can therefore process an enormous quantity of reactants.

A car’s exhaust system contains one. Inside the catalytic converter the exhaust flows through a honeycomb coated with platinum and similar metals. The metal takes the worst of the exhaust, poisonous carbon monoxide included, and turns it into safer gases such as carbon dioxide, nitrogen and water vapour.

Living things depend on catalysts completely. The body’s catalysts are proteins called enzymes, and almost every reaction in a cell has its own. Without them, the chemistry of life would be far too slow at body temperature.

Combining with oxygen does not need a flame. A reaction in which a substance combines with oxygen is called oxidation, and most of it happens slowly and quietly. Iron left out in damp air is oxidised over months and years. Iron, oxygen and water together produce the flaky red-brown solid called rust.

Rust is a different substance from iron, weaker and more bulky. It flakes away and exposes fresh metal underneath, so the damage keeps going. That is why steel bridges and ships have to be painted: the paint keeps air and water off the metal.

A fire is the same kind of reaction running fast. Burning is rapid oxidation, giving out its energy quickly enough to make heat and light, where rusting gives out its heat too slowly to notice.

Burning and rusting give out energy as they go. Other reactions run the opposite way: they take energy in, and they stop the moment the supply is cut off.

The most important of them is photosynthesis. In a green leaf, carbon dioxide from the air and water from the soil are built into sugar, with oxygen released as a by-product. The reaction is driven by sunlight, and it stops in the dark. Nearly all the food on Earth begins there.

Electricity can drive a reaction too. Pass an electric current through water and the water molecules are split into hydrogen gas and oxygen gas, a process called electrolysis. It is the reverse of burning hydrogen, which makes water and gives energy out. Splitting the water takes the same energy back in.