How can anyone tell that a reaction happened? A color change, gas, a new solid and a temperature change are only hints, and the one sure test is to compare the substances after the change with those before it. Melting wax and sugar stirred into coffee are physical changes, which leave the substances as they were, whereas a chemical change produces new ones. Red cabbage juice shows how a color change can work. Add something and the juice can change color, showing a different shade in an acid than in a base, so it qualifies as a natural indicator; it starts out purple, turns pink and on to red with acids, and moves through blue and green on the way to yellow with bases. A solid that suddenly forms when two clear liquids meet, known as a precipitate, is one sign of a reaction; gas bubbles can also hint that mixing made something new.
Salt dissolves in water. Water is called the universal solvent, though it cannot dissolve everything: no other liquid dissolves more substances, yet oil is among those it leaves alone. Oil and vinegar form two layers, since oil floats above the water, and shaking holds them together only for a short spell. Egg yolk changes that; it holds oil and water together as mayonnaise because a yolk molecule named lecithin has one end drawn to water and another drawn to oil. At sea, waves can churn oil and water into an emulsion, and that is one reason spills are hard to clean up. Dissolving can even absorb heat. Some solutions form despite taking in heat, because the mixture grows more disordered, and an instant cold pack turns cold since ammonium nitrate dissolving in water takes heat in.
Chemists track heat with a sign. Heat going out is exothermic and its enthalpy change is negative, while heat going in is endothermic and its enthalpy change is positive. A change in temperature can itself be a clue, because some reactions release energy and some absorb it; some reactions are reversible and can run in either direction, and at equilibrium the forward and reverse rates are equal, so the amounts of each substance stay constant. Warmer conditions generally speed reactions up; fine pieces of a solid react faster than a single large piece of the same substance, since more surface touches the other substance, and a higher concentration of a reactant usually raises the rate as well. A catalyst helps a reaction along without being used up, and enzymes are the catalysts of living things, raising reaction speeds by factors of 100 million to 10 billion.
A catalyst helps a reaction along without being used up, and it need not shorten the route: the catalyzed reaction may have the same number of steps as the uncatalyzed one, but it must provide a path whose slowest step is faster, because that rate-determining step sets how quickly the reaction goes. Enzymes are the catalysts of living things, and each has a preferred window of temperature and acidity. Mild warmth tends to speed their reactions, yet excess heat or an extreme pH alters the enzyme's shape until it stops working well, a change named denaturing. Chemists can now steer enzymes with evolution. The 2018 Nobel Prize in Chemistry honored Frances Arnold for the directed evolution of enzymes, and enzymes made this way are used to manufacture everything from biofuels to pharmaceuticals.
The reaction behind a flame is combustion, a chemical reaction that usually draws on oxygen and releases heat and light. It accelerates itself: more heat is made than can escape, so the reacting material warms up and the reaction runs faster. A flame also needs heat to keep going, and Britannica explains that a burning match goes out when wind carries the heat off, or when the chemicals are moist and the heat falls short. Most flames are more complicated than simple oxygen-plus-fuel, though. Britannica notes that combustion includes other kinds of reactions as well, and that most flames have a region where reduction reactions occur instead of oxidations. Oxidation began as a word for combining with oxygen, but chemists later stretched it to cover every reaction where atoms lose electrons, so combustion counts as one kind of oxidation even though the wider meaning can do without oxygen.
Cooking changes food, and heat changes an egg: its proteins uncurl and bond with each other into a network that holds water, and a white kept at high heat for too long becomes rubbery. Cakes and bread rise because gas bubbles form inside them. Baking soda is a base, vinegar holds an acid, and the pair give carbonic acid, which releases carbon dioxide when it breaks down. Baking powder already includes a dry acid alongside the baking soda, so wetting it starts the fizz, while bread dough gets its gas from yeast, which releases carbon dioxide during fermentation. The Maillard reaction, which happens during baking, is the source of the smell and taste of bread's brown crust, and caramelization, which is what happens first when sugar is heated and turns brown, makes caramelized onions sweet and nutty. Different cleaning products should never be combined, since combining them can create fumes that are poisonous and difficult to breathe.
Corrosion is the slow chemical wearing away of metals, and rust on iron is one example. Iron rusts when oxygen and water reach it, producing a new kind of matter, and the process runs by way of a galvanic cell on the iron surface; left in air and water, iron turns to rust, and rust is a different substance from iron, since the iron has combined with oxygen and water. Silver tarnish and copper's blue-green patina count as corrosion too; some metals shield iron by corroding in its place: galvanization coats iron with a more easily oxidized metal, usually zinc. Coating iron with grease, paint or plastic also prevents rust, because the coating separates the iron from air and water. As of November 2026, corrosion in the United States is estimated to cost more than half a trillion dollars a year, according to a university chemistry textbook.
Before the chemical revolution, many natural philosophers still saw earth, air, fire and water as the basic substances of all matter. Lavoisier and his peers studied compounds and hoped that identifying simple substances would let them explain the properties of compounds. Burning had its own explanation in the phlogiston theory, which said that burnable things such as charcoal and metals held phlogiston, a substance that left them for the air as they burned, and a metal that burned became a calx. The theory was accepted for more than 100 years, and it fit the known evidence until better evidence replaced it. In the 1770s Lavoisier proposed a theory of combustion that led to the discovery of oxygen and ended phlogiston theories, a change so large that it is called the Chemical Revolution.
Lavoisier's wife, Marie-Anne Paulze Lavoisier, worked alongside him as the two showed that burning and breathing both need a newly isolated kind of air, which he named oxygen. He also proved that water is made of hydrogen and oxygen, which contradicted the ancient idea that it was one of the basic elements. He did more than propose a theory about oxygen: he developed an experimentally based theory of oxygen's chemical reactivity and coauthored the modern system for naming chemical substances, so that chemists could describe reactions in a shared language. Philosophers often use the Chemical Revolution as an example of a scientific revolution, and it features in Thomas Kuhn's influential book The Structure of Scientific Revolutions.
Lavoisier is credited with the law of conservation of mass, though he was certainly not the first to accept or teach it; earlier thinkers had speculated about it and even run some measuring experiments. His belief that mass is conserved started as an assumption shared by Enlightenment investigators, not as a result that experiments had revealed. He set out to show his experiments did not break it, though he had difficulty proving it was universally valid, and the episode shows that a principle can guide experiments before it is proved. What he added depended on technology, an accurate and precise balance, because measuring the mass of the metal, the calx and the surrounding air turned chemistry into an exact science based on careful measurement.
Careful weighing gave the principle its evidence. Lavoisier's measuring showed that what comes out of a reaction matches, in mass, what went in, and metal heated in closed vessels gained as much as the air lost. Beer is made from yeast, hops, malt, grains, sugar and water, and no substance is actually lost in the change. In a chemical change atoms get regrouped into new substances, never created or destroyed, so a balanced equation must show the same number of atoms of every element for reactants and products. Whenever substances are mixed or react, matter in total stays unchanged.
Words to know
- equilibrium: steady balance
- caramelization: sugar browning
- corrosion: metal wear
- reversible: runs both ways
- fermentation: yeast making gas
