Not every change makes something new. Melting wax is a physical change. So is dissolving sugar in coffee. A physical change does not make new substances. A chemical change does. Iron that sits in air and water turns to rust. Sorting a change into one of these two groups depends on whether a new substance appears.

Salt dissolves in water. That makes a solution. The parts of a solution are mixed so evenly that the solution is alike all the way through. The salt and the water are still there, mixed evenly.

Gas bubbles are one clue. Baking soda and vinegar fizz, and the bubbles are carbon dioxide, a gas. Bubbles of gas are also why cakes and bread rise. Gas bubbles inside the cake or the dough lift it up. In both cases, a gas does the work.

Acids and bases can be measured. On the pH scale, each step is a tenfold change. That means water with a pH of 5 is ten times more acidic than water with a pH of 6. The steps are not equal in size. Each one multiplies the strength by ten.

Many plants besides red cabbage can make an indicator. Berries and beetroot are two of them. An indicator shows one colour in an acid and a different colour in a base. So the colours that tell acids from bases are not found in only one plant.

Egg yolk holds the oil and water of mayonnaise together. Egg yolk has a molecule called lecithin. One end of the molecule likes water and the other end likes oil. That lets it hold tiny drops of oil in water. The egg yolk is what keeps the oil and water from splitting apart.

Soap molecules also have two kinds of ends. They have a water-loving end and an oil-loving end. In water they bunch into groups called micelles. The oil-loving ends are tucked inside each group. So a micelle is a tiny ball with the water-loving ends on the outside.

Heat and cold can be clues too. Heating or cooling can be a sign of a reaction. Some reactions give off energy, and others take energy in. If a mixture gets warmer or colder by itself, a reaction may be at work. It is one clue among others, along with gas bubbles.

When things mix or react, the total amount of matter stays the same. Nothing is lost. A chemical change does not create or destroy atoms, and it rearranges them into new substances. So even when a reaction makes something that looks new, it is made from the same atoms.

The French chemist Antoine Lavoisier lived from 1743 to 1794. He showed by careful measuring that the mass of the products of a reaction equals the mass of what went in. He weighed everything while he heated metals in closed vessels. The mass the metal gained matched the mass the air lost. His scales showed that nothing disappears in a reaction.

Words to know

• physical: no new stuff
• solution: evenly mixed
• micelles: soap balls
• indicator: colour clue
• mass: amount of matter