A solution has parts, and chemists have names for them. The solvent is the part present in the greater amount, while the solutes are the parts present in smaller amounts. Almost any gas, liquid or solid can act as a solvent, so water is only one of many possibilities. When sugar dissolves in water, its molecules spread evenly among the water molecules. Gravity does not make them settle out over time, which means the sugar stays spread through the water instead of sinking to the bottom.

Water is good at pulling salt apart, and the reason is its shape. Its molecules are polar: one side is slightly positive and the other slightly negative. That lets water attract the charged sodium and chloride in salt. Dissolving can also change the temperature. Some salts take in heat when they dissolve, and an instant cold pack works this way. Dissolving ammonium nitrate in water is an endothermic process, so the pack gets cold. The heat that the salt takes in comes out of the pack, and that is why it feels cold.

Chemists can speed up a reaction in several ways. A higher temperature typically makes chemical reactions go faster, so a warmer mixture usually reacts sooner than a cooler one. Size matters too. A finely divided solid reacts faster than one large lump of the same thing, because more of its surface is in contact with the other substance. A powder has far more surface touching the other substance than a single lump of the same material has.

Amounts matter as well. Reaction rates usually go up when the concentration of one or more of the reactants goes up, so more of a reactant usually means a faster reaction. Then there is the catalyst. A catalyst speeds up a reaction without being used up by it. A catalyst helps the reaction go, and it is still there when the reaction ends, ready to be used again.

Red cabbage juice gives a colour chart, not just a yes or no. It starts out purple, which is the colour it has before you add anything. Acids turn it pink and then red, while bases turn it blue, then green, and finally yellow. Red cabbage juice can change colour when you add something to it, and what you add decides which colour it turns. Each colour along the chart is a step in the change from purple toward red on the acid side, or from purple toward yellow on the base side.

Chemists write reactions as equations. A balanced chemical equation has equal numbers of atoms of each element on the reactant side and the product side. That fits the law of conservation of matter. Atoms are not lost on the way from the reactants to the products. The equation is balanced when every element has the same count of atoms on both sides, and the count on the left matches the count on the right.

Cooking changes food, and an egg is a good example, because heat changes an egg. Heat makes egg proteins uncurl and bond into a network that traps water. The proteins join together, and the network holds the water inside it. An egg white left too long at a high temperature gets rubbery. So the same heat that sets the egg can ruin it when it goes on too long.

Bread dough is another kind of cooking chemistry. In bread dough, yeast releases carbon dioxide gas during fermentation. The gas makes the tiny air bubbles in the dough expand. Cakes and bread rise because of bubbles of gas inside them, so the growing bubbles are what lift the dough. Fermentation is the name for the process in which the yeast gives off that gas.

Some mixes will not stay mixed. Oil and vinegar will not stay mixed, and they split into two layers. Because oil floats on top of water, shaking the two together still leaves two layers. Shaking oil and water mixes them only for a short time, and then they separate again. A shaken jar of oil and water might look blended for a moment, but the layers come back.

Some changes can be kept away. Iron turns into rust after it sits in air and water. A coating of paint, grease or plastic keeps rust away, because it keeps air and water off the iron. The coating does not change the iron. It works by standing between the iron and the air and water that would otherwise turn it to rust.

Words to know

• solvent: the part dissolving
• solutes: dissolved parts
• catalyst: reaction speeder
• endothermic: takes heat in
• fermentation: yeast gas making