Not every mix is the same. Stir salt into water and the salt dissolves. That makes a solution. In a solution, the parts are mixed so well that it is the same all the way through. Oil and water are different. Pour oil onto water and it floats on top. Shake them and you get two layers. They mix for a short time, and then they separate again.

Two liquids may not dissolve in each other, but they can still be held together. An emulsion is a mix like that. Tiny drops of one liquid are spread through the other. Mayonnaise is an emulsion too. It is oil and water, held in place by egg yolk. The job is done by lecithin, a molecule in the yolk. One end likes water, and the other end likes oil. So it holds tiny drops of oil in water.

A reaction makes something new, and it can leave signs behind. Bubbles of gas are one sign. Mix baking soda and vinegar and they fizz. The bubbles are carbon dioxide, a gas. A solid can also give it away. Say two clear liquids are mixed, and then a solid suddenly appears. That solid is called a precipitate. It is a sign of a reaction.

Vinegar contains an acid, and baking soda is a base. When acids and bases meet, they react. The pH scale runs from 0 to 14, and seven is neutral. A number below seven means an acid, and a number above seven means a base. Lemons and other citrus fruits taste sour because of their acids.

Red cabbage juice can change color when you add something to it. In an acid it shows one color, and in a base it shows a different one. That makes it an indicator. Cabbage is not the only plant that works. Berries and beetroot are two others, and there are many more. With a grown-up, you could test an indicator on things in the kitchen.

Rust is a reaction. Iron left in air and water will turn to rust. Once it has rusted, it is no longer iron. Rust is a new kind of matter, made when iron joins with oxygen and water. Even so, when things mix or react, the amount of matter stays the same. Nothing is lost. All the matter that was there before is still there.

Cooks mix things all the time. Baking soda in cake batter reacts with a sour thing, such as lemon juice. This makes gas bubbles, and the batter rises. Bubbles of gas inside cakes and bread make them rise. Cooking changes food in other ways too. Heat changes an egg. Cleaning products are another story. Never mix cleaning products. Together, they can give off poisonous fumes that are hard to breathe.

Words to know

• solution: an even mix
• emulsion: spread-out drops
• precipitate: new solid
• indicator: a color clue
• neutral: in the middle

The short version

Some mixes only blend, like salt in water, and others are reactions that make something new. Gas bubbles and a sudden solid called a precipitate are clues that a reaction happened. When things mix or react, the amount of matter stays the same.

Quick facts

• Salt dissolves in water to make a solution that is the same all the way through.
• Mix baking soda and vinegar and they fizz. The bubbles are carbon dioxide, a gas.
• The pH scale runs from 0 to 14, with seven neutral, acids lower and bases higher.
• An emulsion is a mix where tiny drops of one liquid are spread through another.
• Never mix cleaning products, because they can make poisonous fumes.