What happens when vinegar meets baking soda? It fizzes, and there is a reason for that. Vinegar has an acid in it, and baking soda is a base. Acids and bases react. You can find acids in other places too. Acids are also why citrus fruits, and lemon juice, taste sour. So lemons and vinegar both taste sour for the same reason. When vinegar meets baking soda, the two of them react, and the fizz is what you see.

The two things have other names. The sour acid in vinegar is called acetic acid. Baking soda is also called sodium bicarbonate. When baking soda reacts with an acid, it makes carbonic acid. Then it breaks down, and it gives off carbon dioxide gas. That gas makes the bubbles. So the fizz goes in steps. First the new acid forms. Then it breaks down, and then the gas shows up as bubbles.

Scientists sort acids and bases with a number scale. The pH scale runs from 0 to 14. Seven is neutral, lower numbers are acids, and higher numbers are bases. So a pH of 2 is an acid and a pH of 12 is a base, but a pH of 7 is neither one. The lowest number is 0 and the highest is 14, so 7 sits right in the middle, and you can picture a line of numbers, with the acids at the low end and the bases at the high end.

Can you see whether something is an acid or a base? Red cabbage juice can show you. It shows one color in an acid and a different color in a base. This is why it counts as a natural indicator. An indicator is a way to find out which kind you have. The juice does the telling, because it changes color when something is added to it. One color means an acid, and another color means a base.

Salt dissolves in water because of the shape of water molecules. A water molecule has a slightly positive side and a slightly negative side. Those sides pull the salt apart, and then the salt is in the water. Oil is different, because it does not dissolve in water. Water molecules cling to each other, and oil molecules are not polar enough to pull them apart. So the water holds on to itself, and the oil stays separate from the water.

What if two liquids will not mix? An emulsion is a mix of two liquids that do not dissolve in each other. In it, tiny drops of one are spread through the other. Emulsions are everywhere, and milk and mayonnaise are emulsions, and so are many cosmetics. So milk has tiny drops spread through it, and so does mayonnaise, so each one has tiny drops inside it. In each of them, the drops are small.

Some changes make a whole new kind of matter. Rust is one of them. Rust is no longer iron. It is a new kind of matter that iron makes with oxygen and water. The iron has become something else. So rust and iron are two different kinds of matter. It takes iron, oxygen and water to make rust, and what comes out of that is not iron at all. Rust is a good example of what a chemical change can do.

Here is another sign of a reaction. Mix two clear liquids. If a solid suddenly appears, it is called a precipitate. Think about what went in and what came out, because that is how you can tell. Two clear liquids went in, and a solid came out, so the solid is the sign. The solid is new, and it was not in either of the clear liquids that went in.

Changes like these also happen inside your own body. Enzymes are special molecules that speed up chemical reactions in living things. Amylase, one of these enzymes, helps us digest carbohydrates. So a reaction does not have to happen in a bowl or a cup. It can also happen in a body, and it can happen in any living thing, not only in a person. Living things use enzymes to make theirs go faster. It is just one of them.

It is fun to read about these changes, but some are not safe to try. Never mix cleaning products. Poisonous fumes that are hard to breathe can come from mixing them. That is why cleaning products are for grown-ups to handle. Bubbles are one thing, and fumes that are hard to breathe are another. If you want to see bubbles at home, ask a grown-up to be with you.

Words to know

• acid: sour kind
• indicator: acid tester
• emulsion: drops in liquid
• enzymes: reaction speeders
• neutral: in the middle