If you put a cube of sugar into a saucer of water it will soon break up and disappear into the water. We say the sugar has dissolved in the water.

Dissolving does not cause the sugar to change in any way. It just causes the sugar to break up into microscopic particles called molecules, and as they are now too small to see, they seem to have disappeared. Of course, if you tasted the water, you would still taste the sugar.

All liquids are made of tiny water particles. Many of them are molecules. But the particles simply touch. Think of it as like a pile of balls. There are spaces between the balls. When the sugar touches the water, the sugar particles let go of each other and enter the spaces between the water particles. The sugar and water particles become mixed up. They form a mixture - a solution.

Dissolving is a reversible change. If you evaporate the water, the sugar will reappear. It won't be a cube, but it will be a thin layer of sugar crystals at the bottom of the saucer.

The same happens when you put coffee granules or powder into hot water. The coffee granules are made from particles of a substance which is brown. When coffee dissolves it does not stop being brown. As a result, unlike sugar, which is transparent, the coffee particles make the water brown.

Not everything can dissolve in other things. Take water and oil. They don't mix at all, and the oil floats on the surface. If you shake it up, the oil forms little globules. So oil cannot dissolve in water. But it will dissolve in petrol. Something that makes something else dissolve is called a solvent.

Gases can also dissolve in liquids. There is oxygen gas in water, which is why animals with gills can live in water.

Water will let many things dissolve in it. It is a very good solvent. This is very useful, but it also has its problems. For example, water can easily dissolve harmful substances in it, which is why we have to purify water before drinking it.

Here is a rule: if you can see bits of something in water, it has not dissolved. Mud and sand don't dissolve. If you can no longer see individual bits or globules, then it has dissolved. Salt dissolves.

What this tells you is that if you filter something you do not remove the substances dissolved in it. Just the little bits suspended in it. So when you pour real ground coffee through a filter to remove the tiny bits of bean, you simply make the part of the coffee that dissolved easier to drink.

By the way, water with nothing dissolved in it is called distilled water. Spring water, and other bottled waters, are not pure water. They all have many minerals dissolved in them. They even advertise themselves that way.

Blood has many substances and gases dissolved in it. Blood is mostly water. The red colour is due to the blood cells that are suspended (not dissolved) in the water. The rest is water, dissolved and digested food, salt and carbon dioxide. (The oxygen is not dissolved in it. It is carried by blood cells).

You make something dissolve more easily by stirring it and by heating it. In hot water the particles have more energy and separate the sugar, salt, coffee etc particles more quickly. They also jostle them away more quickly so the speed of dissolving is higher.

Also, remember that if you dissolve something, you add it in an invisible way. But that does not mean it takes up no space. A cup of coffee with sugar in takes up more space (and weighs more) than a cup of coffee without sugar.