Soluble and insoluble
You may stir sugar into your tea or coffee without giving it much thought. Just for a moment let's think about what is going on in your drink. You cannot see because the tea or coffee is colouring the water. To investigate you would need to stir the sugar into just plain water.

If you dipped a spoon slowly into the liquid you could watch the process of dissolving taking place. The sugar that is dipped under the water would quickly disappear and the sugar above it would crumble and tip into the water too. If you had got eyes like powerful electron microscopes, you would be able to see the particles in the sugar crystals and in the water. As you looked at a sugar crystal you would see the particles stacked one above the other like cars in a multistorey car park. When you looked at the water, you would see particles sliding over each other but you would also see spaces between the particles like holes in a sponge. When a sugar crystal dips into the water, you would see its particles separate from each other. They would enter the gaps between the particles of water.

At first you would see the sugar particles in the water around the crystals. In time they would spread throughout the liquid. If you stirred up the water, you would see how this helps dissolving. It moves water containing sugar particles away from the sugar crystals. This water is replaced by water, which does not have sugar particles in it. Its gaps are free to take up any more sugar so the crystals continue to break up and dissolve.

Stirring, then, brings fresh supplies of water to the crystals to help them dissolve. When a substance dissolves it is not destroyed. It is just broken up into tiny particles that we cannot see. Just think about these two facts next time you give your drink a stir.

Is there a limit to how much sugar can be dissolved in water?
Yes, there is. Water will continue to dissolve sugar until all the gaps between the particles are full. You could think of the sugar water like a soaking sponge. We say a soaking sponge is saturated with water. A liquid which cannot dissolve any more of a substance is also called saturated. It is called a saturated solution.

What is a solution?
A solution is a liquid with a substance dissolved in it. A liquid, which dissolves a substance, is called a solvent.

Aren't solvents dangerous?
You may be thinking about the solvents in some glues. They are made from chemicals, which can damage the brain and other parts of the body, if they are inhaled too frequently. Glues with these kinds of solvents are only sold to adults. There are many other kinds of solvents which are much safer. The most widely used solvent is water.

What happens if a solvent cannot dissolve any more of a substance?
The substance just sinks to the bottom of the liquid. If it is a crystal, like sugar, it just remains as crystals in the liquid.

Can a liquid dissolve another liquid?
Yes, it can. This is what happens when you dilute a fruit drink. The particles of the fruit juice spread out into the gaps between the particles of water. Some liquids do not dissolve in another. For example, if you stir some oil into water, you will see shining, golden droplets form. When you stop stirring, you will see the droplets stick together to form larger globules of oil. These, too, will stick together to form larger balls of oil. Eventually all the oil joins up to form a layer on top of the water. The oil will not dissolve in the water.

Do all substances that don't dissolve in water float on it?
No, Oil floats on water because it is lighter in weight than water. Most substances that do not dissolve in water are heavier than water and sink below it. If you mix sand and water you will find that the sand is insoluble and forms a layer at the bottom of the container. This layer is called a sediment.

When a little clay is mixed with water, it makes the water yellow. Has the clay dissolved?
No. Clay is made from tiny particles that we can just see. They spread out in the water and seem to float for a while. In fact they are slowly sinking but because they are so small any upward current in the water will make them rise. The clay particles are described as being suspended in the water and are said to form a suspension. Over a few hours they will eventually sink and form a sediment.

If a substance dissolves in one solvent will it dissolve in them all?
Substances that dissolve in water tend not to dissolve in some other solvents such as white spirit. White spirit is used as a solvent for paint.

Can a solvent dissolve more than one substance?
Yes. Water can dissolve a large number of substances. It can even dissolve many substances at once. Sea water contains many different minerals. They have been dissolved out of rocks by rainwater and swilled into the sea by streams and rivers.