How much will dissolve?

If you pour some water into a glass and add a few grains of salt you may see them sink through the liquid. Each salt grain is a cube-shaped crystal. Inside the crystal, the particles are holding firmly together. After a while you may see the salt grains disappear. The salt has not really gone. It has just dissolved. When the crystal dissolves, its particles let go of each other and slip into the spaces between the water particles around them.

As you look at the clear water in the glass, you may ask yourself, "How does anyone know when all the spaces between the particles have been filled?" After thinking about this for a few moments you may decide that the only way to find out is to add more crystals until no more will dissolve. When you do this you find that after a time a group of crystals remains in the bottom of the glass no matter how long you leave it. You have reached the point at which no more salt will dissolve because all the available spaces are full.

When a substance such as salt is added to a liquid such as water a mixture is formed which is called a solution. When the solution is absolutely full of the dissolved substance the solution is called a saturated solution. There are some other terms connected with dissolving that it can be useful to know. A substance which dissolves in a liquid is said to be soluble. A substance which does not dissolve is said to be insoluble. A liquid, such as water, in which things dissolve is called a solvent.

Water is sometimes called the universal solvent because so many substances dissolve in it. You may think that all substances will dissolve the same amount in it. If you do, you can check out your idea by performing a fair test. To make your test you need two jars with the same amount of water in them. The water in both jars needs to be at the same temperature and needs to be collected from the same source. For example, you should not use rainwater for one test and tap water for the other. It is best to use tap water for both. People often stir water when they add sugar to their coffee and you may like to feature stirring in your test. If you do, you should make the same number of stirs in each jar or stir for the same amount of time. Finally you should add the same small amounts of each substance to each jar before you stir.

When the test is complete you will find that one substance is more soluble in water than the other. This is because the size of the particles differs. Some particles are more soluble than others because their particles are smaller and pack more tightly into the spaces between the water particles. You may be amazed to find out that there is more to dissolving than just stirring sugar into your tea.

Why is something insoluble in water?

Its particles are held together so firmly that they will not separate when surrounded by water particles. Many substances will dissolve in water although some of them may only be very slightly soluble. Silica from which sand is made is not soluble in water. If it were you would never find sand on a beach.

Are there any other solvents beside water?

Yes, there are. You may be familiar with a very smelly solvent, which is used for removing nail varnish. Another solvent is oven cleaner. This is a very hazardous substance and should only be used by an adult. It dissolves grease. Solvents are also used in the manufacture of other substances such as glues.

Can more than one substance dissolve in a solvent at the same time?

Yes, they can. Water can have many different substances dissolved in it. Look on the side of a bottle of mineral water to see the different minerals it contains. Sea water contains a wide range of substances from common salt to tiny amounts of dissolved metals such as copper and gold.

Why are there so many substances dissolved in seawater?

As rainwater passes through the rocks in the land and then down the rivers, it dissolves small amounts of a wide range of minerals. The water carries these dissolved substances into the sea and then may evaporate. The dissolved substances are left in the sea. There is no way they can return to the land. The water vapour that has formed during evaporation condenses to form clouds. These produce rain when they move over hills and more water carries more minerals to the sea again. This constant repeating of the water cycle has brought large amounts of dissolved minerals into seawater.

Can things dissolve in blood?

Yes, it can. We think of blood as a red liquid but this colour is due to red blood cells. If they are removed it is found that blood is mostly made from a straw-coloured liquid called plasma. This is mostly water. A wide range of substances is dissolved in it, including digested food, salt and carbon dioxide. The large amounts of oxygen that blood carries are not dissolved in it. The oxygen is carried by a substance in the red blood cells.

Don't acids dissolve substances?

Yes, they do, but an irreversible change also takes place. Strong acids dissolve metal. When they do a gas called hydrogen is produced and the metal forms a new substance in the acid. If you evaporate the water in the acid, the metal does not reappear because it is now part of the new substance.