Dissolving adds volume

When you add a spoonful of sugar to a drink, you not only make your drink sweeter you make it larger. This may seem odd but when you think about if for a moment it does make sense.

If you take a jar of currants and a jar of peanuts and pour the currants into the peanuts the currants scatter all over the table. They do this because the peanuts are filling the whole space in their jar and there is no room for the currants. When you have put the currants back in their jar you could try another experiment. This time you need a tall jar such as a spaghetti jar. When you pour the peanuts into the spaghetti jar, you could note the level at which the nuts come to rest. This time, when you pour the currants on the nuts, the currants do not scatter on the table. They pile up above the peanuts because there is room in the jar. If you have stored the nuts and currants in jars of the same size you will see that the volume of the mixture is about twice the volume of the nuts on their own. When you shake the spaghetti jar the level of the mixture may sink a little as the currants and peanuts pack down together but the volume of the mixture will not change much.

The peanuts, currants and spaghetti jar provide a model of what happens when sugar dissolves in water. Think of the peanuts as particles of water and the currants as particles of sugar. All particles, whether they are in a solid, a liquid or a gas, take up space. When sugar dissolves its particles separate and go into the water. The particles do not shrink. They stay the same size and still need space in which to exist. They find this space between the particles of the water. If you shake up the spaghetti jar, you will see the peanuts and currants mix but they still take up space. Sugar and water particles do exactly the same and as more sugar dissolves in the water, its particles require more space. This results in the sugar particles pushing on the water particles. The water particles have nowhere else to go but upward so the level of the solution rises.

The increase in size of the solution depends on the solubility of the substance that is dissolving. Sugar is very soluble in water. This means that a large amount of sugar can dissolve in any volume of water before the water can take no more. When this happens we say that the mixture of sugar and water is a saturated sugar solution. If a person keeps a check on the volume of the solution, as more and more sugar is added it will be found that the level of the solution steadily and greatly increases.

Salt is much less soluble in water than sugar. Only a small amount dissolves in water before the mixture becomes a saturated salt solution. This means that the level of the salt solution only rises a little to accommodate the extra particles.

Next time you put sugar in your coffee or salt on your soup think about how their volumes change. Both changes are very tiny but very real because every particle has to be somewhere and in that place it takes up space.

What is a scientific model?

It is a model that is used to try and help people understand a scientific process. Anything can be used in a model as long as it fits in with the process being studied. Peanuts and currants are suitable to use in a model about particles because they are roughly the same shape as particles although millions of times larger. When using a model it is vital to remember what the items in the model represent.

Do scientists use models?

Yes, they do. They use them to try and explain their discoveries. The idea of substances being made of particles came from a model. Scientists thought they could explain the difference between solids, liquids and gases by thinking of them as being made of tiny ball-shaped particles. At that time the scientists did not have very powerful microscopes so they had no way of knowing whether their model was correct. All they had was the knowledge that their model of substances being made from particles seemed to explain how solids, liquids and gases were made and how a substance changed from one state to another. It was much later, when electron microscopes were invented, that scientists could see that substances really were made of particles.

When a substance changes from one state to another does it change into another substance?

No, it does not. Water exists in three states of matter. They are solid water, called ice, liquid water and gaseous water which we call water vapour. When it melts, freezes, evaporates or condenses it is still water.

What is a solution?

It is a mixture of a liquid and a dissolved substance. The dissolved substance can be a solid such as sugar or a gas such as carbon dioxide.

What is a saturated solution?

It is a solution in which the dissolving substance has taken up all the space. There is no room for any more particles from the dissolving substance.

How do you know when a solution is saturated?

When you add some of the substance to it and the substance will not dissolve. For example, if you add a spoon of salt to a saturated salt solution, the crystals will just settle at the bottom of the jar. They will not dissolve.