What is dissolving?

If you put a cube of sugar into a saucer of water you can watch the sugar collapse. It breaks up and disappears into the water. In a way it looks a little like a piece of candle wax that has become too hot and has melted. Some people say that sugar melts into the water but they are wrong.

Melting and dissolving are two different processes. To understand how these processes work we have to look deep inside the substances where the processes are taking place but first we have to think about the different forms that substances can take. All substances can exist in three different states. We call them the states of matter. There is the solid state, liquid state and gas or gaseous state. Not only can a substance exist in three different states it can also change from one state to another. The process of changing a solid to a liquid is called melting. It is a reversible reaction because no new substance has been made. If the liquid is cooled it freezes to make the solid again. Dissolving does not cause a substance to change its state. It allows a substance to mix very closely with another substance.

If you could look deep inside any substance you would find that it is made from particles. These particles are so small that they can only be seen by powerful microscopes called electron microscopes. In a solid substance the particles hold onto each other firmly. They are stacked one on top of another and each stack holds onto the next. The way these particles are arranged makes the solid have a fixed shape. In a liquid the particles are close together but are free to move. They slide around each other and this allows the liquid to flow. In gases the particles do not stay close to each other. They move quickly and bounce about. They hit each other frequently but then move off again to take part in more collisions.

When a solid is heated, the energy it receives makes the firmly held particles shake about. This shaking is called vibrating. If the solid gets hot enough, the particles shake themselves loose and flow around each other. When this happens, the solid has turned into a liquid. It has melted.

If we look at the sugar cube in the water we can see that something different is taking place. The particles in the sugar cube are held firmly together and the particles in the water are sliding round each other. At the place where the sugar cube dips into the water, the sugar particles and the water particles meet. There are spaces between the water particles and they are vital to the dissolving process. 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, which we call a solution.

Are the particles in a substance the same as particles of sand or clay?

No. The particles from which all substances are made are much smaller. Millions of them would make the smallest grain of sand that you could see. Some of these particles are atoms and others are groups of atoms called molecules. They are all incredibly small and incredibly numerous. For example you are looking through billions of air particles to read the words on this screen.

Has the sugar changed into a liquid in the water?

No. Its particles have just separated and mixed with the water particles. They have partially lost their grip of each other and are sliding round each other. They have moved away from each other and gone among the water particles. Sugar can be changed into a liquid if it is heated. When this happens, the sugar crystals lose their shape and form a sticky liquid.

Does the sugar change into another substance when it dissolves?

No. All substances have properties. For example, sugar has a sweet taste. When a substance dissolves it does not lose its properties. If you dissolve some sugar in water in a clean cup, then sip the water, you will find that the water tastes sweet. The dissolved sugar still has its sweet-tasting property.

Why do coffee granules make water brown?

The coffee granules are made from particles of a substance which is brown. When that substance dissolves it does not lose this property. It keeps it and makes the water brown.

Can the particles of one liquid mix with another?

Yes, they can. If you add water to orange juice you mix two liquids. When two liquids mix together they are said to be miscible. Some liquids, however, do not mix. If you stir water and oil together the oil eventually floats on top of the water. When two liquids do not mix they are said to be immiscible.

Can gases dissolve in liquids?

Yes, they can. If they didn't there would never have been any life on Earth. Water life needs oxygen just like life on land. The first forms of life developed in water. They could do this because oxygen dissolves in water. When you watch a goldfish move its gill covers, it is letting water flow over its gills and taking oxygen from the water.