Solutes, solvents and solutions

These two glasses contain the same amounts of different liquids. The first one contains water, the second one, oil. Let’s add a spoonful of sugar to each of these substances, stir, and observe what happens. First, the water. After stirring, the added sugar seems to disappear!

But what is really happening? The sugar molecules that make up the sugar crystals interact with the molecules of water. This causes the molecules to separate and spread evenly throughout the water… until the sugar crystals are no longer visible. The sugar has dissolved in the water. Even if we look very closely at the mixture we just made, we can’t easily identify the individual parts.

We say the mixture is homogeneous. A homogeneous mixture is also called a solution. We made this solution by dissolving a small quantity of sugar in a larger quantity of water. Most solutions are made this way — a small amount of a substance, called a solute, is dissolved in a larger amount of another substance, called a solvent. Solutes are substances that dissolve in solvents, forming solutions.

In our case, the solute, sugar, is a solid, and the solvent, water, is a liquid. But all three common states of matter — solids, liquids, and gases — can act both as solutes and solvents. Let’s move on to the next glass, containing oil. Add a spoonful of sugar and stir. What happens?

Nothing really! The ability to dissolve depends on the interaction between the molecules of the substances. Sugar molecules do not interact with the molecules that make up oil. Therefore, sugar doesn’t dissolve in oil. We say that sugar is insoluble in oil, and that it is soluble in water.

This little experiment shows us that the same substance might dissolve very well in one solvent, but not so well in another. Similarly, some solvents can dissolve many different substances, others just a few. Water, for example, is a very good solvent, because it can dissolve many substances. To sum up, a solution is a homogeneous mixture of at least two substances. It is formed by mixing a substance that dissolves — a solute, with another substance — a solvent.

Whether a solute dissolves in a solvent depends on the interaction between the molecules in each of them.