Saturated solutions and solubility curves

If we mix some sugar into a glass of water, the water and sugar will form a homogeneous mixture — a solution. A solution is always made up of at least two substances – a solvent and a solute that dissolves in it. Let’s see what happens if we keep adding sugar to that glass of water. The first few spoonfuls dissolve easily. But eventually, the sugar crystals don’t dissolve anymore.

They sink to the bottom instead. No more sugar can dissolve in this amount of water. The solution is saturated. There is always a limit to how much of a certain solute will dissolve in a given amount of solvent. The maximum amount of solute that can dissolve in a certain amount of solvent is known as the solute’s solubility.

Is there a way to make even more sugar dissolve in this glass of water? Let’s try heating it up and see what happens! Look! The little pile of sugar on the bottom that wouldn’t dissolve before, disappears as the temperature rises. The maximum amount of sugar that dissolves in cold water is different from the amount that dissolves in hot water.

The solubility of a substance can change depending on temperature. Let’s look at this graph. The vertical axis shows the amount of solute per 100 grams of water — from 0 grams to 500 grams. The horizontal axis shows the temperature — from 0 to 100 degrees Celsius. This line here shows the solubility of sugar.

At 0 degrees Celsius, the solubility of sugar is marked as 180 grams. This means that, at 0 degrees Celsius, the sugar water solution becomes saturated once you mix 100 grams of water and 180 grams of sugar. If you add more sugar, it won’t dissolve. But as you increase the temperature, the solubility changes. The amount of sugar that dissolves increases.

At 20 degrees Celsius, it’s about 200 grams, at 40 degrees Celsius, it’s about 220 grams. At 60 degrees Celsius, you can dissolve 300 grams of sugar in 100 grams of water. And at 100 degrees Celsius, it’s nearly 500 grams! Graphs like this one, showing the relationship between the temperature and solubility of a substance, are known as solubility curves. Let’s look at the solubility curve for another substance: table salt.

At 0 degrees Celsius, you can dissolve about 35 grams of salt in 100 grams of water, and at 100 degrees Celsius, about 40 grams. Let’s put this curve on the same graph as the sugar water solution. Now, we can easily compare how much of each substance dissolves at a certain temperature. The curve for sugar starts higher up and rises more steeply, while the curve for salt is nearly flat. From this, we can conclude that sugar dissolves more easily in water than salt.

To sum up, we’ve learned that solubility is the maximum amount of solute that can dissolve in a certain amount of solvent. Solubility changes depending on temperature, which we can show with a solubility curve. By showing more than one solubility curve on the same graph, we can compare the solubility of different solutes in the same solvent.