Chemistry Unit 3 Topic 3

C3-3 : Water & Solubility

The Water Cycle
The Sun supplies rivers, lakes and oceans with energy, allowing the water there toevaporate. The water vapour formed rises into the atmosphere, where is cools and condenses to form droplets which clouds are made of. Eventually the water droplets fall as rain, replenishing the water sources they originally came from. This is the water cycle. Water covers around two thirds of the Earth’s surface, and is absolutely essential to all life.

Solubility
We call the amount of solute which we can dissolve in a certain amount of solvent thesolubility of that substance. This is usually measured in grams (of solute) per 100g (of solvent). The most common solvent used is water. Generally speaking, solubility of solidsolutes increases with temperature. A saturated solution is a solution in which as much solute as possible has been dissolved. Heating the solution will allow more of the solvent to be dissolved until it becomes saturated again. Of course, this means when the hot saturated solution cools, some of the solute will have to come back – so itcrystallises back out of the solution.

We can show the different amounts of solute which will dissolve into solution at different temperatures using special graphs called solubility curves. These can be used to a) predict how much solute will dissolve into a solvent at any given temperature, and b) predict how much solute will form again when we cool down a hot solution.

The solubility curves for potassium nitrate, sodium nitrate and sodium chloride are shown here. As you can see, the solubility of each one increases with temperature as the rule states – but the rate of increase differs between solutes. As you can see, sodium chloride barely increases in solubility between 0°C and 100°C, whereas potassium nitrate increases eightfold in the same period.

The thing that all of these solutes have in common is that they are all solid solutes. The solubility of gases works in exactly the opposite way – as temperature increases, the amount of solute which will dissolve into solution decreases. However, pressure is another factor affecting the solubility of gases. So gas solubility only decreases with temperature as long as the pressure is kept constant – but if temperature is kept constant – solubility of gases increases as pressure increases.

The solubility curve here (note: solubility curves may be straight lines) shows the solubility of oxygen in water at 10°C. Temperature has to be kept the same here, because if temperature is not kept constant, it has a knock-on effect of the changing solubility. We measure the pressure in atmospheres.

The Importance of Solubility
There are a large number of reasons why we need to know about the solubility of solvents. For example, rivers, lakes and reservoirs are contaminated by chemical fertilisers on crops which are dissolved by rainwater – so the nitrate levels of the water…