Soluble or insoluble solids in water
Some solids are soluble in water while some are insoluble in water. What do we mean by that?
When sand is added into water, sand do not dissolve in water. If we leave it to stand, sand will settle at the bottom of the water. We can separate sand from water using a physical method, called filtration.
When copper(II) sulfate is added into water, it dissolves to form a blue solution. We say that copper(II) sulfate is soluble in water. When a salt is soluble in water, we cannot simply filter to separate the solute from the solvent. We will have to carry out crystallation.
Separating sand from copper(II) sulfate solution
Now let’s say we have a mixture of sand and copper(II) sulfate solution. There are 3 components in this mixture – we have sand, copper(II) sulfate salt and water.
We can first start by carrying out filtration to separate the sand from copper(II) sulfate solution.
We will pour the mixture through the filter paper. There are small pores on the filter paper. The size is so small, only copper(II) sulfate solution can pass through. The sand solid particles that are bigger than the pores of the filter paper will not be able to do so.
Upon filtration, sand will remain in the filter paper. We call this the residue. The component that passes through the filter paper is called the filtrate. In this case, copper(II) sulfate solution is the filtrate.
Crystallisation to obtain copper(II) sulfate salt.
We know that many salts are soluble in water. [Link to salt solubility video]. Copper(II) sulfate salt is one of them. To obtain the pure solid salt from the solution, we carry out crystallisation.
Crystallisation is a physical method used to separate a soluble salt from a liquid.
We heat the filtrate until it is saturated. Saturated solution simply means that this solution cannot allow anymore solid to dissolve in it. In other words, it is a solution in which no more solute can be dissolved in the solvent.
To test whether a solution is saturated, we will dip a clean glass rod in the solution. As the solution cools, saturated solution should leave small crystals on the rod. If not, we will need to continue heating until it is saturated.
Still, we need to be careful in this step to ensure we do not overheat the solution. If we overheat, we will end up with white solid instead of blue crystal. This white solid is anhydrous copper(II) sulfate.
When the solution is saturated, we leave it to cool. Do not disturb the process by stirring or shaking the mixture. Crystallisation will occur. You will see very pretty blue crystals forming on the evaporating dish.
Filter to collect the crystals. To remove the remaining impurities, we will wash with a little amount of cold water. We use cold water because copper(II) sulfate salt is soluble. We want to avoid dissolving it again.
Lastly, we dry the crystals between filter paper.
Tada, there you go!
In a nutshell, to separate an insoluble solid from a liquid, we can simply carry out filtration. To separate a soluble salt, which we call the solute, from the solvent, we will carry out crystallisation.