Filtration is a way of separating a mixture of an insoluble solid and a liquid. Because the solid never dissolved, its grains are still whole inside the liquid — so we can strain them out, a bit like using a sieve with unbelievably small holes. Use it whenever the solid does not dissolve: sand in water works, but salt in water does not.
The solid that gets stuck on the paper is called the residue. The liquid (or solution) that makes it through is the filtrate. In this experiment the sand is the residue, and the blue copper(II) sulfate solution is the filtrate.
Filter paper is covered in microscopic gaps called pores. Water molecules and dissolved particles are far smaller than these pores, so they slip through easily. A grain of sand is millions of times bigger than a pore, so it cannot fit — it stays behind. Filtration is really a sorting machine based on particle size.
Fold the circle in half, then in half again to make a quarter. Pull one flap open and you get a cone that sits snugly inside the funnel. A neat cone matters: if the paper tears or gaps, solid sneaks past and the filtrate is spoiled.
A coffee machine is a filtration experiment you can drink. The ground coffee beans are the residue trapped in the paper; the brewed coffee dripping into the pot is the filtrate. Sieves, vacuum-cleaner bags and aquarium filters all work the same way.
At a water treatment works, reservoir water passes through a series of filters with decreasing pore size — coarse screens catch leaves and litter, then finer beds trap smaller and smaller particles. Chlorination is added afterwards to kill bacteria, because microbes are too small for ordinary filters.
NEWater goes further: reverse osmosis pushes used water through a membrane with pores so tiny that essentially only water molecules squeeze through — an ultra-fine filtration that removes even dissolved substances. Ultraviolet (UV) light then sterilises the water as a final safety step.
Once a solid dissolves, it splits into particles about the same size as the liquid's own particles. These sail straight through filter paper — which is exactly why our filtrate is still blue: the copper(II) sulfate is still in there, dissolved. To get a dissolved solid back you need a different method, such as evaporation or crystallisation.
| Term | Meaning |
|---|---|
| mixture | Two or more substances mingled together but not chemically joined. |
| residue | The insoluble solid left behind on the filter paper. |
| filtrate | The liquid or solution that passes through the paper. |
| insoluble | Unable to dissolve in the liquid. |