Filtration is a laboratory technique used to separate an insoluble solid from a liquid. It matters because many chemical reactions produce mixtures that must be purified before the product can be tested, weighed, or used. A filter paper or porous barrier traps larger solid particles while the liquid passes through.
The trapped solid is called the residue, and the liquid that passes through is called the filtrate.
Gravity filtration uses the weight of the liquid to pull it through folded filter paper in a funnel, so it is simple and gentle. Vacuum or Büchner filtration uses reduced pressure below a flat filter surface to pull liquid through faster, making it useful when collecting a solid product. Hot filtration keeps the mixture, funnel, and filter paper warm so unwanted insoluble impurities can be removed without crystallizing the desired solute too early.
Choosing the right filtration method depends on whether the goal is to collect the solid, collect the liquid, prevent crystallization, or save time.
Understanding Chemistry: Filtration Techniques
Filter paper works because it contains a tangled network of tiny spaces between cellulose fibres. Liquid can move through these spaces, but particles larger than the spaces are held back. Very fine particles may initially pass through and make the liquid cloudy.
As more solid collects, it forms a layer called a filter cake. This layer can trap smaller particles, so the liquid often becomes clearer after the first few drops. A thick filter cake eventually slows the process because the liquid must force its way through both paper and solid.
In gravity filtration, good setup prevents spills and loss of sample. The paper should fit the funnel closely, with no open channel around its edge. Wetting the paper with a little of the same solvent helps it stick to the glass.
A glass rod guides liquid from a beaker into the funnel and stops it running down the outside. The liquid level should stay below the rim of the paper.
If it rises too high, unfiltered liquid can flow over the edge. Students should pour slowly, especially when the mixture contains solid that can block the narrow funnel stem.
Vacuum filtration needs equipment that can safely handle lower pressure. A Büchner funnel sits on a rubber adapter connected to a side arm flask. The flat paper must cover every hole in the funnel plate.
It is moistened before suction begins to create a seal. Air leaks reduce the pressure difference and make filtration much slower. The vacuum should be released before the water aspirator or pump is turned off.
Otherwise, water can be drawn backwards into the flask and contaminate the collected material. The solid can be rinsed with a small amount of cold solvent while suction continues. Cold solvent removes dissolved impurities while reducing loss of a solid that might dissolve.
The choice of method affects the purity and amount of product obtained. Fast suction is useful for collecting crystals, but it is not ideal for a hot solution that must stay warm. Rapid cooling in the funnel can cause crystals to form too early and clog the paper.
For this situation, gravity filtration through a warmed funnel removes dust or insoluble material before crystals are allowed to grow. Filtration appears outside school laboratories in water treatment plants, coffee making, swimming pool systems, air filters, and industrial drug production. In practical work, students should record whether the goal was a clean liquid or a dry solid, inspect the liquid for cloudiness, and avoid throwing away either part until they are certain which one contains the intended product.
Key Facts
- Filtration separates an insoluble solid from a liquid using a porous barrier.
- Residue = solid trapped on the filter paper.
- Filtrate = liquid that passes through the filter paper.
- Gravity filtration works best when the filtrate is the desired product or when gentle separation is needed.
- Vacuum filtration increases flow rate because pressure below the filter is lower than atmospheric pressure.
- Flow rate generally increases when pressure difference increases: larger ΔP gives faster filtration.
Vocabulary
- Filtration
- Filtration is a separation method that uses a porous material to remove insoluble solids from a liquid.
- Residue
- Residue is the solid material left behind on the filter paper after filtration.
- Filtrate
- Filtrate is the liquid that has passed through the filter paper and collected in the container below.
- Büchner funnel
- A Büchner funnel is a flat-bottomed funnel used with filter paper and vacuum pressure to speed up filtration.
- Hot filtration
- Hot filtration is filtration performed while the mixture and apparatus are warm to prevent premature crystallization of dissolved substances.
Common Mistakes to Avoid
- Using vacuum filtration when the liquid is the desired product, because splashing or leaks can reduce filtrate recovery and contamination control.
- Forgetting to wet the filter paper in a Büchner funnel, because dry paper may not seal tightly and air can leak around the edges.
- Overfilling the funnel above the filter paper edge, because the mixture can bypass the paper and carry solid into the filtrate.
- Letting a hot filtration setup cool too much, because crystals may form in the funnel and block the filter before impurities are removed.
Practice Questions
- 1 A student filters 50.0 mL of a suspension and collects 47.5 mL of filtrate. What volume of liquid was retained in the filter paper and residue?
- 2 A vacuum filtration collects 3.20 g of dry crystals from a mixture that originally contained 4.00 g of product. What is the percent recovery?
- 3 A reaction mixture contains sand, dissolved salt, and water. Explain which part becomes residue and which part becomes filtrate during gravity filtration, and why the salt is not trapped by the filter paper.