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.
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.