Filter Funnel and Paper

Description

A filter funnel is a cone-shaped piece of glass or plastic, open at the wide top and narrowing to a stem at the bottom. It holds a piece of filter paper through which a mixture is poured to separate insoluble solids from liquids.

Filter paper is a porous paper that allows liquid to pass through while trapping solid particles. It comes in various grades (pore sizes) and as circles that fit inside the funnel.

How to set up

1. Fold the filter paper in half, then in half again, creating a quarter circle.
2. Open one fold to create a cone: three layers of paper on one side, one layer on the other.
3. Place the cone into the funnel. Press gently to fit against the glass surface.
4. Moisten the paper with a small amount of distilled water from a wash bottle. This helps the paper stick to the funnel and creates a seal.
5. Place the funnel in a filter funnel stand or in the neck of a conical flask.
6. Position the stem of the funnel so it touches the inside wall of the flask below. This allows the filtrate to run down the wall without splashing.

The filtration process

1. Pour the mixture into the funnel slowly, directing the liquid against the glass rod into the paper (not directly onto the paper, which can tear it).
2. The liquid (filtrate) passes through the paper and collects in the flask.
3. The solid (residue) collects on the paper.
4. If needed, wash the residue by pouring small amounts of distilled water over it to remove soluble impurities.
5. Allow the residue to dry: leave in the air, place in a warm oven, or press between dry filter papers.

Uses in IGCSE Chemistry

• Separating an insoluble solid from a liquid: Sand from salt water, precipitates from the solution they formed in
• Collecting a precipitate: After a precipitation reaction, the insoluble product is collected by filtration
• Removing excess reagent: In salt preparation by the excess method, unreacted metal or metal oxide is filtered off
• Purification: Filtering a recrystallised product from the mother liquor

Common errors

Confusing residue and filtrate: The residue is ON the paper (the solid). The filtrate is IN the flask (the liquid). Getting these terms wrong loses marks.

Tearing the filter paper: Pouring too quickly or directly onto the paper without a glass rod can puncture it, allowing solid into the filtrate.

Not washing the residue: The residue often contains soluble impurities from the solution. Washing with distilled water improves purity.

Using the wrong size paper: If the paper is too small, the mixture overflows. If too large, it folds unevenly. The paper should fit neatly inside the funnel about 1 cm below the rim.