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Gravity filtration separates an insoluble solid from a liquid by pouring the mixture through filter paper in a funnel and letting the liquid drain under its own weight. At the bench it is used to remove solid impurities (for example a drying agent or a by-product) from a solution, and in its hot form to remove solids that stay undissolved in a hot solution before recrystallization. This page is about the laboratory technique, not household gravity-fed water filters. It is general bench information, not a substitute for your procedure, supervisor or safety data sheets.
Which filtration method fits the job
| Goal | Method | Why (per the sources) |
|---|---|---|
| Remove a solid impurity from a liquid you want to keep | Gravity filtration (room temperature) | Used to separate insoluble solids such as a drying agent or by-product from a liquid; the liquid is the product. |
| Remove solid impurities that do not dissolve in a hot solution | Hot gravity filtration | The funnel must stay hot so the dissolved compound does not crystallize in the paper or stem. |
| Collect the solid product | Vacuum filtration | Vacuum filtration is faster and is the usual choice for collecting a solid product; see the Buchner flask guide. |
What you need
- A conical funnel, preferably stemless or short-stemmed for hot work. Long-stemmed funnels are avoided in hot filtration because crystals can form in the stem.
- A ring clamp on a stand to hold the funnel over the receiving vessel.
- A clean receiving vessel, usually an Erlenmeyer flask (a beaker is also used for room-temperature work).
- Filter paper, folded into a fluted (pleated) cone.
- A glass rod to guide the pour, and a small amount of clean solvent for rinsing.
Funnel and paper choices
| Choice | Option | What the sources say |
|---|---|---|
| Funnel stem | Stemless or short-stemmed | Preferred for hot filtration; long stems let crystals form and clog. |
| Paper fold | Fluted | Fluted paper exposes more surface area, filters faster and lets displaced air escape, compared with a quadrant fold. |
| Paper height | At or just reaching the funnel rim | Paper should reach at least the top edge of the funnel but must not protrude above the rim. |
| Paper retention | Whatman Grade 4, 5, 201, 202 | Manufacturer listings (Whatman, Cytiva, via Avantor) give nominal particle retention of 20-25 μm for Grade 4, 2.5 μm for Grade 5 (listed as 98% efficiency in liquid), 7-14 μm for Grade 201 and 15-19 μm for Grade 202. A finer grade retains smaller particles; check your procedure for the grade it names. |
Steps
- Fold the filter paper into a fluted cone and seat it in the funnel so it does not stand above the rim.
- Clamp the funnel in a ring over the clean receiving vessel, with the funnel outlet inside the vessel mouth.
- Pour the mixture slowly in portions, using a glass rod as a guide, and never fill past the paper edge.
- Let the liquid drain by gravity.
- Rinse the solid on the paper with a small amount of solvent so retained liquid is carried through.
- Process the filtrate as your procedure directs.
Hot gravity filtration
Hot gravity filtration is used during recrystallization when the solution contains solids that do not dissolve even when hot. As LibreTexts puts it, it is important that the funnel is kept hot during filtration, because a cooling solution can deposit crystals in the paper and funnel.
- Set a stemless or short-stemmed funnel in a ring clamp over a clean Erlenmeyer flask, and leave a small gap between the funnel and flask mouth so displaced air can escape.
- Preheat: pour a few millilitres of hot solvent through the funnel and keep the receiving flask boiling gently until the apparatus is hot. If your procedure calls for decolorizing charcoal, add it to the solution at this stage.
- Pour the hot solution through the fluted paper in portions while the solution keeps boiling, touching the flask lip to the paper, and reheat the source flask between pours.
- If crystals appear on the paper, rinse them through with a few millilitres of boiling solvent.
- Cool the filtrate as your recrystallization procedure directs.
Gravity vs vacuum filtration
Gravity filtration is slow, copes poorly with very fine particles and large volumes, and can clog when the solid load is high. Vacuum filtration is faster and is the usual way to collect a solid product, so gravity filtration is the method for removing a solid impurity from a liquid you want to keep.
What can go wrong
| Problem | Likely cause | What to do |
|---|---|---|
| Crystals form on the paper or in the stem | Funnel or solution cooled during hot filtration; long stem | Rinse with a few millilitres of boiling solvent; use a stemless funnel and preheat the apparatus. |
| Grey filtrate or fine black particles (charcoal) | Charcoal passed through a hole or the paper grade is too coarse | Repeat with new paper and a clean receiving flask. |
| Overflow | Liquid above the paper edge or paper above the rim | Pour in smaller portions; keep paper at or below the rim. |
| Solvent evaporates during hot filtration | Open heating of volatile solvent | Use a reflux condenser for flammable organic solvents as your procedure directs. |
| Very slow drainage | High solid load or fine particles clogging the paper | Gravity filtration is poor for very fine particles and large volumes; ask your supervisor about another method. |
Safety points from the sources
- Hot vapours can scald: pour sideways, not leaning over the funnel.
- Use a ring clamp rather than holding the funnel; this reduces the risk of tipping.
- Do not heat an empty flask, which may crack.
- A strip of paper towel under the flask lip is described as a way to manage drips.
When to ask a supervisor
Ask the lab supervisor or safety officer before hot-filtering flammable solvents, when the compound or solvent is unfamiliar, or when the filtrate fails the expected check (discoloured, cloudy or low yield). Follow the written procedure and the safety data sheet for the solvents in use.
FAQ
What is gravity filtration used for?
It removes insoluble solid impurities from a liquid by draining the liquid through filter paper under gravity, keeping the filtrate.
What is the difference between gravity and vacuum filtration?
Vacuum filtration pulls liquid through using reduced pressure and is faster; it is the usual choice for collecting a solid product. Gravity filtration relies on the liquid’s own weight and is slower.
Why use fluted filter paper?
A fluted fold gives more surface area than a quadrant fold, filters faster and lets air escape from the funnel.
When is hot gravity filtration used in recrystallization?
When the hot solution contains insoluble impurities or charcoal that must be removed before the solution is cooled to crystallize the product.
Why avoid a long-stemmed funnel for hot filtration?
Crystals can form in the cooling stem and block it.
Sources
- LibreTexts Chemistry, Nichols, Organic Chemistry Lab Techniques, 1.5E Hot Filtration: chem.libretexts.org (read 2026-11-07).
- Chemistry Online, Gravity filtration: chemistry-online.com (read 2026-11-07).
- Whatman qualitative filter papers, Cytiva, product listing via Avantor: avantorsciences.com (read 2026-11-07).
Related: types of laboratory glassware.
