As an example of how to separate a supernatant from a precipitate by gravity filtration, the movie explains the separation of iron(III) hydroxide yielded in the following reaction which is carried out in "Inorganic Qualitative Analysis Experiment".
Fe3+ + 3OH- → Fe(OH)3↓
The movie also explains how to dissolve iron(III) hydroxide on a filter paper by adding hydrochloric acid.
Fe(OH)3 + 3HCl → Fe3+ + 3H2O + 3Cl-
The precipitate is separated from the filtrate by gravity filtration using a filter paper and funnel.
This technique is suitable for separation of a coarse precipitate.
Choose a filter paper suitable for the particle size of the precipitate.
Vacuum filtration rather than gravity filtration is suitable for the separation of a precipitate consisting of light and fine particles.
Unless the filter paper tightly sticks to the funnel, air comes into the funnel leg, and the filtration speed decreases.
Keep the sample surface about 5 mm lower than the upper rim of the filter paper to prevent the sample from overflow.
How to Operate Gravity Filtration
Preparation
Fold the filter paper into four and fit it into the funnel.
When the filter paper does not fit well into the funnel, slightly adjust the folding in order to fit it perfectly.
In order to make the filter paper fit perfectly, cut the edge of the over lapping filter paper in contact with the funnel obliquely. Wet the paper with the solvent and press with fingers to fit the paper in.
Place a container in a manner that the leg of the funnel touches its inside wall.
Filtration and Washing of a Precipitate
Put a stirring stick on the mouse of the container and decant a sample slowly.
The filtration proceeds smoothly when the supernatant and then the mixture containing the precipitate are filtrated. Such operation is suitable when the filtrate is required.
When the precipitate is needed, quickly pour the swirling sample onto the filter paper. Next, add the filtrate to the remaining precipitate in the container, and then pour the mixture onto the filter paper again.
Wash the collected precipitate on the filter paper with a washing liquid a few times.
Dissolving the Precipitate
Dissolve the precipitate on the filter paper by adding an appropriate reagent.
Alternatively, put the filter paper on the inner wall of a beaker and pour the reagent repeatedly on the precipitate.