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Washing the solid is important whether the solid or filtrate will be collected. Washing will remove soluble species from the solid. If the solid is to be collected, washing will remove any soluble impurities. If the filtrate is to be collected, washing will ensure that all of the solute (the compound that you want that is dissolved in the solvent) is removed from the solid and collected in the filtrate. If the filtrate is to be collected, the volume of washes should be minimized so that it does not take a lot of effort to remove the solvent.
To wash the solid, add a small amount of solvent to the funnel. Stir the solid to disperse it into the solvent. Be careful not to dislodge or tear the filter paper as you stir the sample, or some of your solid sample is likely to go through the funnel into the filtrate.
After the solid has been suspended in the solvent allow the liquid to pass through the funnel. Repeat this washing process until the all soluble species are removed. If the filtrate is to be collected, the volume of washes should be minimized so that it does not take a lot of effort to remove the solvent.
How can you tell when "all soluble species are removed"?
Answer
You should test either the filtrate or the precipitate to ensure that all of the soluble species have been removed. Simple tests might include:
- visual inspection (look to see if the filtrate is still discolored or if the amount of solid is still decreasing-though you may not want the solid to dissolve)
- measure the pH
- measure the conductivity
- determine the melting point
- perform a chromatography experiment (paper, tlc, or gas chromatography)
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