# 1.5C: Gravity Filtration


When there is a need to separate a solid-liquid mixture, it is common that the particles are so fine that they swirl and disperse when the flask is tilted. These mixtures cannot be decanted, and an alternative method is gravity filtration. Gravity filtration is generally used when the filtrate (liquid that has passed through the filter paper) will be retained, while the solid on the filter paper will be discarded.

A common use for gravity filtration is for separating anhydrous magnesium sulfate ($$\ce{MgSO4}$$) from an organic solution that it has dried (Figure 1.68b). Anhydrous magnesium sulfate is powdery, and with swirling in an organic solvent creates a fine dispersal of particles like a snow globe.

To gravity filter a mixture, pour the mixture through a quadrant-folded filter paper (Figure 1.69) or fluted filter paper in a funnel and allow the liquid to filter using only the force of gravity (Figure 1.68c). It is best to pour as if attempting to decant, meaning to keep the solid settled in the flask for as long as possible. When solid begins to pour onto the filter paper, it has the possibility of clogging the filter paper pores or slowing filtration. After finished pouring, rinse the solid on the filter paper (and in the flask) with a few portions of fresh solvent to remove residual compound adhering to the solid.

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