Pure Solids Washing and Filtering Precipitates Quiz

Precipitates often carry ions from the original solution on their surface. Washing with a suitable liquid removes these contaminants ensuring that the final mass represents only the pure chemical compound being measured.

Ashless filter paper is treated with acid to remove minerals. When it is burned during the ignition step it leaves a negligible amount of residue meaning the weight of the paper does not interfere with the mass of the precipitate.

Peptization occurs when the electrolyte responsible for clumping the particles is washed away. This makes the particles so small they can pass through the filter leading to a loss of analyte and inaccurate results.

Mathematical models of washing show that multiple small volumes are much more efficient at reducing the concentration of impurities than a single large volume of the same total amount of liquid.

The liquid must be volatile enough to evaporate during drying. It should also minimize solubility often by including a common ion or an electrolyte to prevent the precipitate from dispersing or dissolving.

Using a glass rod ensures that the liquid enters the funnel smoothly. This prevents splashing which could cause tiny particles of the precipitate to land outside the filter paper leading to a loss of mass.

A Buchner funnel is used with a vacuum flask. The suction pulls the liquid through the filter much faster than gravity alone which is especially helpful when dealing with large volumes or slow filtering solids.

The paper must be fitted properly and moistened with a small amount of solvent. This creates a seal against the funnel walls preventing the liquid from bypassing the filter and taking solid particles with it.

Pure water can remove the ionic layer that holds small particles together. Adding a volatile electrolyte like ammonium nitrate keeps the particles coagulated while being easy to remove later during the heating process.

Sintered glass and Gooch crucibles are reusable and can be heated to high temperatures. Ashless paper is used when the precipitate must be ignited in a porcelain crucible to reach a stable weighing form.

By taking a small sample of the liquid passing through the filter which is the filtrate and adding a reagent that reacts with known impurities the chemist can confirm when the washing process is complete.

Gelatinous precipitates are notorious for clogging the pores of filter paper. Crystalline precipitates are much preferred because their larger more defined shapes allow the liquid to pass through much more freely.

When the stem of the funnel is full of liquid the weight of that column creates a slight suction effect. This gravity driven suction helps pull the rest of the liquid through the filter paper more quickly.

Every step of the transfer and wash process must be handled with extreme care. Even a tiny amount of solid left in the beaker or dissolved by excess wash liquid will decrease the final mass.

Dilute nitric acid provides the necessary ions to keep the silver chloride particles from becoming colloidal and is easily evaporated away during the subsequent drying step in the oven.