Some salts are soluble in water but others are not. For example, all sodium salts are soluble and most sulfates are soluble. Some combinations of soluble salts will react together to form an insoluble salt, the precipitate. For example, silver nitrate and sodium chloride are soluble. Their solutions react together, when moixed, to form a precipitate of silver chloride:
silver nitrate + sodium chloride -> silver chloride + sodium nitrate
AgNO3 + NaCl -> AgCl + NaNO3
Notice how the ions involved appear to have 'swapped places' in this precipition reaction. Silver ions from the silver nitrate solution combine with chloride ions from the sodium chloride solution to form insoluble silver chloride. The precipitation can then be separated from the unreacted ions in solution by filtration, washed on the filter paper, them dried in a warm oven.
It is possible to work out which substances are needed to make a particular precipitate, and also to work out what precipitate will form when two solutions are…