Rapid formation of precipitates
When crystals form slowly, each ion or molecule finds its correct place in the crystal
lattice. Very careful crystallization is the best method of preparing materials that are
extremely pure. In environmental engineering, there is no need for pure crystals or
precipitates. The rate of precipitation depends, in part, on the degree of supersaturation. You can select the time for the crystal to form in the following Java applet.
If the material that precipitates is fairly insoluble and forms crystals at a moderate rate, a long time to form the crystal can deliver high purity. Note that a few red circles
stand for a low level of impurities.
When the precipitate is highly insoluble and precipitates very rapidly (e.g., ferric
hydroxide), there is not enough time for ions to find their place in the lattice,
and foreign ions can take their places. The crystal can be quite impure.
D O N E