Ordinarily, the precipitate produced by mixing two chemicals in solution is formed more or less slowly. But in the following experiment we have a ease where two different speeds of reaction are shown. The first takes several seconds before it is complete, while the second is quicker than a flash. The experiment also shows just when that action takes place.

When iodine is added to a starch solution, or vice versa, the solution is colored bine. This is a well-known test for either free iodine or starch.

Prepare a solution, A, of starch in water, and add to it some sulphurous acid. Prepare a separate solution, B, of potassium iodate, and add to it a little sulphuric acid. These last two substances form potassium sulphate and iodic acid, in accordance with the following reaction:

2KIO, + H1SO4 = K2SO4 + 2HIO3. The iodic acid is the only constituent which plays any part and need be considered in the experiment proper. Pour A + B together quickly into a glass vessel and watch it closely. In a few seconds the solution will change like a flash to a deep blue. The explanation and reactions are as follows:

HIO3 + 3SO2 + 3H1O = HI + 3H1SO4 5HI + HIO3 = 5I + 3H1O SO2 + 2H1O + I = H1SO4 + 2HI. The iodic acid unites with the sulphurous acid to form hydriodic acid and sulphuric acid. The iodic acid then unites with some of the hydriodic acid and forms iodine. But the iodine does not get a chance for existence, because it is immediately taken up by some of the sulphurous acid and formed into iodic acid. This process keeps on as long as there is any sulphurous acid present, and the solution remains clear, for there is also no free iodine present. But just as soon as the last molecule of sulphurous acid disappears, the whole solution turns blue so quickly that one looks twice before believing. The blue color generally flashes in from 20 to 40 seconds, depending upon the concentration and strength of the solutions. With a little experimenting the solution may be made to change in a predetermined time. It is a good idea to use a small part of the solution, and time it before mixing the remainder. Care should he taken not to use too much sulphurous acid.