The following experiment is easily performed, even by those who have had little or no previous chemicai training. It gives an idea of the infinite diversity of chemical and physical changes which one and the same reagent may induce in other substances. Fourteen glasses, arranged in seven pairs, are brought before the spectators, who are requested to examine them and their contents. The experimenter then pours one and the same liquid into the fourteen glasses. The following contradictory results become at once apparent: Substance contained in glass A becomes so hot that the glass cannot be kept in the hand. Substance contained in glass Ar becomes so cold that frost promptly collects outside of the glass. Liquid contained in glass B was blue; it becomes colorless. Liquid contained in glass B' was colorless; it becomes blue. Glass G contained a clear liquid which becomes muddy. Glass C" contained a muddy liquid which becomes clear. If a lighted match be introduced into glass D, several inches above the liquid, noisy flames are seen to spring in every direction. If the same experiment is made in the same manner with glass D'', the match is quietly but immediately extinguished. Liquid in glass E had a burning, suffocating smell; it becomes odorless. Liquid in glass E' was odorless; it acquires such an offensive smell that it becomes necessary to carry it outside. Liquid in glass F was red; it becomes blue. Liquid in glass F' was blue; it becomes red. Glass G contained a solid substance which becomes liquid. Glass G' contained a clear liquid which becomes instantaneously solid. Every transformation is the result of the action of common hydrochloric acid over some chemical. Here is the nature and proportions of these; capacity of glasses being supposed to be about one pint. Glass A contains 50 grammes of sodium hydrate dissolved in 100 cubic centimeters of water. This solution fills about one-half of the glass. During the experiment the other half must be entirely filled with hydrochloric acid. This should be poured slowly while the liquid is agitated with a glass or wooden stick. The last additions of acid cause the liquid to boil. The glass will then be found to contain common table salt mixed with an excess either of acid or of sodium hydrate. Glass A' is filled with sodium sulphate in small crystals, such as is sold by pharmacists to be taken internally. Enough hydrochloric acid must be poured to cover the salt. Temperature goes at once much below 32 deg. F. Cold becomes still more intense if the mixture be agitated. Three-quarters of glass B are filled with, water; then one decigramme of copper sulphate is dissolved in it. Ammonia is added in small portions until an intense blue color appears. The addition of hydrochloric acid will cause it to vanish. A solution of two centigrammes of potassium ferricyanide in 100 cubic centimeters of water is made in glass B' and, in this, another solution of 3 centigrammes of ferrous sulphate in 100 cubic centimeters of water is poured. A little ammonia is then added until the beautiful blue color vanishes. Hydrochloric acid will cause it to reappear instantaneously. Glass G contains the ordinary solution of lead acetate sold by druggists for local or external application. Hydrochloric acid forms in it a dense precipitate of lead chloride. Three-quarters of glass C" are filled with water to which about a quarter of a teaspoonful of slaked lime, free from coarse particles, is added. Calcium chloride, resulting from the action of hydrochloric acid on lime, is exceedingly soluble in water. A few pieces of zinc are deposited in glass D. Bubbles of hydrogen will set noisily on fire when a match is introduced into the glass. The experiment is free from danger so long as no attempt is made to close the glass. One-third of glass D' is filled with wood ashes with enough water to make a thin paste. Carbon dioxide is the gas which promptly extinguishes the match. A hundred cubic centimeters of water, fifty cubic centimeters of ammonia, and enough litmus solution to give the liquid a bluish tint are poured into glass E. During the experiment, hydrochloric acid is ardded, little by little, until the bluish color suddenly becomes reddish. The odor will then be found to have vanished. One-third of glass E' is filled with water, and 8 grammes of powdered iron sulphide are thrown into it. Hydrochloric acid will generate hydrogen sulphide in the glass. This gas is the active agent found in some mineral waters, to which it gives a characteristic smell of putrefied eggs. It is poisonous, but the amount evolved during the experiment is small, and the smell is so strong that it makes the air apparently irrespirable long before there is any danger. It should not, however, be smelled directly over the glass, nor should the glass remain in the room longer than is necessary to detect the odor. Glass F contains the same liquid as used in glass B, with the addition of one centigramme of aniline red scarlet. Glass F' contains the ordinary solution of blue litmus. Calcined magnesia is the solid substance which fills about one-third of glass G, and the ordinary syrupy soluble glass or sodium silicate solution sold by druggists is the liquid substance which will become instantaneously solid, in glass G', if mixed with about one-third its volume of hydrochloric acid.
This article was originally published with the title "A Chemical Puzzle" in Scientific American 97, 26, 472 (December 1907)