Synthetic Inorganic Chemistry

66 WATER AND SOLUTION

in the simple compounds, and that it is only some sort of "secon-
dary" valence which holds the substances in combination.
When elements react with water, however, a change in the
primary valence is usually involved.
A rather unusual instance of a hydrate of an element is chlorine
hydrate, C1 2 -8H 2 O, which crystallizes from ice-cold water which
is saturated with chlorine. This substance is clearly a molecular
compound, for if these crystals are placed in a watch glass and
allowed to come to room temperature, chlorine gas escapes and
water containing only the amount of chlorine corresponding to
an ordinary saturated solution is left.

5. Sodium and Water. Danger. Pour about 50 cc. of
   water into a 4-inch, porcelain dish. Select a piece of metallic
   sodium half the size of a pea and press it in a folded filter
   paper to remove the adhering kerosene. Drop the metal on
   the surface of the water. Stand at least 3 feet from the dish.
   Note that the sodium floats; that it almost immediately melts
   to a globule with a bright metallic surface; that a gas is given
   off freely under the impulse of which the globule races about
   over the surface of the water; that finally the globule entirely
   disappears, and then that the remaining solution is strongly
   alkaline, turning litmus blue and making the fingers slippery
   when wet with it. Often, after the sodium has melted and
   become hot, there is a violent explosion which throws the
   caustic solution and the burning metal about, hence the
   danger.

The gas evolved is hydrogen; the alkaline character of the

solution is due to sodium hydroxide which is left dissolved in the

water

2Na + 2H0H -» 2NaOH + H 2

Sodium is a much more active element than hydrogen and has dis-

placed an equivalent amount of it from water. Sodium hy-

droxide then may be regarded as water in which sodium has taken

the place of one-half of the hydrogen. A gram atom of sodium

can replace a gram atom of hydrogen; in other words, the valence

of sodium is 1.

6. Calcium and Water. Drop a few pieces of metallic
   calcium (in the form of turnings) into a test tube of clear