Synthetic Inorganic Chemistry

ELEMENTS AND WATER 67

water. Note that a gas is evolved; collect a little of it, and

note that it burns like hydrogen. Note that the water re-

mains clear for a little time but that soon it becomes cloudy

owing to the separation of a finely divided white soh'd sub-

stance. The solution colors litmus blue but produces hardly

a noticeable slippery feeling between the fingers.

This experiment is similar to the preceding one. Calcium dis-
places hydrogen from water

Ca + 2H0H -» Ca(OH) 2 + H 2

but the reaction is less violent; calcium is thus shown to be a less
active metal than sodium. Furthermore, the product obtained
by substituting calcium for hydrogen in water, calcium hydroxide,
is sparingly soluble and appears as a solid substance as soon as
more than enough of it has been formed to saturate the solution.
The formula Ca(OH) 2 shows that one gram atomic weight of
calcium can take the place of two gram atomic weights of hydro-
gen; therefore, the valence of calcium is 2.
It is only the most active metals that displace hydrogen freely
from cold water; nevertheless, many of the metals do react with
water but for one reason or another the reaction does not progress
far. We just saw that calcium hydroxide was only sparingly
soluble. The hydroxides of magnesium, zinc, aluminum, lead,
iron are even less soluble. Thus, although a freshly cleaned piece
of metal may react with water, the hydroxide which is produced
adheres to the surface as a coating which separates the metal and
the water. This is the main reason why most of the fairly active
metals seem to be without action on water.
The following few experiments illustrate the action of such
metals with water. In every case it is a question of removing or
breaking through the film of insoluble material coating the surface.

7. Magnesium and Water, (a) Note the appearance of
   the surface of some magnesium ribbon. Scrape the surface
   with a knife and note the bright metallic luster. Note also
   that this luster quickly grows dim. Put the magnesium into
   a test tube half filled with water and note that no reaction
   takes place. Heat the water to boiling and note that hydro-
   gen is not evolved.