292 ELEMENTS OF GROUP IV
affected. Add Na 2 CO 3 solution and warm gently, noting
that there is no effervescence.
(6) Collect a little of a mixture of anhydrous potassium
carbonate and sodium carbonate (the mixture melts more
easily than either salt alone) in a loop on the end of a platinum
wire and melt it in the Bunsen flame to a clear bead. Dip
the bead into powdered silicon dioxide and melt it again.
Note that the liquid bead effervesces until the silica has dis-
solved.
(c) Dilute 5 cc. of water glass, sodium silicate solution,
with 5 cc. of water in a beaker, and add 6 N HC1, drop by drop,
with stirring, noting that the solution coagulates to a stiff,
apparently dry jelly.
Repeat, diluting 5 cc. of water glass with 100 cc. of water
and noting that coagulation does not take place on acidify-
ing.Silicon dioxide is the anhydride of silicic acid, but when it is
entirely dehydrated its action with water is almost imperceptible.
It does not react with a solution of the salt of the weak carbonic
acid. Silicic acid is in fact a far weaker acid than carbonic.
With melted sodium carbonate, however, silicon dioxide reacts,
SiO 2 + Na 2 CO 3 —> Na 2 Si0 3 + CO 2 |, but this does not necessarily
show that silicon dioxide is more strongly acidic than carbon di-
oxide; the effect is due rather to the greater volatility of the carbon
dioxide. Water glass is obtained by dissolving the sodium silicate
melt in hot water under pressure. From it ordinary acids dis-
place the weak silicic acid in colloidal form. This colloidal silicic
acid appears as a jelly in concentrated solutions. In dilute solu-
tion it remains dispersed so that its formation is not apparent.
When the colloidal silicic acid is heated it is changed to the anhy-
dride, which will not again take up water.
- Hydrolysis of Stannous Salts. Dissolve 0.5 gram crys-
tallized SnCl 2 -2H 2 O in a few drops of water. A clear solution
can be obtained if the preparation is fresh. Dilute the solu-
tion with water and note the white precipitate. Add a little
HC1 and note that the precipitate redissolves.
Stannous salts in which tin displays the lower valence of 2
are derivatives of the hydroxide Sn(OH) 2. That salts, such as