216 ELEMENTS OF GROUP IIIIs the solution clear or cloudy? Is the salt in the dilute aqueous
solution extensively hydrolyzed?
- Place another 3 cc. of the carbon disulphide solution in a
watch glass. After the carbon disulphide evaporates let the residue
stand on a hot plate until it appears dry. Is this residue soluble
in water? Of what does it consist? - How could hydrated aluminum bromide be prepared?
- Suggest a method of making anhydrous aluminum bromide,
using hydrogen bromide instead of bromine, and state why you
think the method might be feasible.
Experiments- Acid Strength of Boric Acid. Dissolve 3 grams of
H3BO3 in 50 cc. of water, thus making a formal solution.
Add a few drops of a solution of blue litmus and compare the
color with that produced by dilute HC1. Add 12V NaOH,
1 cc. at a time, noting the gradual change in color, until
the litmus is completely blue, and note the amount taken.
It would take 150 cc. of 12V NaOH to give Na 3 BO 3 , 50 cc
to give NaBO 2 , and 25 cc. to give Na 2 B4O7. The fact that the
solution becomes alkaline when there is still a large excess of boric
acid in the solution (even when it is figured as tetraboric acid,
H2B4O7) shows how very weak the acid is.
AMPHOTEKIC SUBSTANCES
A substance which can behave both as an acid and a base is
known as amphoteric. In water such a substance would yield
both H+ and OH~ ions, but the product of the concentrations of
these ions could not exceed the ion product of water;
[H+] X [OH"] = 10"^7 X 10"^7 = 10-"
Amphoteric substances are necessarily extremely weak both as
acids and as bases, but they possess the property of reacting with
strong acids on the one hand and with strong bases on the other
hand.
Aluminum hydroxide is amphoteric; it is itself insoluble, but
its salts, such as A1C1 3 and Na 3 A10 3 , are very soluble; it is therefore
most easy to observe whether the aluminum hydroxide reacts with
an acid or a base, because if it does it is seen to dissolve.