Video Transcript
To determine the pH of various
oxides, an experiment was set up. Three beakers were filled with 0.5
liters of deionized water and a few drops of universal indicator were added. A spatula of the following oxide
was then added to each beaker. What color will each solution
change to following the addition of the oxide? (A) A: blue, B: green, and C:
red. (B) A: green, B: red, and C:
blue. (C) A: blue, B: red, and C:
green. (D) A: red, B: green, and C:
blue. Or (E) A: red, B: blue, and C:
green.
An oxide is a compound consisting
of oxygen bonded to another element. P2O10, which was added to the first
beaker, is a nonmetal oxide because it is composed of the nonmetal phosphorus bonded
to oxygen. MgO and Al2O3 are both examples of
metal oxides because Mg, magnesium, is a metal and Al, aluminum, is also a
metal. And these metals are bonded to
oxygen. In general, when a nonmetal oxide
reacts with water, an acid is formed. This usually occurs when a nonmetal
is from groups 14 to 17 of the periodic table. When the metal in a metal oxide is
from groups one or two of the periodic table, for example, magnesium, and the oxide
reacts with water, a base or alkali solution is usually formed.
Note, however, that there are
always exceptions to the rule. For example, beryllium oxide is not
soluble in water and does not react with water under normal conditions. Other metal oxides containing
metals which are not in groups one or two of the periodic table, for example,
copper, zinc, lead, aluminum, and tin, when they are placed in water, they do not
usually react and are usually insoluble. Now, magnesium oxide is also not
very soluble in water, but tiny amounts of it will dissolve and react with water to
produce a base or alkaline solution.
We were told that universal
indicator was added to each beaker to determine the pH. Universal indicator is red in the
highly acidic region of the pH scale, then orange-yellow, then green around neutral
point, then blue, and at the far end of the spectrum in the highly basic region, it
is purple. We have seen that when a nonmetal
oxide reacts with water, an acid forms in the case of beaker A. So the indicator
will turn red in beaker A. In beaker B, a base or alkali will form as the metal
oxide reacts with water, so the indicator will turn a purple-blue color. And in beaker C, when the metal
oxide is added, no reaction occurs.
Aluminum oxide is an example of an
amphoteric oxide. And again, these are not usually
soluble, nor do they usually react with water, although they can react with acids
and bases. Because there is no reaction with
water, the pH of the water in this beaker will remain neutral, and the indicator
will appear green. So the color change in each beaker
because of the addition of an oxide will be A: red, B: blue, and C: green.