acid + barium dioxide = barium sulphate + hydrogen dioxide
-------------- -------------- --------------- ----------------
hydrogen barium barium hydrogen
sulphur oxygen sulphur oxygen
oxygen oxygen

In other words, the barium and hydrogen in the two compounds exchange
places. By this method a dilute solution of the dioxide in water is
obtained. It is possible to separate the dioxide from the water by
fractional distillation. This is attended with great difficulties,
however, since the pure dioxide is explosive. The distillation is
carried on under diminished pressure so as to lower the boiling points
as much as possible; otherwise the high temperature would decompose the
dioxide.

~Properties.~ Pure hydrogen dioxide is a colorless sirupy liquid having a
density of 1.49. Its most characteristic property is the ease with which
it decomposes into water and oxygen. One part by weight of hydrogen is
capable of holding firmly only 7.94 parts of oxygen. The additional 7.94
parts of oxygen present in hydrogen dioxide are therefore easily
evolved, the compound breaking down into water and oxygen. This
decomposition is attended by the generation of considerable heat. In
dilute solution hydrogen dioxide is fairly stable, although such a
solution should be kept in a dark, cool place, since both heat and light
aid in the decomposition of the dioxide.

~Uses.~ Solutions of hydrogen dioxide are used largely as oxidizing
agents. The solution sold by druggists contains 3% of the dioxide and is
used in medicine as an antiseptic. Its use as an antiseptic depends upon
its oxidizing properties.

EXERCISES

1. Why does the chemist use distilled water in making solutions, rather
than filtered water?

2. How could you determine the total amount of solid matter dissolved in
a sample of water?

3. How could you determine whether a given sample of water is distilled
water?

4. How could the presence of air dissolved in water be detected?

5. How could the amount of water in a food such as bread or potato be
determined?

6. Would ice frozen from impure water necessarily be free from disease
germs?

7. Suppose that the maximum density of water were at 0 deg. in place of 4 deg.;
what effect would this have on the formation of ice on bodies of water?

8. Is it possible for a substance to contain both mechanically inclosed
water