ery powerful. There is one, called _hydrofluoric
acid_, that will eat through glass and has to be kept in wax bottles;
and all acids tend to eat or corrode metals. You saw what hydrochloric
acid did to the zinc shavings when you wanted to make a balloon; or,
to be more accurate, you saw what the zinc shavings did to the acid,
for the hydrogen gas that bubbled off was driven out of the acid by
the zinc. Then the zinc combined with the rest of the acid to form
what chemists call a _salt_.

If we were to let the soft metal, sodium, act on hydrochloric acid,
we should get hydrogen also; but the salt that formed would be regular
table salt (NaCl). You cannot do this experiment, however, as the
sodium does its work so violently that it is dangerous.

EXPERIMENT 105. _To be done by the teacher before the class.
If acid spatters on any one's skin or clothes, wash it of
immediately with ammonia or a strong soda solution._

Drop a little candle grease on a piece of copper about 3/4
inch wide and 2 or 3 inches long. In the flame of a Bunsen
burner, gently heat the end of the copper that has the candle
grease (paraffin) on it, so that the paraffin will spread out
all over the end. Let it harden. With a nail, draw a design in
the paraffin on the copper, scratching through the thin coat
of paraffin to the copper below. Pour a couple of drops of
concentrated nitric acid on the paraffin-covered end of the
piece of copper, and spread the acid with a match so that
it can get down into the scratches. Let it stand by an open
window for 5 or 10 minutes. Do not inhale the brown fumes
that are given off. They are harmless in small amounts, but if
breathed directly they are very irritating. Now wash off the
acid by holding the copper under the hydrant, and scrape off
the paraffin.

[Illustration: FIG. 182. Etching copper with acid.]

The nitric acid did to the copper in this experiment exactly what
the hydrochloric acid did to the zinc shavings when you made the toy
balloon. The copper drove the hydrogen out of the nitric acid and
incidentally broke down some of the nitric acid to make the brown gas,
and then the copper joined the rest of the nitric acid to make a
salt called _copper nitrate_. This salt is green, and it dissolves in
water. When you washed the copper, the green salt was washed away and
a dent remained in the copper where the copper salt had been.

H