liberated at the cathode
collect. The hydrogen escapes by lifting the cover, and the
sodium, protected from the air by the hydrogen, is skimmed or
drained off from time to time. Oxygen is set free upon the
anode and escapes into the air through the openings O without
coming into contact with the sodium or hydrogen. This process
is carried on extensively at Niagara Falls.

[Illustration: Fig. 76]

~Properties.~ Sodium is a silver-white metal about as heavy as water, and
so soft that it can be molded easily by the fingers or pressed into
wire. It is very active chemically, combining with most of the
non-metallic elements, such as oxygen and chlorine, with great energy.
It will often withdraw these elements from combination with other
elements, and is thus able to decompose water and the oxides and
chlorides of many metals.

~Sodium peroxide~ (NaO). Since sodium is a univalent element we should
expect it to form an oxide of the formula Na_{2}O. While such an oxide
can be prepared, the peroxide (NaO) is much better known. It is a
yellowish-white powder made by burning sodium in air. Its chief use is
as an oxidizing agent. When heated with oxidizable substances it gives
up a part of its oxygen, as shown in the equation

2NaO = Na_{2}O + O.

Water decomposes it in accordance with the equation

2NaO + 2H_{2}O = 2NaOH + H_{2}O_{2}.

Acids act readily upon it, forming a sodium salt and hydrogen peroxide:

2NaO + 2HCl = 2NaCl + H_{2}O_{2}.

In these last two reactions the hydrogen dioxide formed may decompose
into water and oxygen if the temperature is allowed to rise:

H_{2}O_{2} = H_{2}O + O.

~Peroxides.~ It will be remembered that barium dioxide (BaO_{2})
yields hydrogen dioxide when treated with acids, and that
manganese dioxide gives up oxygen when heated with sulphuric
acid. Oxides which yield either hydrogen dioxide or oxygen when
treated with water or an acid are called peroxides.

~Sodium hydroxide~ (_caustic soda_) (NaOH). 1. _Preparation._ Sodium
hydroxide is prepared commercially by several processes.

(a) In the older process, still in extensive use, sodium carbonate is
treated with calcium hydroxide suspended in water. Calcium carbonate is
precipitated according to the equation

Na_{2}CO_{3} + Ca(OH)_{2} = CaCO_{3} + 2NaOH.

The dilute solution of sodium hydroxide, filtered from the calcium
carbonate, is evaporated to a p