Sodium oxide

Preparation

Sodium oxide is produced by the reaction of sodium with sodium hydroxide, sodium peroxide, or sodium nitrite:

2 NaOH + 2 Na → 2 Na₂O + H₂

To the extent that NaOH is contaminated with water, correspondingly greater amounts of sodium are employed. Excess sodium is distilled from the crude product.

A second method involves heating a mixture of sodium azide and sodium nitrate:

5 NaN₃ + NaNO₃ → 3 Na₂O + 8 N₂

Burning sodium in air produces a mixture of Na₂O and sodium peroxide (Na₂O₂):

4 Na + O₂ → 2Na₂O

and

2 Na + O₂ → Na₂O₂

A third, much less common, method involves a single displacement reaction when heating sodium metal with iron(III) oxide (rust):

6 Na + Fe₂O₃ → 3 Na₂O + 2 Fe

the reaction is done in an inert atmosphere to avoid the reaction of sodium with the air instead.

Applications

Glassmaking

Glasses are often described in terms of their sodium oxide content although they do not really contain Na₂O. Furthermore, such glasses are not made from sodium oxide, but the equivalent of Na₂O is added in the form of "soda" (sodium carbonate), which loses carbon dioxide at high temperatures:

Na₂CO₃ → Na₂O + CO₂

Na₂O + SiO₂ → Na₂SiO₃

Na₂CO₃ + SiO₂ → Na₂SiO₃ + CO₂

A typical manufactured glass contains around 15% sodium oxide, 70% silica (silicon dioxide), and 9% lime (calcium oxide). The sodium carbonate "soda" serves as a flux to lower the temperature at which the silica mixture melts. Such soda-lime glass has a much lower melting temperature than pure silica and has slightly higher elasticity. These changes arise because the Na₂[SiO₂]_x[SiO₃]-based material is somewhat more flexible.

Reactions

Sodium oxide reacts readily and irreversibly with water to give sodium hydroxide:

Na₂O + H₂O → 2 NaOH

Because of this reaction, sodium oxide is sometimes referred to as the base anhydride of sodium hydroxide (more archaically, "anhydride of caustic soda").