An Elementary Study of Chemistry

by William McPherson; William Edwards Henderson

Fig. 70

Explanation of the reaction. To understand the reaction which occurs, it must be remembered that a volatile acid anhydride is expelled from its salts when heated with an anhydride which is not volatile. Thus, when sodium carbonate and silicon dioxide are heated together the following reaction takes place:

Na₂CO₃ + SiO₂ = Na₂SiO₃ + CO₂.

Silicon dioxide is a less volatile anhydride than phosphoric anhydride (P₂O₅), and when strongly heated with a phosphate the phosphoric anhydride is driven out, thus:

Ca₃(PO₄)₂ + 3SiO₂ = 3CaSiO₃ + P₂O₅.

If carbon is added before the heat is applied, the P₂O₅ is reduced to phosphorus at the same time, according to the equation

P₂O₅ + 5C = 2P + 5CO.

Physical properties. The purified phosphorus is a pale yellowish, translucent, waxy solid which melts at 43.3° and boils at 269°. It can therefore be cast into any convenient form under warm water, and is usually sold in the market in the form of sticks. It is quite soft and can be easily cut with a knife, but this must always be done while the element is covered with water, since it is extremely inflammable, and the friction of the knife blade is almost sure to set it on fire if cut in the air. It is not soluble in water, but is freely soluble in some other liquids, notably in carbon disulphide. Its density is 1.8.

Chemical properties. Exposed to the air phosphorus slowly combines with oxygen, and in so doing emits a pale light, or phosphorescence, which can be seen only in a dark place. The heat of the room may easily raise the temperature to the kindling point of phosphorus, when it burns with a sputtering flame, giving off dense fumes of oxide of phosphorus. It burns with dazzling brilliancy in oxygen, and combines directly with many other elements, especially with sulphur and the halogens. On account of its great affinity for oxygen it is always preserved under water.

Phosphorus is very poisonous, from 0.2 to 0.3 gram being a fatal dose. Ground up with flour and water or similar substances, it is often used as a poison for rats and other vermin.