Note 9 — Chapter XII · SODIUM

According to Pickering's experiments (1886), the molecular weight in grams (that is, 142 grams) of anhydrous sodium sulphate, on being dissolved in a large mass of water, at 0° absorbs (hence the - sign) -1,100 heat units, at 10°–700, at 15°–275, at 20° gives out +25, at 25° +300 calories. For the decahydrated salt, Na2SO4,10H2O, 5° - 4,225, 10° - 4,000, 15° - 3,570, 20° - 3,160, 25° - 2,775. Hence (just as in Chapter I., Note 56) the heat of the combination Na2SO4,10H2O at 5° = +3,125, 10° = +3,250, 20° = +3,200, and 25° = +3,050.

It is evident that the decahydrated salt dissolving in water gives a decrease of temperature. Solutions in hydrochloric acid give a still greater decrease, because they contain the water of crystallisation in a solid state—that is, like ice—and this on melting absorbs heat. A mixture of 15 parts of Na₂SO₄,10H₂O and 12 parts of strong hydrochloric acid produces sufficient cold to freeze water. During the treatment with hydrochloric acid a certain quantity of sodium chloride is formed.