Physical Properties of Water 273 



EXPERIMENT 124. Thoroughly clean a flask of 

 about 500 c.c. capacity by rinsing it with tap-water 

 several times, and following this by two or three 

 rinsings with distilled water. Measure out 200 c.c. of 

 distilled water into the clean flask. Weigh out 100 g. 

 of crystallised sodium sulphate Glauber's salt and 

 place the crystals in the flask. Stand the flask in a 

 water-bath, heating the latter gently so that the 

 temperature of the water remains practically constant 

 at 33 C. Shake the flask and its contents frequently. 

 When all the crystals have dissolved examine the 

 liquid produced. This should be perfectly clear. Any 

 crystals undissolved or any particles of other substances 

 floating in the liquid may be removed by filtering (see 

 Experiment 126), the liquid filtrate being received in 

 another thoroughly cleaned flask standing in the water 

 in the water-bath. When a clear solution has been 

 obtained remove the flask from the water-bath, closing 

 the mouth lightly with a wad of cotton -wool. Let the 

 flask cool slowly down to the temperature of the 

 laboratory. It now contains a much larger quantity 

 of dissolved sulphate than is required to form a saturated 

 solution at ordinary temperatures, but, if the experiment 

 has been carried out with sufficient care, no separation 

 of crystals will occur. Wash the bulb of a thermometer 

 with tap and distilled water and insert it in the sodium 

 sulphate solution. Note the temperature recorded by 

 the instrument. Should its introduction not have been 

 followed by the immediate formation of crystals, as is 

 sometimes the case, drop a very small piece of sodium 

 sulphate into the solution. This will start the crystallisa- 

 tion, and the whole mass of liquid will apparently 

 become solid in a few seconds. Read the thermometer 

 as soon as this has happened. 



H. D. s. 18 



