274 Domestic Science 



Exercise for Student. 



How do you explain the change of temperature noted as a result 

 of the rapid crystallisation of the sodium sulphate ? Test your 

 explanation by finding whether any change of temperature occurs 

 when 100 g. of the sulphate are dissolved in 200 c.c. of water. 



189. EXPERIMENT 125. Grind up some potassium 

 chlorate with a clean pestle and mortar. Half fill a 

 beaker with distilled water and place it in a water-bath 

 partly filled with water. Drop some of the chlorate 

 into the water and, by shaking and stirring, try to 

 dissolve it. Should all this first portion dissolve, add 

 more and shake and stir again, repeating the addition 

 till part of the solid refuses to dissolve, in spite of 

 agitation for several minutes. Allow the undissolved 

 portion to settle. Weigh a clean porcelain dish and 

 pipette 20 c.c. of the clear solution into this dish. 



Weigh the dish and solution. Evaporate the liquid 

 to dryness over a water-bath. When dry, cool and 

 weigh the dish and contained residue. The difference 

 between the weight of the dish and residue and that 

 of the dish alone is the weight of solid dissolved in the 

 weight of solution given by the subtraction of the weight 

 of the empty dish from that of the 

 dish and solution. By gently heating 

 the water-bath, a second determina- 

 tion of the weight of dissolved sub- 

 stance in a known weight of solution 

 maybe made at a higher temperature, 

 say 30 C. Other determinations at 

 different temperatures may be per- Fi 89 



formed, care being taken to agitate 

 the solid thoroughly all the time it is being dissolved. 

 By allowing one of the solutions made at a higher 

 temperature to evaporate slowly, beautiful crystals 



