FILTERING AND PRECIPITATING 33 



Then add the same amount of dilute sulphuric acid and of 

 potassium permanganate to 50 cu. cm. of hydrant or well water, 

 and to 50 cu. cm. of ditch, pond, or aquarium water. Cover 

 the 3 vessels with cardboard or glass covers, and set them aside 

 in a warm place. Let them stand several hours. In which 

 case is the pink color changed most? In which least? If it 

 is the organic matter that changes the potassium permanganate, 

 which water has the most of it? 



6. Hardness of Water. Put a small piece of a white soap, 

 such as Ivory, Pearl, or Fairy soap, into a test tube half full of 

 distilled water. Close the tube with your thumb, and shake it 

 vigorously for 30 seconds. Then let it stand for 5 minutes. 



Do the same with a test tube half full of hydrant or well 

 water, and with one half full of a very hard water. Make the 

 very hard water by shaking some powdered calcium sulphate 

 (gypsum or plaster of Paris) with water and then filtering the 

 solution. Compare the results in the 3 tubes; what differences 

 do you see? Which forms the best and most lasting lather or 

 suds? Which leaves a scum? If the ability to form a lasting 

 suds is a test for a good laundry water, which of the 3 tried is 

 the best? Which water would waste the most soap? Do you 

 think the scum would be a good thing for the clothing washed? 



EXERCISE 31 

 FILTERING AND PRECIPITATING 



Apparatus and Materials. Fine sand, salt, funnel, filter paper, 

 watch glass or glass plate, cup of boiling water, potassium dichromate 

 solution, solution of lead nitrate or of lead acetate (sugar of lead). 



a. Mix thoroughly half a teaspoonful of fine sand with the 

 same amount of powdered salt. Put the mixture into a dish, 

 and add to it half a test tube full of hot water. 



Make a filter as shown in Fig. 68, 84, of the text. Fold the 



