the object and note the result. What causes the paste to harden? For what is plaster of Paris used? b. Shake 1 g. of calcium sulfate with 10 cc. of water in a test tube, filter, and divide the solution into two parts. Test one part for the presence of sulfates. Is calcium sulfate soluble in water? Shake the other part with 1 or 2 cc. of soap solution. Does a froth form? How could such a solution be softened? Distinguish between temporary and permanent hardness of water. To what is each due? 170. Barium chloride. Weigh out accurately in a porcelain crucible from 1 to 2 g. of small crystals of barium chloride. Place the lid on the crucible and heat the crucible gently for a few minutes, holding the burner in the hand and moving it about so as to apply the heat uniformly. Finally, apply a strong heat for five minutes. When the crucible is cool, weigh. The residue is BaCl. From your results determine the number of molecules of water of hydration in the crystals. 171. Analytical reactions. Place in separate test tubes solutions of a compound of each of the following elements: calcium, barium, strontium. Test the effect of each on the flame by means of a platinum wire, as in § 164. Add a few drops of a solution of potassium dichromate to each. Test the solubility of the precipitates in acetic acid. Add an equal volume of a saturated solution of calcium sulfate to solutions of each, heat to boiling, and set aside until cool. Note all the results. How could you distinguish between compounds of the three elements? CHAPTER XXII THE MAGNESIUM FAMILY 172. Magnesium carbonate. Place 2 or 3 g. of magnesium carbonate (preferably the mineral magnesite) in a hard-glass tube fitted with a delivery tube, and gently heat in the Bunsen flame, passing the gas evolved through clear limewater. What would you judge as to the ease of decomposition of magnesium carbonate in comparison with that of calcium carbonate? Increase the heat, and when the evolution of gas becomes slow, cool the solid product and shake it with water, testing the reaction toward litmus paper. 173. Magnesium chloride. a. What is the formula of crystallized magnesium chloride? Place a little of the solid in a test tube and heat it gently. Is water given off easily? With blue litmus paper keep testing the drops which condense on the sides of the tube. How do you account for the reaction (R)? After most of the moisture is driven off, add water to the residue. Is it soluble? What is it? What is the industrial importance of this fact? b. Add a few drops of ammonium hydroxide to a solution of magnesium chloride (R). Repeat the experiment, first diluting the magnesium chloride with an equal volume of a solution of ammonium chloride. Does a precipitate form? Since there are many magnesium ions present, what other ion must have largely disappeared? Can you account for this by mass action? To the solution add disodium phosphate. The precipitate has the formula MgNH ̧PO (R). 4 174. Oxygen equivalent of magnesium. In a fair-sized crucible or a small evaporating-dish weigh accurately about 0.4 g. of magnesium oxide which has previously been heated, first |