CHAPTER XIX

GENERAL METHODS FOR PREPARATION OF COMPOUNDS

154. General methods. a. Chlorides. How can you prepare zinc chloride from zinc ? from zinc carbonate? from zinc oxide (R)? How can you prepare calcium chloride from calcium carbonate? What chlorides are insoluble? Prepare small amounts of each (R) and note their physical properties.

b. Sulfides. Describe the different ways in which sulfides have been prepared in the laboratory. Devise a method for preparing the following insoluble sulfides: lead sulfide, silver sulfide, antimony sulfide, zinc sulfide, manganese sulfide. Prepare (hood) small amounts of each in test tubes (R).

c. Nitrates. How can you prepare copper nitrate from copper? from copper carbonate? from copper hydroxide (R)? Are any of the nitrates insoluble? What is the effect of heat on copper nitrate? on lead nitrate? on ammonium nitrate ?

d. Nitrites. Recall the preparation of sodium nitrite (R). e. Sulfates. The following sulfates have been prepared in previous exercises: zinc sulfate, sodium sulfate, copper sulfate, iron sulfate. Write the equations for the reactions involved in the preparation of each. What sulfates are insoluble? Prepare small amounts of each, noting the color and writing the equations for the reactions involved in each case.

f. Sulfites. Recall the method used in the preparation of sodium sulfite. How can you distinguish between sulfates and sulfites?

g. Carbonates. What carbonates are soluble? Give a general method for the preparation of the soluble carbonates; give a general method for the preparation of the insoluble carbonates. Prepare calcium carbonate by two different

methods (R). How could you prepare from calcium carbonate the following compounds: calcium chloride, calcium sulfate, calcium nitrate, calcium oxide (R)? Why are carbonates so readily decomposed by acids?

h. Phosphates. Recall the formulas and the names of the three phosphoric acids, also the action of silver nitrate on their salts.

i. Oxides. Burn a bit of magnesium wire 1 or 2 cm. in length (R). Heat a crystal of lead nitrate in a test tube (R). Heat a small piece of limestone on a wire gauze (R).

j. Hydroxides. Hold a piece of lime the size of a walnut in water for a few seconds, then place it on a watch glass and set it aside for a half hour. Note the change (R). Pour into separate test tubes about 1 cc. of a solution of each of the following compounds: ferric chloride (FeCl); magnesium sulfate (MgSO); copper sulfate (CuSO). Add to each solution 2 drops of a solution of sodium hydroxide (R). Now add a few drops of hydrochloric acid to each test tube, and explain the results.

155. Equilibrium. a. To 3 or 4 cc. of a solution of barium chloride add dilute sulfuric acid, drop by drop, as long as a precipitate is produced. Is the reaction a complete one? In a similar way, to a solution of calcium chloride add a solution of oxalic acid as long as calcium oxalate (CaC ̧O) is precipitated. To test the completeness of the reaction, filter off the precipitate and add ammonium hydroxide to the filtrate until it is alkaline in reaction. Does an additional precipitate form? Can it be ammonium oxalate? What is it? Why did it not appear before the ammonium hydroxide was added? If ammonium oxalate is added in excess to calcium chloride, is the reaction complete? Why is it incomplete when the free acid is used?

b. Make a list of about a dozen acids which you have studied. What insoluble salts do they form? Prepare an insoluble salt of each (if it forms one) and try its solubility in hydrochloric acid. Which ones should be insoluble? Why? Do your experiments confirm your predictions?

CHAPTER XX

THE ALKALI METALS

156. Sodium. a. Recall § 20. Obtain a small piece of sodium from the instructor. Cut it and note the rapidity with which the freshly cut surface is tarnished. Half fill an evaporating-dish with water, then drop the sodium into it and quickly cover the dish with a glass plate. After the action (R) has entirely ceased, test the liquid with red litmus paper. What compound is dissolved in the water? Convert it into common salt. Describe the method (R).

b. Dissolve 5 g. of sodium carbonate in 20 cc. of water. Prepare common salt from this (R). How can you be sure that the product contains no unchanged sodium carbonate? Describe the method (R). Treat some of the salt so prepared with sulfuric acid. What gas is evolved (R)?

157. Sodium hydroxide. a. Weigh out about 5 g. of good quicklime and moisten it with about an equal weight of water in an evaporating-dish, warming it, if necessary, to make it slake (R). When it has ceased slaking, dilute to 100 cc. and add the required weight of sodium carbonate to produce complete double decomposition (R). Boil for a few minutes to render filtration easier, and then filter. What is the filtrate ? How is this obtained commercially in solid form? Test a little of the filtrate to see if an excess of carbonate has been added (how will you do this?), and if an excess is found, devise a way to remove it.

b. Try the effect of the solution of sodium hydroxide upon a soluble salt of each of the following: barium, potassium, magnesium, zinc, copper (R). In each case add a few drops at first, and if a precipitate forms, add an excess to see