A Pupil Project with Carbonates CHARLES H . STONE Vermont Junior College, Montpelier, Vermont
C
AREiONATES, salts of carbonic acid, have the general formula
M representing any bivalent metal. The carbonate ion contributes no color to compounds, so colors of carbonates are due to the cation. Most carbonates are colorless, but there are some interesting exceptions. All carbonates are insoluble except those of the alkali metals. Bicarbonates, however, are generally soluble. Since textbooks and manuals do not cover the subject of the carbonates as fully as might be desired, the following suggestions are submitted for experimental purposes. Formation of carbonates. Dissolve 10 g. each of mercurous nitrate, copper sulfate, cobalt chloride, manganese chloride, and nickel sulfate, all finely powdered, in 30 ml. of water in separate beakers. Add to each liquid, filtered if necessary, a clear solution of sodium carbonate until precipitation is complete, noting the color of the product in each case. Filter off the solids and, when the liquid has completely drained through, wash with a little cold water. Drain, dry, powder, sift, and preserve the five products in neatly labeled vials or test tubes. Write the equation for each reaction. The effect of heat on carbonates. In separate test tubes put two or three grams of the carbonates of lead, copper, cadmium (how would you pre are this?), and nickel. Provide the test tubes with oie-hole stoppers carrying a delivery tube in each, the delivery tubes leading down into tubes containing clear lime water. Heat each tube separately until no more gas escapes. Then remove the lime water tubes. What results were observed? What is shown by these results? When the solid residues in the original tubes have cooled, pour them out on small squares of white paper. What are the colors of the products? From these experiments what general statement can be made concerning the effect of heat on carbonates? (A) In a long dry test tube heat a little mercuric carbonate until all solid matter has disappeared. What products were you able to identify during the reaction? Where is mercury in the displacement series? How would silver carbonate a d if heated? These are exceptions to the general statement made in (A). Add a drop of dilute hydrochloric acid to a little anhydrous sodium carbonate in an evaporating dish or test tube. What gaseous product escapes? How can this product be identified? Now beat two or
three grams of the anhydrous sodium carbonate in an evaporating dish strongly for a t least two minutes. Let it cool. To the cold residue add a few drops of the acid; identify the gas which escapes. Note another exception to the general statement made in (A). Now make a comprehensive statement concerning the effect of heat on carbonates. What other carbonates will behave as did sodium carbonate? The action of organic acids on carbonates. All carbonates react with mineral acids. The action of slightly dissociated organic acids is somewhat diierent. In separate small beakers warm 20 ml. of formic add; this is the acid secreted by certain insects such as ants (Latin, formica). Do not get this acid on the hands as it produces blisters. Stir in powdered lead carbonate, copper carbonate, and cobalt carbonate, respectively, until there is no further evidence of action. Heat a little to assist the action but do not boil. Filter the several liquids into separate flat-bottomed glass dishes with vertical sides and stand these in a steady but not violent current of air. A window sill with the lower sash slightly open is suitable. Dust in the surrounding air is to be avoided. When the-liquid has almost entirely evaporated, remove the aystals and dry them on blotting or filter paper. When perfectly dry preserve the crystals in tightly stoppered small tubes neatly labeled. Note the blue cubes of copper formate. Hold the tube of cobalt formate in a horizontal position so that bright sunlight shines through it, and view the crystals; they glow like live coals. Examine the lead formate crystals similarly. Organic Products from sea shells. Procure two or three oyster, clam, mussel, or whelk shells from the water or fish market and wash them clean. When dry, reduce the shells to powder in a clean, dry, iron mortar. In separate beakers treat five grams of the powder with lactic acid, formic acid, and dilute hydrochloric acid. The first two acids are organic, and their action will be slow. Continue in each case until no further action results. Filter off the undissolved material and evaporate each filtrate to dryness with gentle heat. Calcium lactate is used to stop internal bleeding, calcium chloride is used as a drying agent for certain organic liquids. For what is calcium formate used? Calcium nitrate and calcium acetate may be prepared in a similar manner. Calcium acetate is used to prepare solid alcohol in the laboratory. Preparation of solid alcohol. Pour 3 ml. of saturated calcium acetate solution and 7 ml. of denatured alcohol a t the same time into an evaporating dish. The alcohol may be colored if desired with a little solid
dyestuff. The gel that forms is similar to Sterno. Remove and ignite some of the jelly, supporting i t on a metal or a n asbestos sheet. What is the white residue? Carbon dioxide soap bubbles. Make a strong solution of soap in which some anhydrous sodium carbonate is dissolved. I n t ~ the liquid in a beaker introduce though a pipet some diiute hydrochloric acid so that the acid is delivered a t the bottom of the liquid. What happens? Why? Preparation of sod& bicerbonate. Saturate 50 mL of concentrated ammonium hydroxide with common s a k Pour the clear liquid into an Erlenmeyer flask fitted with a one-hole stopper through which extends the long arm of an L-tube reaching to the bottom ,of the flask. Connect the short arm to a Kipp generator for carbon dioxide. When the stopcock is opened, the gas will flow in only as long as it is absorbed. A 100-ml graduate may be used instead of the flask, prwided the top is dosed in the same way as described .abo~e. Let stand overnight In the morning filter off the white product and dry i t Write the equation for the reaction. Instead of gas from the Kipp, lumps of solid carbon dioxide (dry ice) may be added from time to time. Results can b e obtained in this way in ten to fifteen minutes. Reaction of carbon d i o d e with bases. When carbon dioxide is nassed into clear lime water. the ~ r o d u c t formed is visiile because calcium carbonate ;&ting from the reaction is insoluble. A soluble carbonate may be formed with certain bases. Add a few drops
of acid to five ml. of sodium hydroxide solution. There are no visible products. Why? Now pass carbon dioxide for a t least one minute into sodium hydroxide solution; then add acid. What happens? What is proved? When a mixture of carbon monoxide and carbon dioxide is passed into sodium hydroxide solution, the carbon dioxide is retained but the monoxide passes on without reaction. Problem: prepare sodium carbonate from sodium hydroxide, and prove that sodium hydrogen carbonate is absent. Comparison of carbonates and sulJites. How closely the carbonates of a metal resemble the sulfites of that metal is shown by a comparison of their structural formulas : Na-0 Na-0
>-
and Na-** Na-0
Treated with a strong acid such as diiute sulfuric acid, these solids yield carbon dioxide and sulfur dioxide, respectively. Dissolved in water, these gases form weak unstable acids. Water solutions of these solids treated with carbon dioxide and sulfur dioxide, respectively, yield the acid salts: Na-4>C_0 H 4
and Na-O%~ H-0
Whiie the sulfites can be oxidized to sulfates. the same is not true of the carbonates, for carbon has'only the higher valence of four while sulfur can be oxidized from valence four to valence six.
