CHAPTER X. ALKALIS; AND ALKALINE HYDROXIDES. Exp. 1. To two quantities of distilled water (about 100 c.c. each) in basins add sodium and potassium, respectively. Cut the sodium and potassium into small pieces, and add these one at a time, until about 5 grams of each metal has been added. Never touch sodium or potassium with wet hands. Test a little of each solution with litmus; both solutions turn red litmus blue. These solutions contain the compounds NaOH and KOH, respectively. [M + H2O+ Aq = MOHAq +H; M = Na or K.] Exp. 2. Into each of two test tubes pour a little solution of ferric chloride; into two other tubes, a solution of manganese sulphate; into two others, a solution of zinc sulphate; and into two others, a solution of mercuric chloride. To one of each pair of solutions add some of the solution of potassium hydroxide, and to the other some of the sodium hydroxide solution, you prepared in Exp. 1. In every case a pp. is produced; observe that addition of a considerable quantity of the sodium or potassium hydroxide to the liquid containing zinc sulphate brings about a solution of the pp. which was at first produced. The pps. produced have the following compositions ;— The chemical changes which occurred may be represented thus ;— Fe,Cl2Aq + 6MOHAq= Fe2OH+6MClAq. M. P. C. where M = Na or K. 4 In each case reaction occurred between a salt of a heavy metal and a solution of an alkali; in three cases an hydroxide of the heavy metal was pptd., in the other case an oxide of the heavy metal was pptd.; in every case a salt of the metal of the alkali was formed. Exp. 3. Into portions of the solutions of potassium and sodium hydroxides prepared in Exp. 1, pass carbon dioxide for a few minutes. The carbon dioxide reacts with the alkali Fig. 19. therefore is carbon present to form carbonate of sodium or potassium. To prove that a carbonate is formed, pour each solution into a small flask with a well-fitting cork carrying tubes as shewn in fig. 19. Let the exit tube from each flask pass into a clean beaker, in which you place a little lime water. Pour a little hydrochloric or sulphuric acid through the funnel tube into each flask. A gas comes off which reacts with the lime water to produce a white pp.; this gas. dioxide. But carbonates are recognised by the fact that they interact with acids to evolve carbon dioxide gas. Carbon dioxide is an acidic oxide. Alkalis in solution react with many acidic oxides to produce salts. Exp. 4. Place two small pieces of suet, each weighing about 50 grams, in basins; into one basin pour a solution of about 10 grams of potassium hydroxide, and into the other a solution of about 10 grams of sodium hydroxide. Heat the contents of the basins with constant stirring until clear liquids are obtained; continue heating for 10-15 minutes; then allow to cool. A semi-solid crust forms on the surface of each liquid. Break the crusts and pour out the liquids. The crusts are soaps; in the one case potash-soap, in the other case soda-soap. The liquids contain glycerin; prove this by moistening a borax bead with the liquid and bringing the bead into the upper part of a Bunsen-flame, when a green colour is imparted to the flame. Alkalis react with fats to form soaps and glycerin. The change of composition which occurs is similar to that noticed in Exp. 2. Both classes of change may be represented by the general equation; RX + MOH = ROH + MX, where RX is a compound of a positive and a negative radicle (a salt), and MX is a compound of the metal of the alkali used with the negative radicle of the salt RX. In Exp. 2 the compounds RX were metallic salts; R was Fe, Mn, or Zn, X was SO or Cl. In the present Exp. RX is a fat; both R and X are composed of more than one element. Common mutton suet is almost pure stearin or glyceryl stearate. The reaction which occurs when this fat is saponified by potash may be expressed thus ; (C3H5) (C18H35O2)3 + 3KOH = C2H5.O2H3 + 3K.C1H35O2 [Here R = C,H,, and X = C1H3502.] 18 Recall Exp. 2, Chap. VII. where you neutralised acids by the interactions of these acids with solutions of potassium and sodium oxides. Solutions of these oxides contain the alkalis potassium and sodium hydroxide [M,O + H2O + Aq=2MOHAq.]. The hydroxides of the metals lithium, sodium, potassium, rubidium, and cæsium, are alkalis. Alkalis then have the composition MOH where M = Li, Na, K, Rb, or Cs; they are very soluble in water; aqueous solutions of alkalis neutralise acids forming salts and water; they react with many acidic oxides to form salts and water; they react with solutions of salts of many heavy metals to form pps. of hydroxides, or sometimes oxides, of these metals, and at the same time to produce compounds of the metal of the alkali used with the acid radicle of the salt of the heavy metal; they saponify fats. The hydroxides of the metals calcium, strontium, and barium, resemble the alkalis in most of their properties: the composition of these hydroxides is MOH, where M = Ca, Sr, or Ba. 2 Exp. 5. Prove experimentally (1) that aqueous solutions of calcium and barium hydroxides are alkaline to litmus; (2) that these solutions react with solutions of ferric chloride, manganese sulphate, zinc sulphate, and mercuric chloride in a way similar to that in which the alkalis react; (3) that these solutions react with carbon dioxide to form carbonates of calcium and barium respectively; (4) that these solutions very slightly and slowly saponify fat; (5) that the solutions neutralise acids forming salts. Exp. 6. Weigh out approximately equal masses of the alkali caustic potash (KOH), and the alkaline hydroxide caustic baryta (BaO,H,), say about a couple of grams of each, and add each to the same quantity of water, say to 10 c.c. The alkali dissolves rapidly and a great deal of heat is produced during the solution; only a little of the alkaline hydroxide dissolves, and comparatively very little heat is produced during the solution. Exp. 7. Heat a little solid dry caustic potash, and a little solid dry calcium hydroxide, in separate dishes, covering each with a dry funnel; the potash melts at a high temperature but does not give off water; the calcium hydroxide does not melt, but it is decomposed into calcium oxide and water. Now tabulate the results of your experiments on alkalis and alkaline hydroxides, shewing clearly the resemblances and differences between these compounds. Recall Exp. 8, Chap. VIII., in which you proved that ferric hydroxide reacts with acids to form salts; ferric hydroxide is insoluble in water; it is neutral to litmus; it does not saponify fats; it does not react with carbon dioxide to form a carbonate of iron. The term basic hydroxide may be used to include all hydroxides which interact with acids to form salts. You have now learned to contrast acidic with basic oxides; and to contrast alkalis, alkaline hydroxides, and neutral hydroxides, with acids. You have learned some of the relations of composition and properties between salts, acids, basic hydroxides (including alkalis, alkaline hydroxides, and neutral hydroxides), basic oxides, and acidic oxides. The term base is frequently used to signify a compound which interacts with acids to form salts and water, only; the term includes basic oxides and basic hydroxides, and also some other compounds, such as ammonia (NH), ethylamine (NH.CH), &c. Reference to "ELEMENTARY CHEMISTRY"; Chaps. VIII. and IX. CHAPTER XI. REACTIONS BETWEEN ACIDS AND SALTS. Exp. 1. To the salt barium chloride add a little concentrated sulphuric acid. Arrange the experiment as shewn in fig. 20. Put about 3 or 4 grams of the salt in the small flask, and pour the acid down the funnel tube. Allow the gas which comes off to pass into a little distilled water; the gas dissolves rapidly. When the reaction has moderated, heat the flask gently and continue to heat for 5 minutes or so. Remove the exit tube from the water before you take away the lamp. When the contents of the flask are cold add some water; shake up well; pour off the water; add more Fig. 20. water, and shake up again; pour off; continue to wash the white solid by decantation until the washings are no longer acid to litmus paper. Now prove that the white solid is barium sulphate (s. Chap. VII. Exp. 5). Prove that the aqueous solution of the gas produced in the reaction, (1) is acid to litmus, (2) gives the reaction with silver nitrate characteristic of a chloride, (3) neutralises caustic potash forming potassium chloride, (4) dissolves metals-say zinc and magnesium-with evolution of hydrogen. The liquid is an aqueous solution of hydrochloric acid. |