CHAPTER VII.

CHEMICAL CLASSIFICATION.

ELEMENTS are placed in the same class because they form compounds having similar compositions and similar properties. In this chapter are given merely the outlines of some experiments on classification.

I. The three metals Calcium, Strontium, and Barium are placed in the same class.

Exp. 1. Given the oxides, CaO, SrO, and BaO; examine quantitatively the interaction of each with water. To a weighed quantity, 2 or 3 grams, of each oxide add water drop by drop as long as there is any apparent reaction. Dry the products at about 150° and weigh. Assuming the oxides to have the composition MO (M = Ca, Sr, Ba), and the atomic weights of M and O to be known, calculate the composition of the product of the interaction in each case.

Exp. 2. Examine quantitatively the action of heat on the three hydroxides MOH. Heat weighed quantities (about 5 gram in each case) of the compounds prepared by combining the oxides with water, in weighed small hard glass tubes open at one end, to about 200o, 300°, and 400°, weighing the tubes with their contents after each experiment. Also heat weighed quantities of the compounds to about the same temperature in a stream of dry air, freed from CO,, and weigh the products.

Exp. 3. Examine quantitatively the action of heat on the carbonates MCO. Heat weighed quantities (about 5 gram) of the three carbonates to redness over a blowpipe-flame for 10

minutes, in open crucibles, and determine the amounts of unchanged carbonates, by dissolving out the oxides produced, by repeated washing with cold water (until the washings do not affect the colour of turmeric paper), drying at 100o, and weighing.

Exp. 4. Prepare the sulphates, MSO, and determine their solubilities in water. Add the oxides to moderately dilute warm sulphuric acid until the acid is nearly saturated; remove the remaining acid by washing with cold water, and dry at 150°. Digest an excess of each sulphate in water at the temperature of the air for some hours; then withdraw a measured volume of each solution, evaporate to dryness in water-baths, dry at 150°, and weigh.

Exp. 5. Examine quantitatively the interactions between the sulphates MSO and saturated solutions of sodium carbonate. To weighed equal quantities of the sulphates add equal volumes of a saturated solution of sodium carbonate; boil for equal times; wash the residual mixtures of sulphates and carbonates with cold water as long as the washings contain sulphates; dry at 150°; weigh; determine the carbon dioxide obtainable from each solid, and thus estimate the masses of sulphate and carbonate in each. State your results so as to shew the percentage of each sulphate MSO, changed to carbonate MCO,.

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Exp. 6. Determine, roughly, the heats of neutralisation of aqueous solutions of the hydroxides, MO,H,; the acid being hydrochloric. Prepare a considerable quantity of a normal hydrochloric acid solution, i.e. a solution containing exactly 36.5 grams HCI per litre. Prepare cold saturated solutions of the three hydroxides; keep these solutions in bottles fitted with greased stoppers, each bottle being filled with the liquid; determine the quantity of hydroxide MO2H, in a specified volume of each solution, by titration with standardised oxalic acid solution, using turmeric paper as an indicator. Calculate the volume of each solution which contains that mass of the hydroxide which is equivalent to the mass of HCl in a specified volume, about 250 c.c. of the normal solution, assuming the interaction to be MO,H,Aq + 2HClAq = MCl, Aq + 2H2O. Measure out this volume of the normal hydrochloric acid solution into each of three beakers; measure out the solutions of the three hydroxides into stoppered bottles of a size such that

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each is as nearly as may be filled with the solution; place these bottles and the beakers in a box lined and covered with felt 30 to 40 mm. thick, and place this box alongside the calorimeter described below, in a room the temperature of which varies but little. After an hour or two, pour the contents of one of the bottles into the calorimeter; quickly add the hydrochloric acid from one of the beakers; stir the liquid, observing the course of the thermometer, and note the highest temperature reached. Wash out and dry the beaker of the calorimeter, and repeat the Exp. with each of the hydroxides.

From your results find the quantity of heat produced when an equivalent, in grams, of each hydroxide (MOH ̧ grams) interacts with an equivalent, in grams (2HCl grams) of hydrochloric acid. In the calculation the water-equivalent* of the beaker may be taken as the product of the weight of the beaker multiplied by the specific heat of hard glass (19); the solution in the beaker may be assumed to have the same specific heat as water.

The quantity of heat is about 27,500 gram-units in each case.

Calorimeter.

Line the inside, bottom, and cover, of a cylindrical pasteboard box with felt about 30 to 40 mm. thick. The box must be of such a size that when a beaker of about 600-700 c.c. capacity is placed in it a space of about 15 to 20 mm. remains between the beaker and the felt. The lid of the box is pierced by two holes; one to admit a thermometer graduated to degrees, the other to admit a stirrer formed of a glass rod reaching to the bottom of the beaker and then bent into a spiral. Weigh a thin beaker of about 650 c.c. capacity; place the beaker in the box, and pack cotton wool round it.

II. The elements Sulphur and Chromium belong to the same group, although they exhibit considerable differences: Exp. 7. Prepare the trioxides SO, and Cro, and prove that both are acidic oxides. Arrange an apparatus as shewn in Fig. 51. The bottle A contains a concentrated aqueous solution of sulphur dioxide; B contains concentrated sulphuric

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* Or the water-equivalent may be more accurately determined by adding a known weight of hot water at a known temperature to a known weight of cold water at a known temperature in the calorimeter, and noting the highest temperature which the mixture attains.

acid; C is a piece of hard glass tubing with some platinised asbestos loosely packed into it; D is a small dry flask. The

asbestos is covered with finely divided platinum by soaking it in concentrated platinic chloride solution and then heating strongly. A stream of oxygen is passed into A; the gas issuing from A is a mixture of oxygen and sulphur dioxide; the mixed gases are dried by bubbling through B, and are then passed over the heated finely divided platinum whereby chemical union occurs; the sulphur trioxide, SO,, thus produced is condensed in D. When a few grams of the white snow-like compound SO, have collected in D, stop the process, and at once dissolve the contents of D in about 500 c.c. water; exactly neutralise a portion of the solution by potash; evaporate over a low flame and allow to crystallise; purify the salt which separates by re-crystallisation from warm water; dry the crystals and label them potassium sulphate.

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Dissolve 30 grams of commercial potassium dichromate, KCr,O,, in a warm mixture of 50 c.c. water and 42 c.c. concentrated sulphuric acid; allow to stand for some hours; pour off from the pp. of potassium-hydrogen sulphate, KHSO; warm to 80°-90°; add 15 c.c. concentrated sulphuric acid, and then water drop by drop until the pp. of chromium trioxide which has formed is just dissolved; then evaporate the liquid to the crystallising point and allow to cool; after some hours pour off the mother liquor, collect the crystals in a funnel fitted with a small platinum cone pierced with very small holes, and drain by means of the pump; then spread the crystals of CrO, on a dry porous tile covered with a bell-jar; when the crystals are dry remove them by means of a platinum or glass spatula to a beaker, add 5 c.c. of concentrated nitric acid, and spread out the crystals on another porous tile where

they are allowed to remain until dry*. Finally warm the crystals to 60°-80° until nitric acid ceases to be evolved. Dissolve the chromium trioxide thus prepared in water, neutralise a portion of the solution by potash, evaporate, and crystallise. Purify the salt thus obtained by re-crystallisation from warm water; dry the crystals and label them potassium chromate.

Make estimations of potassium and of sulphuric acid in the crystals of potassium sulphate, and estimations of potassium and chromic acid in the crystals of potassium chromate you have prepared. Assuming the atomic weights of potassium, oxygen, sulphur, and chromium, to be known, and assuming the various reactions on which the analytical methods are based to be known, your analyses prove that the compositions of the two salts are represented by the formulæ K,SO, and K ̧CrО, respectively.

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Exp. 8. Dissolve in water portions of each salt prepared in Exp. 7; to the sulphate add an aqueous solution of the sulphur trioxide, and to the chromate an aqueous solution of the chromium trioxide, you prepared; evaporate and crystallise; recrystallise the salts from water; dry, and analyse each. Find the simplest formula which express the compositions of the salts.

The salts are KHSO4, and K,Cr2O7, respectively.

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Exp. 9. Prepare chromium sulphate from chromium trioxide. Heat chromium trioxide with fairly concentrated hydrochloric acid and a little alcohol, until the solution is green; allow to cool; add a slight excess of ammonia; collect and wash the pale greenish blue pp. thus produced. We shall assume that the composition of this pp. is Cr2O,. жH2O. Dry the pp. at 100° and dissolve it in excess of concentrated sulphuric acid; evaporate until fumes of sulphuric acid begin to be evolved; then allow to cool. Collect the pink solid which separates in a funnel fitted with a little platinum cone pierced with very small holes; dry by spreading on a porous tile in an exsiccator; then wash with alcohol and dry again.

Assuming that the salt thus formed is an anhydrous sulphate of chromium, determine its composition by heating

* The object of thus washing with nitric acid is to remove sulphuric acid and KHSO4.