d. Dissolve a crystal of potassium permanganate, KMnO4, in water, and add grape-sugar solution. Result? Explain (cf. Experiment LXIX, e).

e. From Experiment XXI tell the action of manganese dioxide with hydrochloric acid; from Experiment V, with potassium chlorate; from Experiment LIII, with hydrogen peroxide; and from Experiment LVIII, a, tell the color of the manganese bead.

EXPERIMENT LXXV.

CHROMIUM COMPOUNDS.

Apparatus. Chlorine generator, platinum wire, evaporating dish, test tubes.

Materials.

Solutions of potassium chromate and dichromate, of chromic chloride, of potassium and sodium hydroxides; hydrochloric acid, alcohol, borax, barium chloride solution, chrome-alum.

a. What is the color of solutions of potassium dichromate (K2Cr2O7), potassium chromate (K2CrO4), and of chromic chloride (CrCl)?

b. Treat 1 c.c. of potassium dichromate solution with a drop of potassium hydroxide solution. Result? From the color tell what is formed.

Complete the equation,

K2Cr2O7+2 KOH

→? +?

c. To 1 c.c. potassium chromate solution add a drop of concentrated hydrochloric acid. Result? What is formed?

2 K2CrO4+2 HCI-?+?+?

CHROMIUM COMPOUNDS.

105

How can a dichromate be changed to a chromate? A chromate to a dichromate?

d. To 2 c.c. potassium chromate solution add barium chloride solution. Result? Equation? Repeat with chromic chloride instead of the chromate. Result?

e. To 1 c.c. chromic chloride solution add a drop of sodium hydroxide solution. Result? Equation? Now add the alkali in excess, shaking. Result? The solution contains a chromite, NaCrO2. What other elements behave in this way? See Experiments LXV and LXXI. Save for g.

f. Repeat e with potassium chromate instead of chromic chloride. Result? In what three ways can a chromic salt be distinguished from a chromate?

g. To the clear solution of e add chlorine gas until there is no further change. Do this in a gas-chamber. Results? Test the resulting solution as in b, c, and d. Results? How can a chromic salt be changed into a chromate?

h. To 10 c.c. potassium dichromate solution in an evaporating dish add 2 c.c. concentrated hydrochloric acid and 2 c.c. ethyl alcohol. Boil until bright green, but not to dryness. Test a part of the liquid with barium chloride solution. Result? With potassium hydroxide solution. What does the green solution contain? How can a chromate be changed to a chromic salt? See, also, Experiment XLII, f.

i. Refer to Experiment LVIII for the borax bead test. Repeat with a tiny piece of chrome-alum. Result?

EXPERIMENT LXXVI.

LEAD.

Apparatus. File or knife, test tubes, mouth blowpipe. Materials. Lead; hydrochloric, nitric, and sulphuric acids; lead nitrate, solutions of potassium chromate and sodium hydroxide, lead oxide, stick of charcoal.

a. File or cut off the coating on lead. Is it hard or soft? Color? Try to mark on paper with lead. Result? Refer to Experiment LXV, a, for its fusibility. Treat a small piece with hydrochloric acid, both the dilute and the strong. Results? Wash the lead, and add 2 c.c. concentrated nitric acid and 2 c.c. water. Heat gently. Result? Write the equation (cf. Experiment XXXVII).

b. Heat one-fourth of a c.c. of lead monoxide on charcoal in the reducing flame (mouth blowpipe). See Experiment LXV, b. Result? How identify the product?

c. Dissolve 2 c.c. powdered lead nitrate, Pb(NO3)2, in 15 c.c. water, heating. Cool, and add to 2 c.c. of the solution 5 c.c. dilute hydrochloric acid. Result? Equation? Wash the precipitate by decantation, and heat it with 10 c.c. water. Result? Cool the solution. Result?

d. To 2 c.c. of the lead nitrate solution add dilute sulphuric acid. Result? Use potassium chromate solution instead of sulphuric acid. Result? Equation in each case?

From Experiment XLI, d, tell effect of hydrogen sul

phide upon lead nitrate. For the reduction of lead oxide by charcoal, see Experiment XLVIII, a.

e. To 2 c.c. lead nitrate solution add a drop of sodium hydroxide solution. Result? Equation? Now add an excess, shaking. Result? What three other hydroxides behave in the same way? See Experiment LXXV, e.

f. Put into the remainder of the lead nitrate solution a strip of zinc. Leave it at least ten minutes. Result? Equation (cf. Experiment LXIX, f)?

EXPERIMENT LXXVII.

TIN.

Apparatus. Test tubes, stopper and delivery tube, mouth blowpipe.

Materials. Tin (granular and in a bar); concentrated hydrochloric and nitric acids; solutions of mercuric chloride, stannic chloride, and sodium hydroxide; ammonium sulphide, hydrogen sulphide, zinc, sulphur, stick of charcoal.

a. Treat about 2 c.c. of small bits of tin with 10 c.c. concentrated hydrochloric acid in a test tube. Warm gently to start the action, and when the effervescence is vigorous attach a stopper and delivery tube and collect the gas over water. Identify the gas. Result? The solution contains stannous chloride, SnCl2. Equation? Let the action continue at least ten minutes.

b. From Experiment LXV, a, compare the fusibility of tin with that of lead, etc. Hold a bar of tin near your ear, and bend it. Result? What color has bright tin? Is it hard or soft?

c. To 1 c.c. mercuric chloride solution, HgCl2, add 4 or 5 c.c. of your stannous chloride solution, and then heat. Note all the changes. The solution contains stannic chloride, SnCl4. Equation?

d. To 2 c.c. stannous chloride solution add 5 c.c. water and then hydrogen sulphide. Result? Color? Equation? Wash the precipitate by decantation, and add 5 c.c. ammonium sulphide (use an evaporating dish or beaker) and a small lump of sulphur. Warm gently, and stir. Result? Cool, and add dilute hydrochloric acid in excess. Result? Result? Compare the color with that of

the original precipitate.

e. To 2 c.c. stannous chloride solution add 1 c.c. concentrated nitric acid, and heat gently. The solution contains stannic chloride. Dilute with 5 c.c. water, and pass in hydrogen sulphide. Result? Color? Stannic sulphide is SnS2. Equation? Wash the precipitate by decantation, add ammonium sulphide and a bit of sulphur, and warm gently. Result? Add an excess of dilute hydrochloric acid. Result? Compare with the color of the original precipitate, and with that obtained at the end of d. Conclusion?

f. To 2 c.c. stannic chloride solution add sodium hydroxide solution, drop by drop. Result? Add an excess. Result? What other hydroxides have behaved in the same way? See Experiment LXXVI, e.

g. Pour the solution of a from any unused tin, and put into it a strip of zinc. Result? Equation? Compare with Experiment LXXVI, f.

h. Heat a piece of tin on charcoal in the oxidizing flame (mouth blowpipe). See Experiment LXV, b. Results?