EXERCISE 59

A STUDY OF SOME OF THE PROPERTIES OF

ANTIMONY

Apparatus. Blowpipe; beaker; stirring-rod; hydrogen sulfide generator (Fig. 42).

Materials. Piece of charcoal (2 cm. x 8 cm.); 2 pieces of antimony (size of a grain of wheat); hydrochloric acid; nitric acid; strip of zinc; 10 g. ferrous sulfide.

a. Heat a bit of antimony on charcoal as in a, Exercise 58. The product is Sb2O.

b. Introduce into a test tube a bit of antimony no larger than a grain of wheat and add about 3 cc. of hydrochloric acid and then 2 or 3 drops of nitric acid (?). After the metal has dissolved, pour the solution into 50 cc. of water. If a precipitate forms, add hydrochloric acid, drop by drop, with constant stirring, until the precipitate dissolves. Half fill a test tube with the resulting solution and insert a strip of zinc. Note the results (compare Exercise 30). Through the remainder of the solution pass a few bubbles of hydrogen sulfide. The product is Sb,S,. Note its properties.

EXERCISE 60

A STUDY OF SOME OF THE PROPERTIES OF BISMUTH

Apparatus. Blowpipe; beaker; stirring-rod; hydrogen sulfide generator (Fig. 42).

Materials. 2 pieces of bismuth (size of a grain of wheat); piece of charcoal (2 cm. x 8 cm.); nitric acid; 10 g. ferrous sulfide; hydrochloric acid.

2

a. Heat a piece of bismuth on charcoal, as in the case of antimony (Exercise 59). Bi2O, is formed and is deposited on the charcoal. Contrast the effect of heating arsenic, antimony, and bismuth in air.

b. Repeat b, Exercise 59, substituting bismuth for antimony and using nitric acid alone as the solvent. The precipitate formed by hydrogen sulfide is Bi,S. Note its properties.

Bi (NO), is formed when bismuth is dissolved in nitric acid. Upon pouring this into water a reaction takes place in accordance with the following equation:

OH

Bi(NO), +2 H2O→ BiOH →→ BIONO, + H2O

The bismuth subnitrate (BiONO,) is insoluble and separates as a white solid. Upon the addition of nitric acid, a drop at a time, the reaction is reversed, Bi(NO), being formed, which dissolves in the liquid.

c. Define the terms hydrolysis, basic salt, and reversible reaction, and illustrate with examples from this exercise.

EXERCISE 61

COMPOUNDS OF SILICON

Apparatus. Evaporating-dish; ring stand; burner.

Materials. 2 cc. water glass (solution of Na,SiO); hydrochloric

acid.

a. Recall the formulas and names of the important acids of silicon.

b. Place 2 cc. of a solution of water glass (Na,SiO) in an evaporating-dish, dilute with 10 cc. of water, and add 2 or 3 cc. of hydrochloric acid. Note the gelatinous precipitate (R). Evaporate to dryness and heat the dish gently with the bare flame (?). When cool, add water, filter, and examine the residue. What is it?

c. Recall the action of hydrofluoric acid on silica (R).

EXERCISE 62

COMPOUNDS OF BORON

Apparatus. Platinum wire (piece 10 cm. long, fused in glass tube for handle (Fig. 165, p. 414 of text)); burner; beaker; porcelain crucible; stirring-rod.

Materials. 8 g. borax; litmus paper (red and blue); sulfuric acid; alcohol (R.S.); 1 or 2 drops cobalt nitrate solution (R.S.).

a. Write the names and formulas for three important compounds of boron.

b. Make a little loop on the end of a platinum wire and heat it to redness in a Bunsen flame, then quickly bring the loop in contact with some borax and reheat. The borax adhering to the loop will swell up (owing to the expulsion of water from the hydrate) and finally form a clear, glassy bead. Note the color imparted to the flame.

Moisten the bead with a drop of sulfuric acid and again touch it to the edge of the flame. Note the result (see c, below, for explanation). This serves as a simple test for borax.

Moisten the bead with a drop of a solution of a cobalt compound and reheat until the bead is transparent when cold. Note the color of the bead now. This property serves as a simple test for cobalt. (Some of the other metals likewise impart characteristic colors to the bead.)

c. Dissolve 5 g. of borax in 15 cc. of boiling water. Test the solution with litmus paper. Explain. Carefully add to the hot solution 2 or 3 cc. of sulfuric acid and stir. Cool the solution and filter off the precipitate. Compare the precipitate with borax, first, as to solubility in alcohol, and, second, as to the color imparted to a flame when a small portion on a platinum wire is held in the flame.

d. Dry a little of the boric acid prepared in c, transfer to a porcelain crucible, and heat until a clear liquid is formed (?).

EXERCISE 63

COLLOIDS AND EMULSIONS

Apparatus. 6 test tubes; 2 beakers; hydrogen sulfide generator; stirring-rod; tripod and burner.

Materials. 0.5 g. flowers of sulfur; alcohol (R. S.); 1% solution of tartar-emetic (R.S.); ferrous sulfide and hydrochloric acid for preparing hydrogen sulfide; ferric-chloride solution (R. S.); sulfuric acid; 0.5 g. of dried gelatin; ice water; white of an egg; nitric acid; ammonium hydroxide; 0.5 cc. kerosene or cottonseed oil; 1% soap solution (R.S.); 10 cc. water glass, density 1.08 (R. S.).

a. Place in a test tube a small amount of flowers of sulfur (volume of a pea). Pour over this 5 cc. of alcohol and heat to the boiling point for 2 minutes, so that a saturated solution may be formed. Set the tube aside until the undissolved sulfur settles to the bottom of the tube; then pour the clear supernatant solution into 100 cc. of water and stir the liquid. The sulfur separates (?) in the form of a colloid.

b. To 18 cc. of water add 2 cc. of the solution of tartaremetic. (So far as the experiment is concerned, the tartaremetic may be regarded as the antimony salt of tartaric acid; its real formula is KSbOCHO..) Slowly bubble hydrogen sulfide through the solution for 2 or 3 minutes; antimony sulfide (Sb,S) forms. Does it precipitate (explain) ? Now add 3 drops of hydrochloric acid to the liquid; mix and set the tube aside for a few minutes (?). (See last paragraph, p. 383 of text.) Would you expect antimony chloride to give the same results as the tartar-emetic? (Recall that tartaric acid is a weak acid and hydrochloric acid a strong one.)

c. Heat 100 cc. of water to boiling and add 3 drops of the solution of ferric chloride. Ferric hydroxide (Fe(OH),) forms but remains in colloidal suspension. Now add 2 or 3

drops of sulfuric acid, stir, and set the beaker aside for a few minutes. The ferric hydroxide precipitates (?).

d. Place in a test tube a sufficient amount of small pieces of dried gelatin to make a layer 1 or 2 cm. in depth. Add about 10 cc. of water and heat the water until the gelatin forms a colloidal solution (gelatin sol). Now place the tube in ice water and note the formation of a gel. Again heat and cool the substance to see if the coagulation of the gelatin is a reversible process (?). Save the substance for use under f.

e. Dissolve a small portion of the white of an egg in 10 cc. of cold water. Divide the liquid into two equal parts. Heat the one portion to boiling (?). To the other portion add 2 or 3 drops of nitric acid (?). Make suitable tests to determine whether the coagulation of the white of the egg (albumin) is a reversible process (?).

f. To 10 cc. of water glass, density 1.08, add 10 drops of sulfuric acid. Mix the liquids and set aside until the next laboratory period (?).

g. Often a colloid will prevent the formation of a precipitate or will delay crystallization; it is then called a protective colloid. This principle may be illustrated as follows: To 20 cc. of water add 4 drops of a solution of ferric chloride. Mix thoroughly and half fill each of two test tubes with the solution. To the solution in one of the tubes, add 2 or 3 cc. of the gelatin sol prepared in d above, and shake the mixture; then to each of the tubes add 3 or 4 drops of ammonium hydroxide and heat the resulting liquids to boiling. Set the tubes aside for a few minutes (?).

h. Pour 0.5 cc. of kerosene or cottonseed oil into each of two test tubes. Add water to one of the tubes until it is three fourths filled; to the other add an equal volume of a 1% solution of soap. Close the tubes tightly with the thumbs and shake the mixtures violently for 1 minute; then set the tubes aside and note from time to time the changes in the mixtures in each tube (?).