c. Place a small crystal of such nitrates as are available in your laboratory in separate test tubes and test their solubility in water. What nitrates are insoluble in water (p. 394 of text)?

d. How to detect the presence of nitrates. Dissolve a crystal of sodium nitrate in 2 or 3 cc. of water in a test tube, carefully add (recall experiment b, Exercise 8) an equal volume of sulfuric acid, mix, and cool to room temperature or below. The sulfuric acid acts on the nitrate, liberating nitric acid. Now tip the tube slightly and gently pour 2 or 3 cc. of the solution of ferrous sulfate down the side of the tube, so that it floats on the heavier liquid, and set the tube aside, being careful not to mix the two liquids. A brown ring soon forms where the liquids meet. Repeat the experiment, using potassium nitrate. This is a good test for nitrates. The brown ring is due to the presence of a complex compound formed by the action of ferrous sulfate on nitric acid.

EXERCISE 35

THE PREPARATION AND PROPERTIES OF SOME OF THE OXIDES OF NITROGEN

Apparatus. Hard-glass test tube, with delivery tube, as used in preparing oxygen (Fig. 20); 3 wide-mouthed bottles (250-cc.); pneumatic trough; hydrogen generator (Fig. 21).

Materials. 8 g. ammonium nitrate; wooden splints; 5 small strips of copper; 10 cc. nitric acid.

a. Nitrous oxide. Put 6 or 8 g. of ammonium nitrate in the hard-glass test tube used in the preparation of oxygen. Attach a delivery tube and heat gently, applying no more heat than is necessary to cause a slow evolution of the gas. As soon as the gas is regularly evolved, collect two or three bottles of it over water. Notice the water deposited on the sides of the test tube. What is the source of it?

Note the color, odor, and taste of the gas. Test it with a

glowing splint. Account for the result.

How can you

distinguish between nitrous oxide and oxygen?

b. Nitric oxide and nitrogen dioxide. Put a few pieces of copper in your hydrogen generator (hood), just cover them with water, and add 2 or 3 cc. of nitric acid. Collect over water two bottles of the evolved gas, adding more nitrio acid to the liquid in the generator if necessary.

Compare the color of the gas in the generator with that collected in the bottles and account for any difference. Write the equations for all the reactions involved.

Uncover one of the bottles of the gas and account for the result (R). Test the gas in the second bottle with a burning splint. Which is the more stable, nitrous oxide or nitric oxide? Give reasons for your answer.

EXERCISE 36

SPEED OF REACTIONS; EQUILIBRIUM; HYDROLYSIS

Apparatus. 6 test tubes; test-tube rack.

Materials. Hydrochloric acid; ammonium molybdate solution (R. S.). Silver nitrate solution (R. S.); 0.1 g. sodium phosphate; crystal of sodium chloride dissolved in 5 cc. water; 0.3 g. of each of the following salts dissolved in separate portions (3 cc.) of water: potassium nitrate, common salt, borax, sodium carbonate, alum.

a. Speed of a reaction. Add 2 drops of hydrochloric acid to 3 cc. of water and mix the two liquids; then add 1 or 2 drops of a solution of silver nitrate. Is a precipitate formed at once? Does the precipitate appear to increase on standing?

Dissolve a crystal of sodium phosphate the size of a grain of wheat in 10 cc. of water and add 5 cc. of a solution of ammonium molybdate. Is a precipitate formed at once? Set the tube aside until the end of the laboratory period, noting its appearance from time to time. Contrast the results with those obtained by the addition of silver nitrate to hydrochloric acid.

b. Factors affecting speed of a reaction. Recall, from experiments already performed, the effect of temperature; also of concentration upon the speed of a reaction.

c. Reversible reactions. What is meant by a reversible reaction? The reaction used in the preparation of nitric acid (Exercise 33) is reversible. How can it be made to complete itself?

Repeat the first part of experiment a, substituting a solution of potassium nitrate for that of silver nitrate (?). The potassium nitrate acts upon the hydrochloric acid just as the silver nitrate does. In the former case, however, the reaction soon comes to an equilibrium while in the latter it goes to practical completion. Explain.

Recall the process of neutralization. To what extent are such reactions completed reactions?

d. Hydrolysis. Pour into separate test tubes small amounts of solutions of (1) potassium nitrate, (2) sodium chloride, (3) borax (p. 380 of text), (4) sodium carbonate (p. 403 of text), (5) alum (p. 459 of text). Test the action of each on blue and on red litmus paper, note the results, and explain.

EXERCISE 37

THE PROPERTIES OF SULFUR

Apparatus. 3 test tubes; smallest-sized beaker; magnifying glass; porcelain crucible; ring stand and burner; large beaker.

Materials. 5 cc. carbon disulfide; 20 g. powdered sulfur; strip of copper; 5 g. iron powder.

a. Examine the physical properties of a piece of brimstone. Pour 2 or 3 cc. of carbon disulfide (hood) (keep carbon disulfide away from flame and do not inhale the vapor) over 3 g. of powdered brimstone in a test tube. Cover the mouth of the tube with the thumb and shake the contents gently until the sulfur is dissolved, adding more carbon disulfide if necessary. Pour the clear solution into a small beaker, cover it

loosely with a filter paper, and set it aside in the hood. The carbon disulfide soon evaporates, the sulfur being deposited in crystals. Examine these with a magnifying glass (?).

b. Half fill a test tube with powdered brimstone and heat it gently until the sulfur is just melted. Note the properties of the liquid.

Now apply a stronger heat and observe that the liquid becomes darker and at a certain temperature (200°-250°) is so thick that the tube may be inverted without spilling it.

Finally, increase the heat until the sulfur boils (444°), and then pour the boiling liquid into a beaker of cold water. Examine the product. What name is given to this form of sulfur? Expose it to the air for an hour. Are its properties the same as those of the original substance?

c. Fill a porcelain crucible with powdered brimstone and apply a very gentle heat until the sulfur is just melted. Withdraw the flame and examine the liquid carefully as it cools. Crystals soon begin to form on the surface of the melted sulfur, rapidly extending from the circumference toward the center. Before they reach the center, quickly pour off the remaining liquid and examine the crystals. Contrast them with those formed in a. In how many forms have you obtained sulfur?

d. Burn a small piece of sulfur. Note the appearance of the burning sulfur and the odor of the gas formed (?).

e. Boil a little sulfur in the test tube used in b, and drop a small strip of hot copper foil into the boiling liquid. Is there any visible evidence of a chemical change? What is formed?

Grind together some iron powder with an equal weight of sulfur and heat the mixture in a test tube (?).

EXERCISE 38

THE PREPARATION AND PROPERTIES OF HYDROGEN

SULFIDE

Apparatus. Hydrogen generator and tubes, as shown in Fig. 41; 2 wide-mouthed bottles (250-cc. and 60-cc.); funnel; evaporating-dish. Materials. 10 g. ferrous sulfide; 20 cc. hydrochloric acid added to 20 cc. water; 3 cc. nitric acid; blue and red litmus paper; silver coin; filter paper.

a. (Hood.) Attach a delivery tube to the hydrogen generator, as shown in Fig. 41. Put into the generator A a few pieces of ferrous sulfide (FeS)

and insert the stopper. Now pour a little water through the funnel tube of the generator until the end of the tube just dips below the surface of the water; then pour in a few cc. of the hydrochloric acid, adding more from time to time, if necessary, to maintain a gentle evolution of the gas (R). The gas escapes into the bottle B, which gradually

A

FIG. 41

B

becomes filled. Note the odor (CAUTION: the gas is poisonous if inhaled in concentrated form) and color of the evolved gas. Continue the evolution of the gas until it is ignited by a flame held at the mouth of the bottle B. Account for the deposit on the sides of the bottle (?).

b. Replace the bottle B with a 60-cc. bottle half filled with water, and allow the gas from the generator (add more acid if necessary) to bubble through the water for one or two minutes. Test the resulting solution with blue and with red