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When no more bromine comes over, remove first the delivery tube and then the flame. The light-brown solution is "bromine-water."

c. Wave the air from the test tube toward the nose. Odor of bromine? Pour off as much bromine-water as possible without pouring out the bromine, and add more water to the bromine. Pour a few drops of the brominewater upon litmus paper and upon colored calico.

Results?

d. To 3 c.c. water in a test tube add 1 c.c. carbon disulphide, close tube with thumb, and shake vigorously. Results? Where is the carbon disulphide? Now add

c.c. bromine-water and shake again. Result to the color of the water? To that of the carbon disulphide? This effect on the carbon disulphide is a test for free bromine.

e. To 5 c.c. of potassium bromide solution add 1 c.c. carbon disulphide and shake. Result? Now add two or three drops of chlorine-water (make as in Experiment XXI, i), close the tube, and shake it as before. Results? Action of chlorine on potassium bromide? Equation?

f. To the liquid bromine saved from c add sodium hydroxide solution, a cubic centimeter at a time, shaking thoroughly. (Do not close the tube with the thumb!) Result? The equation is,

2 NaOH+2 Br

NaBr+NaBrO+H2O.
sodium

hypobromite

IODINE AND HYDRIODIC ACID.

75

EXPERIMENT LI.

IODINE AND HYDRIODIC ACID.

Apparatus. Test tubes, beaker, flask.

Materials.

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Potassium iodide, manganese dioxide, sulphuric acid, iodine, carbon disulphide, chlorine- and bromine-water, starch, alcohol, hydrogen sulphide, silver nitrate solution, sodium carbonate, litmus.

a. Powder potassium iodide (KI), mix 1 c.c. of it with a c.c. of manganese dioxide, add 2 c.c. water and then 1 c.c. concentrated sulphuric acid. Result? When the action slackens, warm the tube gently, and then let it cool. Describe what you find in the tube. It is iodine. Compare its preparation with that of chlorine and bromine.

b. Warm a crystal of iodine (gently, not to boiling) with 10 c.c. water for a few seconds. Does the iodine dissolve readily? Cool the water and add 3 c.c. of it to 1 c.c. carbon disulphide. Shake the closed tube. Result? This is a test for free iodine. Save the iodine solution.

c. Shake 5 c.c. potassium iodide solution with 1 c.c. carbon disulphide. Result? Add a drop of chlorinewater and shake again. Result? What effect has chlorine upon potassium iodide? Repeat, using brominewater instead of chlorine-water. Write both equations. follows: Mix 2 c.c.

d. Make a starch solution as powdered starch with 5 c.c. cold water, and pour the emulsion into 30 c.c. boiling water. Boil for a minute or

two, and then cool. To 3 c.c. of the solution add a drop of the iodine solution of b, shaking.

Result?

To 3 c.c. of the starch solution add one drop of a potassium iodide solution and then one drop of chlorineor bromine-water. Result?

e. Heat a crystal or two of iodine in a dry, inclined test tube. Result? Let cool. Result? Effect of iodine on the skin? On wood and paper?

f. To the iodine of e add 5 c.c. ethyl alcohol, C2H¿OH. In which is iodine more soluble, water or alcohol? An alcoholic solution is often called a tincture.

g. To one-half a c.c. of powdered iodine in a flask add 20 c.c. water and then pass in hydrogen sulphide (gaschamber!) until the iodine disappears. Results? Boil the solution gently two minutes, and filter it. Identify the precipitate by igniting a little on a piece of porcelain. Odor? Test the filtrate with red and blue litmus. Results? Add a drop of it to 1 c.c. silver nitrate solution. Result? Add some to 1 c.c. solid sodium carbonate Result? What substances are formed from hydrogen sulphide and iodine? Equation?

EXPERIMENT LII.

COMPARISON OF THE HALOGEN ACIDS.

Materials. Potassium chloride, bromide, and iodide; con centrated sulphuric acid, litmus.

a. Three test tubes have small amounts of potassium chloride, bromide, and iodide, respectively; treat each with a few drops of concentrated sulphuric acid. Re

HYDROGEN PEROXIDE.

77

sults? Blow your breath across the mouth of each tube. Result? Test the gas of each with blue litmus. Result? Note carefully the odor of each gas. What odors beside that of the acid do you get in the tube of potassium iodide? Heat this tube. Result?

b. Which tube gives a colorless gas? What colors the gas in each of the two other cases? From the amount of coloration, tell which of the three halogen acids is most easily decomposed into its elements. Which least.

EXPERIMENT LIII.

HYDROGEN PEROXIDE.

Materials. Hydrochloric acid, barium peroxide, starch solution, potassium iodide solution, manganese dioxide, potassium permanganate, ether, potassium dichromate solution, splinter.

a. To 25 c.c. water add 5 c.c. concentrated hydrochloric acid and then 3 grams powdered barium peroxide, BaO2, a little at a time, stirring. Filter the solution; it should contain hydrogen peroxide, H2O2.

b. To 5 c.c. starch solution add a drop of potassium iodide solution, and then a few drops of the hydrogen peroxide solution. Result?

Do

c. To 3 c.c. of the solution of a add 3 c.c. ether. they mix? Is the ether above or below? Now add one drop of potassium dichromate solution. Close tube and shake gently.

Result?

d. To 5 c.c. of the hydrogen peroxide solution add 1 c.c. powdered manganese dioxide. Result? Test gas with a glowing splinter. Result?

e. To three crystals of potassium permanganate in a test tube add 2 c.c. water and then 5 c.c. of the hydrogen peroxide solution. Result? Test with glowing splinter. Result?

EXPERIMENT LIV.

PHOSPHORUS AND PHOSPHORIC ACID.

Apparatus. Test tubes, small ignition tube, tongs, evaporating dish, file.

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Materials. Red and yellow phosphorus, carbon disulphide, filter paper, phosphoric acid, ammonium hydroxide, powdered disodium hydrogen phosphate; magnesium sulphate, ammonium chloride, silver nitrate, and calcium chloride solutions.

Caution. Ordinary, yellow phosphorus must be handled only with tongs, never with fingers! It must be kept and cut under water. No pieces of it must get into your locker; and every dish that has contained phosphorus must be heated, so that the phosphorus may be completely burned.

Do not bring carbon disulphide near a flame!

a. Put half a c.c. of red phosphorus into a test tube, and add 3 c.c. carbon disulphide. Result? Filter, and let the carbon disulphide evaporate, without heating, in a hood, or where its vapor will not get near a flame. Result? Was any phosphorus dissolved?

To 3 c.c. carbon disulphide add a piece of yellow phosphorus not larger than a grain of wheat. Shake carefully a few minutes. Result? Pour the solution, every drop of it, upon a piece of filter paper laid flat on a ring of the ring stand. Let the carbon disulphide evaporate with

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