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4. Test each gas with a burning match; with a glowing splint. Repeat until you are sure what to expect in each case.

5. Determine whether the carbon or the zinc of the battery is connected with the tube in which the larger amount of gas is formed.

Questions. Which gas burns ? What product is formed ? How can you recognize the other gas ? Which terminal gives the larger volume of gas? How are the gases alike? how different ? Trace the path of the electric current from the switch through the circuit. What are the important facts shown by the experiment? What is a compound ? an element ?

Suggestions for report. Make a diagram of the apparatus; name all the parts ; show the direction of the current;

and name each gas

in the tubes. Reference work. Read sections 68 to 72. Make a list of all of the elements that you use in one day. Arrange these in order of their importance.

Optional problems. Fill a dry test tube with hydrogen. Bring a flame to the tube and watch for a new product. Explain. Find out from a chemistry text how hydrogen and oxygen are prepared in commercial quantities.

NATURE OF CARBON DIOXIDE AND ITS RELATION TO

RESPIRATION (VI-3)

Problem. Carbon dioxide is constantly being added to the air by the burning of all fuels, by the process of respiration, and by the decay of animal and plant substances. On the other hand, growing plants use great quantities of carbon dioxide from the air and return some oxygen. In order to find out more about carbon

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dioxide it is best to obtain it in quantity from an experiment other than that of the burning process. How can this be done ?

What to use. Ring stand, burette clamp, marble, hydrochloric acid, special test tube with side neck and one-hole rubber stopper to fit, glass plug, glass jar or sink, three ordinary test tubes or wide-mouth bottles, small candle, wire, charcoal, wood splints, forceps, filter paper or glass plates, test-tube stand, Bunsen burner, and limewater.

What to do. 1. Assemble the apparatus as shown in figure 15. Put some pieces of marble in the test tube and cover them with water. Fill another test tube or a wide-mouth bottle with water and invert it as shown in the figure. Put the end of the rubber tube in this test tube or bottle.

2. Directions for handling such chemicals as hydrochloric acid etc. will be found in the appendix, p. 181. Add some hydrochloric acid to the marble and insert the rubber stopper in the test tube. Fill three test tubes or bottles with carbon dioxide gas; cover them with wet filter paper or glass plates and set them right side up.

3. Study the nature of the carbon dioxide according to the table given in suggestions for report.

4. Pour off some of the liquid above the marble, put it into a test tube, and evaporate almost to dryness. Set the tube aside to cool.

Questions. How do you recognize marble ? hydrochloric acid ? What occurs as the acid is added to the marble ? Is the marble destroyed ? How can you tell when the vessel is filled with carbon dioxide ? How does carbon dioxide behave toward a burning splint ? a burning candle ? a glowing charcoal ?

Suggestions for report. Record the facts in the table below:

a

WEIGHT ACTION ON ACTION ON ACTION ACTION ON SUBCOLOR ODOR TASTE COMPARED BURNING GLOWING ON

LIMEISTANCE

TO AIR SPLINT CHARCOAL CANDLE WATER

Reference work. Read sections 73 to 76.

Optional problems. Weigh a large beaker of air. Fill the beaker with carbon dioxide and weigh it again. Explain. Devise a method for detecting carbon dioxide in a well. How can the carbon dioxide be removed ? Study a fire extinguisher and give an explanation of its action. How is carbon dioxide used in connection with the soda fountain ?

COMPARATIVE STUDY OF THE ELEMENTS OXYGEN

AND HYDROGEN (VI-4)

The problem. Having made a study of the two compounds, water and carbon dioxide, as given off by the candle, it will be interesting to study the elements, oxygen and hydrogen, and to compare them with carbon dioxide. Oxygen was discovered in 1774. It forms eight ninths of water by weight and about 65 per cent of

Sodium Provide

Fig. 16

the human body. It is required for the life processes of all living things. Hydrogen is the lightest element known and was discovered about 1766. About 10 per cent of the human body is hydrogen. Combined with carbon it is found in petroleum and natural gas.

What to use. Ring stand, burette clamp, sodium peroxide, medicine dropper, zinc, hydrochloric acid, test tube with side neck, and one-hole rubber stopper to fit, glass plug, rubber tube, glass jar or sink, three test tubes or wide-mouth bottles, small candle, wire, charcoal, wood splints, forceps, filter paper or glass plates, test-tube stand, Bunsen burner, and limewater.

a

What to do. 1. Arrange the apparatus as shown in Fig. 16. Fill a vessel with water and invert it in the jar or sink. Insert the rubber tube in the vessel. Put about a tablespoonful of sodium peroxide powder on a dry sheet of paper, then let the powder slide into the side-neck test tube. Fill the medicine dropper with water and insert it in the stopper. Drop some water on the sodium peroxide and collect the oxygen which is evolved. (A chemical change between the water and sodium peroxide produces oxygen and caustic soda, or sodium hydroxide. The gas escapes and the caustic soda remains in solution.)

2. Collect samples of oxygen until you have made all of the observations suggested on page 50.

3. Pour the solution from the test tube into a large bottle, which should be stoppered with a rubber stopper and kept for future use. Wash the test tube.

4. Put some zinc into the test tube and cover the zinc with water. Substitute a glass plug for the medicine dropper. Pour some hydrochloric acid into the tube and collect the hydrogen gas. (The chemical change produces free hydrogen, also zinc

. chloride which remains in solution. This solution should be saved.)

5. Collect samples of hydrogen until you have made all of the observations suggested on page 50.

Questions. Does the test-tube generator get warm? Why? How do you recognize the sodium hydroxide solution ?

Suggestions for report. Record the observations in the form given on page 50.

Reference work. Read sections 69 to 72 and 74. Write out a definition for an element, a mixture, a compound, an atom, and a molecule.

Optional problems. Plunge a white-hot piece of picture wire into a bottle of oxygen.

It is best to have some water in the bottle. Pass hydrogen bubbles through a solution of sodium hydroxide, then into a good soap-bubble solution.

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