then remove it from the pan and pour into it, as nearly as possible, 10 cc. of distilled water (measured in the graduated tube included in your laboratory outfit). Reweigh, and note the weight of the water. How do you account for the fact that it does not weigh exactly 10 g.?

II. THE BUNSEN BURNER

The Bunsen burner is a form of apparatus used for burning gas and is commonly employed in the laboratory as a source of heat. It consists of the tube A (Fig. 1), screwed into the base B. The tube has two small openings

near its lower part. A small band C, provided with similar openings, fits around the lower part of the tube in such a way that the openings of the tube may be closed or kept open by turning the band. Gas is admitted through D by means of rubber tubing.

Α

B

Ꭰ

FIG. 1

Unscrew the tube and examine the different parts of the burner; then put them together again and light the gas by holding a lighted match 4 or 5 cm. above the tube and turning on the gas. The supply should be adjusted so as to give a flame about 10 cm. high. The gas flowing through the tube mixes with the air drawn in through the openings in the lower part of the tube and burns with an almost nonluminous flame. If the band is adjusted so as to close the openings, the flame becomes luminous. Always use the nonluminous flame unless otherwise directed.

EXERCISE 2

A STUDY OF SOME OF THE CHANGES TAKING PLACE WHEN A SUBSTANCE BURNS

Apparatus. Porcelain crucible; Bunsen burner; ring stand; pipestem triangle to support the crucible while being heated; pneumatic trough (or dish); wide-mouthed bottle or large beaker; candle.

Materials. 2 or 3 g. powdered iron; 2 or 3 g. granulated tin; pellet of phosphorus, size of a small pea (to be obtained from the instructor when needed); 5 cc. limewater (R.S.).

a. The burning of metals. Place 2 or 3 g. of powdered iron in a porcelain crucible and accurately weigh. Now heat the crucible (Fig. 2) until the iron begins to glow (burn); then withdraw the heat. Does the iron continue to burn? After the crucible is cool, reweigh. Compare the weight of the product with that of the unburned iron (?). The experiment may be repeated, using tin.

b. The burning of phosphorus. (PRECAUTION. Phosphorus must be kept and handled only under water; otherwise it may ignite, and serious results follow.) Cover the bottom of a pneumatic trough with water to a depth of 2 or 3 cm. On the water float a porcelain crucible containing a piece of phosphorus the size of a small pea. Ignite the phosphorus by touching it with a hot wire or the end of a hot file, and quickly invert over the crucible a large

FIG. 2

beaker (or a wide-mouthed bottle), being careful to keep the rim of the beaker below the surface of the water. The white powder formed by the burning phosphorus floats in the air in the beaker but is gradually dissolved by the water.

Leave the beaker in position until the powder has entirely disappeared. Note that the water has risen in the beaker. How do you account for this fact? Is your conclusion in accord with the results obtained by burning iron in air? Suppose it were possible for you to collect and weigh the white powder formed by the burning phosphorus, how would you expect its weight to compare with that of the unburned phosphorus ?

c. The burning of a candle. Hold a cold, dry, wide-mouthed bottle over a candle flame as shown in Fig. 3. Note the film of moisture collecting on the bottle. After one or two minutes remove the bottle quickly and pour into it 5 cc. of clear limewater. Place the palm of the hand tightly over the mouth of the bottle and shake the contents. Note any change in the appearance of the limewater.

FIG. 3

Clean and dry the bottle and repeat the experiment, but omit holding the bottle over the candle flame. What conclusions do you draw from the experiment? How do you account for the fact that a burning candle gradually disappears, while iron on burning increases in weight?

1 The page references are to McPherson and Henderson's "First Course in Chemistry."

EXERCISE 3

CHEMICAL COMPOUNDS; ELEMENTS; CHEMICAL

CHANGES; CHEMICAL AFFINITY

Apparatus. Hard-glass test tube; burner; wooden splint; ordinary test tube.

Materials. 0.5 g. mercuric oxide; 2 to 3 g. sugar; bit of lime or crayon.

a. The action of heat on mercuric oxide. Introduce about 0.5 g. of mercuric oxide (ash of mercury) into a clean, dry test tube. This is done by placing the oxide near the end of a narrow strip of folded paper and introducing it carefully into the tube, as shown in Fig. 4. On inclining the

FIG. 4

tube and gently tapping the paper, the oxide will be deposited in the bottom of the tube. The paper is then withdrawn, leaving the sides of the tube perfectly clean.

Hold the tube between the thumb and fingers (Fig. 5) and apply a gentle heat to the oxide. The tube must be rotated constantly to distribute the heat.

If the upper part of the

(Fig. 6) or by a band of paper wrapped about the tube (Fig. 7). While the heating is continued, insert a glowing splint from time to time into the mouth of the tube. Note the result. Continue to heat as long as any gas is evolved. What is the gas (p. 5 of text)? What remains in the tube? How has the heat affected the mercuric oxide?

What name is applied to substances which, like mercuric oxide, can be decomposed into two (or more) simpler substances differing from the original substances in properties?

Define the following terms, illustrating each by reference to the experiment (pp. 8, 9 of text): (a) chemical compound; (b) element; (c) chemical change; (d) chemical affinity.

b. The action of heat upon sugar. Introduce 2 or 3 g. of sugar into a test tube and heat (as in a above) until no further change takes place. The residue is the element carbon. Describe your results, illustrating the meanings of the terms chemical compound, element, and chemical change.

c. The action of heat upon lime. Try the effect of heat upon a bit of lime or crayon. Discuss the results of the ex periment with your instructor before attempting to draw any conclusions.

EXERCISE 4

THE MANIPULATION OF GLASS TUBING

Apparatus. Burner and wing-top attachment; hard-glass test tube with cork to fit; cork-borers; triangular file; round file.

Materials. Glass tubing (soft), external diameter 6 mm.; about 30 cm. of glass rod.

a. To fit a tube with stopper and glass tube as shown in Fig. 8. In all operations requiring the application of a strong heat to glass, the heat must be applied gently at first. Highly heated glass must be cooled slowly; otherwise it is easily broken.

Cut from one of the lengths of soft-glass tubing a piece about 15 cm. in length. To do this, place the tubing on the