Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

the hot glass to cool first in the smoky flame. When the bend is covered with soot, support it so that it will not touch a cold object. When the tube is cold, wipe off the soot.

e. Make two " ignition tubes" of hard glass by melting a piece of hard glass tubing 18-20 cm. long at its middle in the flame of a blast-lamp. Do this by grasping one end of the tubing between the thumb and forefinger of each hand, and twirling it rapidly in the flame until the glass softens and the walls of the tubing come close together. Then draw the two halves apart, but do not break the connection until the glass becomes stiff. Now break the connecting tube, and melt off the drawn-out glass where the tube becomes narrow. For this use a small blast-lamp flame. When the closed end of the ignition tube is cool, "fire-polish" the open end in the blast-lamp flame.

EXPERIMENT III.

SOLUTION, FILTRATION, AND EVAPORATION.

Apparatus.Glass rod 15 cm. long (unfinished), file, two beakers of about 50 c.c. capacity, ring stand, wire gauze, funnel, funnel support (small ring of ring stand), evaporating dish, test tubes, mortar.

Materials. Marble, salt, dilute hydrochloric acid, filter

paper.

a. Make a glass stirring rod 15 cm. long, cutting off a piece from a larger one, just as in Experiment II, a. Round off both ends in the flame.

SOLUTION, FILTRATION, AND EVAPORATION.

5

Taste a bit of marble, then put it into a test tube and add a drop of dilute hydrochloric acid. Results? Do the same with a pinch of salt, and state results. can you distinguish marble from salt?

How

b. In a mortar powder a lump of marble, add to it about of a test tube of salt, and grind the two thoroughly together. Put the mixture into a beaker with about 20 c.c. cold water, and heat the beaker over the flame until its contents boil. Before heating the beaker see that it is dry on the outside, then place it upon a wire gauze supported on the ring stand. Move the flame about under the gauze until the beaker has become warm; then put the burner under the center of the beaker. The height of the gauze above the burner should be so great that the bottom of the beaker may be a little above the apex of the dark inner region of the flame.

Note.

[ocr errors]

Always follow these directions when you are heating a beaker, an evaporating dish, or a flask, unless there is some special reason for not doing so. What becomes of the salt? Of the marble?

c. Next, filter the solution. You need a funnel, a support (see above), a filter, the glass rod made in a, and a second beaker.

Fold the circular filter twice in lines at right angles to each other. Press the folded edges between thumb and forefinger, but not between the nails. Open the filter so that it shall form an inverted cone which just fits the funnel. One-half of the conical surface is made up of three of the quarters into

FIG. 93.

which the paper was folded; the remaining quarter of the paper makes up the other half of the cone.

d. Hold the filter in place in the funnel, and wet it completely; it should adhere everywhere to the inner surface of the funnel, and its point should extend a little into the stem of the funnel.

Pour the salt solution down the glass rod to the filter. The glass rod should touch the lip of the beaker; and the stem of the funnel should touch the side of the beaker beneath it.

Always follow these directions in filtering an insoluble solid from a solution.

e. Does anything remain on the filter? We call it the residue. What passes through is the filtrate. Test the residue as you did the marble and salt in a. What is it?

A substance which remains mixed with a liquid, but not dissolved in it, is said to be "suspended in," or "held in suspension by " the liquid.

A suspended substance becomes, after filtration, a residue.

f. Pour the filtrate of c into an evaporating dish, and heat (for precautions, cf. b) over the flame. Boil off the water until a solid begins to separate out; then set the dish aside until it is cold, or until the next laboratory period. What is the solid obtained?

Is this separation of the salt from the marble a physical or a chemical operation?

S

EFFECT OF HEAT UPON OXIDES.

7

EXPERIMENT IV.

EFFECT OF HEAT UPON OXIDES.

Apparatus. Small ignition tube of hard glass, rubber connecting tube, delivery tube, pneumatic trough, test tube, ring stand, clamp. Materials.

Pine splinter, mercuric oxide, lead dioxide.

a. In a small tube of hard glass sealed at one end and about 10 cm. long" ignition tube"-place a layer of mercuric oxide

not more than onehalf a centimeter thick.

In a basin containing water, invert a test tube of water. See that no air bubbles remain in the

test tube. Vessels

FIG. 94.

for holding water over which gases are collected are called "pneumatic troughs."

Attach to the ignition tube by means of a piece of rubber tubing a delivery tube long enough to reach to the bottom of the pneumatic trough. Support the ignition and delivery tubes so that the closed end of the ignition tube is only a little lower than its other end, and so that the mercuric oxide may be heated in the hot portion of the Bunsen flame.

b. Begin to heat slowly, keeping the flame in motion. Note any change in color of the oxide. Afterward heat

strongly with a steady flame until all of the powder disappears. Collect over water anything that escapes from the delivery tube by allowing it to displace the water of the test tube. When the operation is over, remove the delivery tube from the water before removing the flame. Why?

c. Cover the mouth of the test tube under water with the thumb, remove tube from water, invert, and introduce a pine splinter with a spark on the end of it. Result? Is the gas in the test tube air? What is it?

d. When the ignition tube is cool, invert it and strike its open end sharply against the table. Result? What substance is this? On what part of the tube did it collect? Why did it not pass out of the tube?

e. Would you call this a chemical change, or not? How is mercuric oxide made? Consult a text.

f. Repeat a, b, c, and d, using lead dioxide instead of mercuric oxide. Does the powder disappear? Does What is the gas? What is the residue?

it give off a gas?
Answer e for this case also.

EXPERIMENT V.

OXYGEN.

Apparatus. Mortar and pestle (?), test tubes, ring stand and clamp, one-holed stopper, delivery tube, pneumatic trough, 4 collecting bottles, glass or cardboard cover, deflagration spoon.

Materials. Powdered potassium chlorate and manganese dioxide, pine splinter, sulphur, iron wire (picture cord) at least 15 cm. long, lime-water, magnesium wire or ribbon.

« ΠροηγούμενηΣυνέχεια »