JUNIOR COURSE OF

LABORATORY PRACTICE.

PART I.

PREPARATION OF APPARATUS,
GASES, &c.

1. Glass stirring-rods.

Divide a piece of glass rod into several pieces about two decimetres in length. This is done by filing the glass rod at each place where it is to be cut off, with a three-cornered file, and then snapping it across. Knock off any projecting pieces of glass which may be left at the newly-cut edges, and then hold each end of the rod in the flame of a Bunsen lamp until the sharp edges are fused and rounded. The glass rods thus made serve for stirring liquids, &c.

2. A wash bottle.

Soften a cork by gently rolling it under the foot, and fit it air-tight into the neck of a flask about one litre capacity. Then, by means of a round file, bore two holes in the cork about three millimetres in diameter,

B

and running parallel to each other and to the longer axis of the cork. Next obtain two pieces of glass tubing of the same diameter, one three decimetres long and the other half that length. Hold one end

of the longer tube in the Bunsen flame until the opening contracts considerably (but take care not to seal it up entirely), and then bend it about half a decimeter from the end, as shown in the figure. This is done by holding the glass tube horizontally in a common batswing gas jet flame, turning it round so as to heat all parts equally, and bending to the proper angle as soon as it feels sufficiently soft. Now round the edge of the wide end of the tube by holding it in the flame till it softens, and when cold fit it into the cork. In a similar way round both ends of the shorter piece of tubing, bend to the angle shown in the figure, and fit into the other hole. Clean out the flask and tubes thoroughly, rinse with distilled water, and then fill up with distilled water.

FIG. 1.

3. Preparation of oxygen from mercuric oxide.

When mercuric oxide is heated it is decomposed into mercury and oxygen.

Mercuric oxide yields mercury and oxygen.

*For the explanation of these symbols a larger work must be consulted. See p. 15 of Roscoe's "Elementary Chemistry."

Place a small quantity of mercuric oxide in a dry test tube, and heat it over a Bunsen lamp. The substance darkens in colour, and a ring of minute globules of mercury soon forms on the cool part of the tube. That the tube contains oxygen may be shown by plunging a glowing chip of wood into it, and observing that the wood will be rekindled. It is advisable to keep the thumb loosely on the mouth of the test tube to prevent the escape of oxygen by diffusion.

4. Preparation of oxygen from potassium chlorate. Potassium chlorate when heated is decomposed into oxygen and potassium chloride. (See Roscoe, p. 16.) Potassium chlorate yields potassium chloride and oxygen:

=

KCIO3 KCl + 03.

122.6

=

74'6 + 48.

Place a few crystals of potassium chlorate in a dry test tube, and heat gently. The salt soon fuses and then begins to effervesce, giving off oxygen, which may be recognized, as before, by its power of rekindling a glowing chip. When oxygen ceases to be evolved, the residue is a white salt called potassium chloride.

5. Tests for potassium chlorate and chloride.

A solution of potassium chlorate is not precipitated by a solution of silver nitrate; but potassium chloride is precipitated, silver chloride being formed.

Potassium chloride and silver nitrate yield potassium nitrate and silver chloride :

Dissolve a crystal of potassium chlorate in distilled water, then add solution of silver nitrate, no precipitate will be formed. Dissolve also the residue of potassium chloride (obtained in 4) in distilled water, and add silver nitrate solution. A curdy white precipitate of silver chloride will at once be formed.

6. Preparation of oxygen from potassium chlorate and manganese dioxide.

Potassium chlorate when mixed with about one-fifth of its weight of manganese dioxide, gives off its oxygen at a much lower temperature than when heated alone, although the oxide itself remains quite unaltered.

Place in a small flask (about 100 cubic centimetres capacity) a mixture of potassium chlorate and manganese dioxide in the proportions already mentioned. Then fit into the neck of the flask a cork through

which a bent conducting tube passes, the lower end of which is placed under water in the pneumatic trough. Fill some gas jars with water and invert them in the trough. Support the flask on a retort stand, and heat the mixture (Fig. 2); as soon as heat is applied, bubbles of gas will begin to rise through the water, consisting of air expelled from the flask by the heat. These are allowed to escape, and the oxygen, which soon begins to come off abundantly, is collected by placing the inverted bottles over the end of the conducting tube, and thus allowing the bubbles of oxygen to ascend into the bottles and displace the water. As soon as the first bottle is filled with oxygen, place a shallow earthenware tray under the neck, and remove it from the trough, taking care that the tray contains enough water to prevent the escape of the gas. When four bottles have been filled in this manner, remove the flask and conducting tube and allow the former to cool.

7. The residue from the preparation of oxygen from potassium chlorate and manganese dioxide is potassium chloride, and unaltered manganese dioxide.

Dissolve in water the residue in the flask from the preparation of oxygen, filter from the insoluble manganese dioxide, and evaporate the filtrate in a porcelain basin to a small bulk; on cooling, crystals of potassium chloride will separate out. Pour off the mother liquor, dry the crystals between filter paper, and keep them in a small bottle for future experiments.

8. Combustions in oxygen.

When a taper is burned in oxygen, the carbon which