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The Specific Gravity Bottle

(1) To determine the Specific Gravity of small Fragments of a Solid by means of the Specific Gravity Bottle.

We shall suppose that we require to know (1) the weight of the solid, (2) the weight of the empty bottle, (3) the weight of water which completely fills the bottle, and (4) the weight of the contents when the solid has been put inside and the bottle filled up with water. Strictly speaking, if the weight of the solid fragments can be independently determined, the difference of (4) and (3) is all that is necessary, and the weight of the empty bottle is not required; but in order to include under one heading all the practical details referring to the specific gravity bottle we have added an explanation of the method of obtaining or allowing for the weight of the bottle. The student can easily make for himself the suitable abbreviation if this is not required.

We shall also suppose the temperature to be the same throughout the experiment.

If it consists of only a few fragments of considerable size we may find the weight of the solid by the method of oscillations; let it be 5.672 grammes.

Dry the bottle thoroughly before commencing the experiment

The necessity of drying the interior of vessels occurs so -frequently in laboratory practice, that it will be well to mention here the different methods which are suitable under different circumstances in order that we may be able to refer to them afterwards. We may take for granted that all the water that can be removed by shaking or by soaking up with slips of filter paper, has been so got rid of.

An ordinary bottle or flask can for most purposes be sufficiently dried by drawing air through it by means of a tube passing to the bottom of the bottle and connected with an aspirator or the aspirating pump referred to in the note (p. 75), and at the same time gently warming the bottle by means of a spirit lamp. If there be any considerable quantity of water to be got rid of, the process can be considerably shortened by first rinsing out the bottle with alcohol. If more careful drying is necessary, as, for instance, for hygrometric experiments, the mouth of the vessel should be closed by a cork perforated for two tubes, the one opening at one end and the other at the other end of the vessel, and a current of perfectly dry air kept passing through the vessel for some hours. The air may be dried by causing it to pass first through U-tubes filled with fused chloride of calcium, which will remove the greater part of the moisture, and finally through a tube containing phosphoric anhydride or fragments of ignited pumice moistened with the strongest sulphuric acid.

If there be no opening in the vessel sufficiently large to allow of two tubes passing, the following plan may be adopted: Connect the tube which forms the prolongation of the plug of a three-way tap1 with an air-pump. The water air-pump before referred to is very convenient for the purpose if there be a sufficient head of water on the water-supply to give efficient exhaustion. Connect the other openings of the tap with the vessel to be dried and the drying tubes respectively. Then, by turning the tap, connection can be made alternately between the pump and the vessel and between the vessel and the drying tubes, so that the vessel can be alternately exhausted and filled with dried air. This process must be repeated very many times if the vessel is to be completely dried.

Having by one of these methods, thoroughly dried the bottle, place it on one of the scale pans of the balance, and counterpoise on the other either with the brass weight provided for the purpose, or by means of shot or pieces of lead. Observe the resting-point of the pointer by the method of oscillations, taking two or three observations.

Meanwhile a beaker of distilled water, which has been freed from air either by boiling or by being enclosed in the exhausted receiver of an air-pump, should have been placed near the balance, with a thermometer in it, in order that the water used may have had time to acquire the temperature of the room and that the temperature may be observed.

Fill the bottle with the water, taking care that no air-bubbles are left in. To do this the bottle is filled up to the brim, and the stopper well wetted with water. The end of the stopper is then brought into contact with the surface of the water, taking care that no air is enclosed between, and the stopper pushed home.

All traces of moisture must be carefully removed from the outside of the bottle by wiping it with a dry cloth.

Observe the temperature of the water before inserting the stopper; let it be 15 C. The bottle should be handled as little as possible, to avoid altering its temperature.

Replace the bottle on the scale pan, and weigh; let the weight observed be 24.975 grammes.

This weighing, like every other, should be done twice or three times, and the mean taken.

This is the weight of the water in the bottle only, for we have supposed that the bottle has been previously counterpoised

Open the bottle and introduce the small fragments of the solid which have been weighed, taking care to put all in.

Again fill the bottle, making sure by careful shaking that no air-bubbles are held down by the pieces of the solid; if any are observed, they must be removed by shaking or by stirring with a clean glass rod; or, if great accuracy is required, by placing the bottle under the receiver of an air-pump and then exhausting.

Replace the stopper, carefully wiping off all moisture, and weigh again, twice or three times; let the weight be 27.764 grammes.

This is clearly the weight of the substance + the weight of the bottleful of water - the weight of water displaced by the substance.

Thus the weight of water displaced is equal to the weight of the substance + the weight of the bottleful of water - 27.764 grammes

Now we require the weight of water which would be displaced were the temperature 4C.; for the specific gravity of a substance is equal to

but the weight of any volume of water at 4

Thus the specific gravity of the substance

Taking from the table (32) the specific gravity of water at 15, we find the specific gravity of the substance to be

If greater accuracy be required, we must free the water used from air by boiling or the use of the air-pump. We should also require to correct the weighings for the air displaced.

(2) To find the Specific Gravity of a Powder.

The process of finding the specific gravity of a powder is nearly identical with the foregoing. The only modification necessary is to weigh the powder in the bottle. The order of operations would then be -

(1) Counterpoise the dry bottle.

(2) Introduce a convenient amount of the powder, say enough to fill one-third of the bottle, and weigh.

(3) Fill up with water, taking care that none of the powder is floated away, and that there are no air-bubbles, and weigh again. If it be impossible to make all the powder sink, that which floats should be collected on a watch-glass, dried, and weighed, and its weight allowed for.

(4) Empty the bottle, and then fill up with water and weigh again.

The method of calculation is the same as before.

(3) To determine the Specific Gravity of a Liquid by the Specific Gravity Bottle.

Fill the bottle with water, as described above, and weigh the water contained, then fill with the liquid required, and weigh again. Each weight should of course be taken twice.

The ratio of the two weights is the specific gravity of the liquid at 4 C. if it expand by heat equally with water.

If we require the specific gravity of the liquid at the temperature of the experiment, we must note the temperature of the water, and reduce its weight to the weight of an equal volume at 4 C.; that is, we must multiply the above ratio by the specific gravity of water at the temperature of the observation.

Thus, the specific gravity of a liquid

Experiments.

(1) Determine the specific gravity of the given solid.
(2) Determine the specific gravity of the given liquid.

Enter as below, indicating the number of observations made of each quantity:

(1) Specific gravity of solid.

Weight of solid 5.672 gm. (3)
Weight of water in bottle 24.975 gm. (2)
Weight of water with solid 27.764 gm. (3)
Temperature 15 C.
Specific gravity 1.966.

(2) Specific gravity of liquid.

Weight of water in bottle 24.975 gm.
Weight of liquid 23.586 gm.
Temperature 15 C
Specific gravity of liquid 0.9430



1 A three-way tap is a simple, but in many ways very useful, contrivance. In addition to the two openings of an ordinary tap, it has a third, formed by a tubular elongation of the plug, and communicating with that part of the conical face of the plug which is on the same cross-section as the usual holes, but at one end of a diameter perpendicular to the line joining them. Such taps may now be obtained from many of the glass-blowers.



Last Update: 2011-03-27