Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

(i) Manufacturing glass from sand, soda, and lime. (j) The rusting of iron.

(k) The growing of a tree.

(1) The assimilation of food by an animal.

All the changes which substances undergo in the processes of nature and of the arts may be divided into the two classes illustrated by the above experiments, viz.:

I. Physical changes, in which no new substance is formed. Changes in state of motion, such as that involved in throwing or catching a ball, are purely physical. So also are changes in form, such as are produced by cutting, grinding, or hammering. Changes in temperature and changes in electrical condition may bring about chemical changes, but they are not in themselves chemical.

2. Chemical changes, in which one or more new substances are formed. We recognize new substances by observing such physical properties as color, physical state (solid, liquid, or gaseous), density (weight of a given volume, "heaviness"), solubility in water or in other solvents, etc. In some instances it is difficult to decide whether a new substance has been formed or not. But in many others, as we have seen in the experiments, the product of the change is readily recognized as a new substance.

Chemistry is the science which treats of those changes or "reactions" in which new substances are formed.

Chemical changes seldom, if ever, occur without the accompaniment of physical changes. In Experiment 2, for example, not only is a new substance formed in place of the magnesium, but heat and light are given out. Again, the heat evolved produces an upward current in the air, and some of the white, solid product of the chemical change is carried up as a smoke. There are thus a number of physical changes occurring concomitantly with the chemical change. In studying chemical changes we cannot ignore these concomitant physical changes, but we direct our attention- partic

ularly to the question whether new substances have been produced, and, if so, what those new substances are.

It must not be thought, however, that chemical changes are the only objects of study in the science of Chemistry. The science includes also the study of the properties of substances and of their "composition," a term which will be understood when the next three chapters have been read.

CHAPTER II

DECOMPOSITION AND COMBINATION

THE following experiments illustrate chemical changes of two classes. The chief point to be studied in each experiment is the alteration in the number of substances present.

Experiment 13.

Materials:

Mercuric oxide (best the red modification prepared by ignition).

Matrass or glass tube closed at one end, prepared in Experiment 4.

Splints of pine or other soft wood.

Heat mercuric oxide in a matrass or glass tube closed at one end. Test the escaping gas with a glowing splint. Note what collects on the inner surface of the tube. How many substances were put into the matrass? How many new substances were formed? Is this a chemical or a physical change?

The gas which affects the splint is called oxygen.

Experiment 14.

Materials:

Potassium chlorate.

Splints.

Heat potassium chlorate in a test tube until bubbling ceases. Apply a glowing splint to the gas escaping during the bubbling. Determine whether the sub

stance left in the tube is potassium chlorate or not. For this purpose not only may the effect of heat on this residual substance be compared with that on the original potassium chlorate,

FIG. 16.- A test tube holder.

but also a portion of each may be dissolved separately in water and treated with silver nitrate solution.

In the main experiment how many substances were put into the tube to be heated? How many new substances were found to be formed?

Experiment 15.

Heat a little sugar (about 0.5 gram) in a test tube. Note what

escapes from the tube and
what is left in it. Test the

escaping gas with blue lit-
mus paper.
How many
substances were put into
the tube? Were new sub-
stances formed? How
many were detected?

Experiment 16.*

Materials:

[graphic]

Sulphuric acid solution, about 10 per cent.

Apparatus:

Hofmann's electrolysis

apparatus (Fig. 18) or the simpler apparatus represented in Figure 17.

3 accumulators in series (or other appropriate source of direct current).

With the Hofmann apparatus full of the dilute sulphuric acid solution up to the bottom of the reservoir (or with the test tubes of the simplified apparatus full of the acid and inverted in the beaker of acid) connect the battery to the binding posts and allow the current to pass until a considerable quantity of gas has collected in each branch of the apparatus. Note that about twice as much gas collects in one branch of the apparatus as in the other. Invert a small test tube over one of the outlet

FIG. 17.- A simple apparatus for

the electrolysis of water.

tips, open the cock, and allow the gas to escape into the test tube. Cover the tube with the thumb, invert, and immediately apply a flaming splint. Make the same test upon the gas from the other limb of the apparatus. Also test each with a splint with a glowing, but not flaming, end. Compare the behavior of the two gases. One of these gases is the substance already met with in Experiments 13 and 14. What is its name? The other is hydrogen. Which of the gases is obtained in the larger quantity? Careful experiments have shown that none of the sulphuric acid is used up in this experiment. Indeed, several other substances can be substituted for the sulphuric acid without altering the result. The amount of water in the apparatus is, however, a little less after the experiment than before. The diminution of the quantity of water is not visible, unless the current has been passed for some hours. But it has been found by accurate measurement that the water lost weighs exactly the same as the sum of the weights of the hydrogen and oxygen produced. The use of the sulphuric acid is to make the water a better conductor of electricity. Pure water conducts electricity so badly that, using it, we should require years to obtain as much gas as we do in a few minutes with the mixture of sulphuric acid and water.

[ocr errors]

FIG. 18.-Hofmann's apparatus for the electrolysis of water.

If the hydrogen and oxygen produced in the electrolysis of water are both collected and mixed and set on fire, an explosion takes place, and if the experiment is conducted in a closed apparatus strong enough to stand the shock of the explosion, there is found in the apparatus after the explosion a quantity of water weighing just the same as the hydrogen and oxygen, and therefore just the same as the water' which was destroyed by the electrolysis that produced the hydrogen and oxygen.

Experiment 17.*

In a hard-glass test tube fitted with a cork and bent delivery tube and supported in a clamp on a ring stand heat a piece of

« ΠροηγούμενηΣυνέχεια »