A practical chemistry for high school students

Preface

This book is intended for those schools desiring a more practical and interesting course in chemistry. Its purpose is to bring the pupil into closer contact with the things of everyday life, to give him an insight into the nature of those chemical processes with which all of us come more or less in contact, to furnish him with some chemical knowledge of the things necessary for his health and comfort and thus better adjust him to his environment.

In doing this no attempt is made to treat the subject in a theoretical way ; in fact, but little of the theory of chemistry is mentioned. The idea of atoms and the use of symbols and formulae have, however, been introduced after a few substances have been studied.

The mathematical side of chemistry finds no place in a text of this sort, but a chapter on chemical arithmetic has been included in the Appendix.

The book is the result of classroom experiment and has grown out of an attempt in the Boys ' High School to adapt chemistry teaching to the needs of second-year pupils. The subjects chosen, however, have not been those which appeal particularly to boys, but are those which are of most vital importance to both girls and boys and, in general, are of equal interest to both.

Charles G. Cook.

New York City.

Contents

CHAPTER PAGE

I. Introduction . 1

II. Water 5

Chapter I Introduction

Children are fond of pulling things apart ; perhaps they wish to try to see how things are made and of what they are made. Older people are in some respects like children; they also want to know how things are made and of what they are made. But the objects of their curiosity are not, as a rule, articles manufactured by other men, but rather the vast number of things which are found in nature, such as water, air, wood, stone, coal, flour, fruits, eggs, etc. Grown people have done much at this taking things apart and have found out much about the composition of many things in nature and how they are made. They have called the process of taking things apart to learn their composition analysis. In many cases also men have learned to put things together to make substances and this is called synthesis. The people who do this kind of work are said to be chemists, and the study connected with the work is called chemistry. It is a very large study and its importance is very great, for it has done much to improve many of our manufactures.

Physical and Chemical Properties and Physical and Chemical Changes. — When a new toy is given to a little boy he does not generally break it up right away to see where "the squeak comes from," but he looks at it and tries to see what he can do with it in every way he can think of. So the chemist, also, first of all carefully examines any new substance to learn its physical properties, that is, what qualities it will show without being treated

Fig. 1. — The Chemist at Work. The interior of a laboratory at a leadworks.

with acids and other chemicals. Later he studies the behavior of the new substance when in contact with other substances, and the properties thus exhibited are chemical properties. Now the chemist in bringing out the physical properties of a substance does not change its composition, but in showing its chemical properties he does change the composition. All changes, then, which do not change the composition of a substance are said to be physical

INTRODUCTION 3

changes, while those which do change the composition of a substance are called chemical changes. You may think of many examples of these changes : when a tree is blown down by a storm, when it is cut into firewood, or when it is made into lumber, all of these are physical changes, for the composition of the wood is not changed. On the other hand, if the wood is burned in the fire or rots through exposure to the weather, its composition is changed and the change is chemical.

Elements. — When the small boy has torn his toy apart he has left bits of wire, cloth, wood, sawdust, etc., and these he is unable to break up into anything more simple, and, if he has found out all he wished to know, he is satisfied. Likewise the chemist finds that he can go only so far in his analysis, and, like the small boy, comes to substances which he can tear apart no further. These substances he calls elements. You have often heard the names of some of them, as iron, lead, sulphur, gold, silver, carbon. There are about eighty of these elements known, but many of them are rare.

What Chemistry Teaches. — You will understand, then, that, in our study of chemistry, we wish to learn these elements, in what substances they are found, how we can get them, what they will do, what laws govern them.

Curiosity has led to many discoveries. The composition of substances is ascertained by both analysis and synthesis.

Analysis — Analysis is the separating of a substance into its constituents for the purpose of finding its composition.

Synthesis. — Synthesis is the building up of a substance from its constituents.

Physical Properties. — The physical properties of a substance are those which may be discovered without changing the composition of the substance.

Physical Changes. — Physical changes are those which do not

change the composition of the substance. Chemical Properties. — The chemical properties of a substance are

those made manifest by its behavior when in contact with

other substances.

Chemical Changes.— Chemical changes are those which change the

composition of the substance.

Elements. — Elements are substances which cannot be decomposed

into any simpler form of matter by known means. Chemistry. — The science of chemistry teaches the sources, properties, and uses of the elements and the laws which govern

them.

1. Why is curiosity sometimes desirable?

2. What is analysis?

3. What is synthesis?

4. Germany has many chemists; what effect has this had on the development of that country?

5. What is a physical change?

6. A chemical change?

7. Is the rusting of iron a chemical or a physical change?

8. What are elements?

9. Why is it possible that some of the substances now known as elements may not be elements?

10. What does chemistry teach?

Chapter II

General Properties. — We shall begin our study with water. Perhaps there are some facts about this veryfamiliar substance which will be new to you. "What becomes of water when it dries, or evaporates, as we sometimes say? If our eyes were sharp enough we might see it flying off into the air as little particles. If the air rises to a colder part of the sky, the little particles run

Fig. 2. — Snow Crystals Photographed by W. A. Bentley. Mr. Bentley has made many hundreds of pictures of snow crystals and among these no two are alike. Perhaps no two snow crystals ever were alike. They are all, however, symmetrical in form and each has six rays. The student will find the study of snow a fascinating pastime.

together to form drops and may fall as rain. Sometimes the water vapor in the air (called moisture) freezes before it can form raindrops, and then we have snowflakes. If snowflakes are examined, it will be seen that they all have six points. (See Fig. 2.)

Pitchers often crack when water freezes in them. The water increases in volume when it freezes ; if you should c F measure carefully, you would find it

expands about one-tenth of its volume.

You might also find that hot water while cooling gradually grows smaller, or contracts, until the ■ temperature of four degrees centigrade (written 4° C.) is reached. It then begins to expand and continues to expand till frozen. Water is therefore heaviest at 4° C. At this temperature its density is said to be 1.

Now just a word about the thermometer with which we measure temperature. Fahrenheit thermometers are used at home, but in chemistry centigrade thermometers are used.

The two kinds differ only in the figures and marks upon them. (See Fig. 3.) If both thermometers are put into melting ice, the mercury will come to the 0° mark on the centigrade, and to the 32° mark on the Fahrenheit. Likewise, if they are put into steam over freely boiling water, the temperature of the steam will be found to be 100° centigrade and 212° Fahrenheit. On the centigrade thermometer there are 100 degrees between the freezing and boiling points of water, while on the Fahrenheit there are 180 degrees.

Fig. 3. — Thekmometers.

It will be noticed that 0° Centigrade corresponds to 32° Fahrenheit and likewise 100° C. to 212° F.

Water is used as a standard for temperatures. It is also a standard for a number of other measurements. We compare the weight of other objects with water. If, for instance, we find a substance seven times as heavy as water, we say it has a specific gravity of 7. In the same way we compare the weights of liquids with water ; if a liquid is said to have a specific gravity of 1.8, we mean it is 1.8 times as heavy as water.

Solution. — Perhaps you have noticed that when sugar or salt is dropped into water it slowly disappears. What becomes of it? It has not been destroyed, for when the water dries away the salt or sugar is left behind. What happens is that the substance simply separates into little particles, called molecules, and these move about all through the water and are said to form a solution with the water. Many other substances act in the same way, that is, dissolve when put into water. Nearly all solids dissolve faster and in larger quantities in hot water than in cold, while gases dissolve in larger quantity in cold water and under pressure. You have noticed how the gas escapes from soda water when drawn and allowed to stand in a glass. This was because it was under pressure and this pressure was removed when the soda came into the glass. If you let cold water stand in a glass in a warm room, you will soon find little bubbles of air sticking to the inside of the glass. This air was dissolved in the cold water, but could not remain in solution after the water became warm. The molecules of gas move so fast that they cannot remain in the warm water; if the water is cooled or if the gas is put under pressure, the molecules will not move so fast, and therefore more gas will go into solution under these conditions. When a solid dissolves its molecules must move faster, and we may cause them to do this by heating the solution. If a substance dissolves without heating, the solution is often found to grow cool, since the molecules in this case take up heat from the solution.

The dissolved substance is called the solute, while the liquid in which it dissolves is the solvent. The product is a solution. By the concentration of a solution is meant the quantity of the solute dissolved in a given volume of the solvent; this is usually expressed in grams per liter. (See Metric Tables.)

Everyone knows that not all substances are equally soluble, but even the most soluble reaches a limit when no more of the substance will go into solution without heating. "When this condition is reached. the solution is said to be saturated. Anything short of this concentration is called an unsaturated solution. A hot saturated solution is sometimes cooled without depositing any of its solute. A solution in this condition is said to be supersaturated. It is easy to tell which of these conditions a solution is in, since an unsaturated solution will dissolve more of the solute, a saturated one will not dissolve more without heating, while a supersaturated solution on the addition of more of the solute will deposit its excess and become a saturated solution. Heat is evolved during this process.

Water is not the only liquid which will dissolve substances ; many others, as alcohol and kerosene, will do the same thing. Water, however, is the best solvent known; it will dissolve the greatest number of substances. Few things are entirely insoluble in water, though many substances are nearly so.

Drinking" Water. — It is this great solvent power of water which is responsible for much of the impurity of drinking water. Rain falls upon the earth and, sinking down into the ground, dissolves many things which render it impure. But there is another trouble with drinking water ; all over the ground everywhere, and often in the air, are innumerable little organisms so small that they can be seen only with a good microscope. Some of these bacteria are the causes of disease, others cause decay in food and other things, some produce the souring of milk, fruit, juices, etc. These little plants grow and multiply very fast. This is particularly true where animal matter is decaying. You see from this why it is so dangerous for drinking water to become contaminated by the drainage from stables, sewers, and the like, and that impure water is often the cause of sickness. All drinking water, then, unless known to be pure, should be purified.

Purification of Drinking Water. — The simplest way to do this is to boil the water, since boiling kills the bacteria, but this is not the most desirable method, for boiled water does not have a pleasant taste. It must also be remembered that boiling will not remove all of the mineral matter dissolved in the water, though sometimes it will remove part of it. A process

which will remove both the bacteria and mineral matter, and which is known as distillation, consists of changing water into steam and then condensing the steam into water again by passing it through long coils of cold pipe. The pipe is kept cold by surrounding it with cold water.

=jLO Faucet-Water ^rO 3 inlet

Distilled Water Outlet

Fig. 4. — Sectional. Diagram of a Still, fob Distilling Water in Small Quantities. Water in a boiler is heated by a gas burner and converted into steam. This steam passes out through a long pipe which is cooled by the faucet-water in the surrounding jacket. The faucet-water thus becomes partly heated before entering the boiler.

Since it requires more water to condense the steam than is required for the boiler, the excess flows away through an outlet pipe.

Distillation will not remove gases or volatile substances, since they will pass over with the steam.

The water supply of large cities is often purified by letting it sink slowly through large filters. The filter forms the bottom of a reservoir. The water in the reservoir soaks down through a bed of sand, or of sand and charcoal, then through gravel and broken stone, and col-

Fig. 5. — Explanation of Sand Filter. R, river from which the water supply comes through the inlet pipe, I. SR, settling reservoir in which the water is allowed to stand till the sediment has fallen to the bottom. The water next passes into the filtering reservoir 3 FR, where it sinks slowly through the thick layers of sand, S, and gravel, G, and collects among the large rocks, B. The outlet pipe, O, conducts the water into the distributing reservoir, DR, from which it flows into the city mains.

lects in the open spaces between large stones at the bottom. Then it drains away into a reservoir below, from which it flows into the waterpipes of the city. Of course, the sand soon becomes dirty and must often be replaced by a fresh supply.

Crystals. — If you examine the "rock candy' ' sold in the stores, you will find it made up of little pieces clustered together. These little bits are all much alike in shape, they all have the same number of sides, and these sides are bright and shining. These bits of candy are sugar crystals. They were formed in a sugar solution by the molecules of sugar coming together as the water evaporated into the air. We may say that crystals are regular geometrical solids formed by natural processes.

Fig. 6. — Sand Filter at Washington, D. C, Before the Sand Was Put In. Loosely jointed pipes instead of large stones are placed at the bottom of this filter. The water after passing through the sand enters these pipes and flows into a storage reservoir. The filter is covered by an arched roof level with the ground and serves as a small park.

Among these natural processes may be mentioned first the evaporation of a solution as illustrated by the sugar solution given above. Again it has been intimated that solutions on cooling often deposit crystals. Many solids, when melted and allowed to cool slowly, solidify as a mass of crystals. Illustrations of this are found in many rocks, quartz being an excellent example. Another way in which crystals are formed is by the sudden cooling of a vapor.

Snow is an illustration of this ; the water vapor in the upper air crystallizes before it has time to form raindrops.

Water of Crystallization. — Sometimes, when crystals form, one or more molecules of the water of the solution combine with each molecule of the dissolved substance to form the crystal. "Water thus combined in a crystal is called water of crystallization, and has lost the properties of ordinary water. The crystals are perfectly dry to the touch. If the crystals are heated, however, the water will escape as steam. Some crystals containing water of crystallization lose this water without heating if they are exposed to the air. As the water escapes the crystal falls into a powder. Crystals which thus lose their water are said to be efflorescent and the process is called efflorescence. Many kinds of crystals do not contain water of crystallization.

Many substances absorb water from the air in considerable quantity and dissolve in it; such substances are called deliquescent.

Composition of Water. — If we are to follow the plan of the small boy with a new toy, as mentioned above, having now examined water to some extent and found out a little of what it will do, we shall next try to break it up. To do this we shall need the help of an electric current and some sulphuric acid, also the apparatus shown in Figure 7. The tubes, H and 0, are united at the bottom with the reservoir tube, R, and through R are filled with a mixture of water and sulphuric acid in about the proportion of one part of acid to twenty parts of water. Fused through the glass at E and E' are two wires terminating in platinum plates. These wires lead back to a source of electric current, such as a battery or dynamo. When the current is turned on, small bubbles of gas form on the platinum plates and pass up the tubes. Soon it is found that the gas is collecting twice as fast in one tube as in the other. When a flame is brought near to this gas it catches fire and burns with an almost colorless flame. The gas in the other tube will not catch fire, but many things will burn more freely in it than in the air. In many respects the two gases are quite different, but neither can be separated into anything simpler; so Ave must regard them as elements. The one which is in larger volume is hydrogen and the other one is called oxygen.

Many other experiments have been made to find out the composition of water, and these all lead to the conclusion that two volumes of hydrogen are combined with one of oxygen to form water.

Oxygen, however, is about sixteen times as heavy as hydrogen, so that the composition of water by weight is about one part of hydrogen to eight parts of oxygen. Before proceeding to the further study of these gases, which will be our next duty, we must stop to consider how they exist in water, that is, whether they are combined or only mixed — whether water is a compound or a mixture.

Compounds and Mixtures. — There are three important differences between compounds and mixtures: (1) a compound always has the same composition — it is always made of the same elements in the same proportions; (2) the properties of a compound are not the same as those of the elements in the compound; (3) you cannot take a compound apart by mechanical means. In the case of

Fig. 7. — The Electrolysis of Water.

mixtures the reverse of these statements is always true. Let us try water with reference to these points. "When we decomposed water by the electric current we found it separated into two gases, and there was twice as large a volume of one gas as of the other. This result has been verified by many repetitions. This shows that water has a definite composition. The properties of the gases are very different from those of water. Lastly it requires a chemical process to decompose water. It must follow, then, that water is a compound, since it answers all these three requirements of compounds. Air, on the other hand, is a mixture ; its composition is always changing, its properties are those of the gases of which it is composed, and these gases may be separated by physical means.

General Properties of Water. — Water evaporates, forming water vapor which may condense to rain and snow. It freezes at 0° C, or 32° F., and boils at 100° C, or 212° F. Water is at its greatest density at 4° C. and expands as the temperature changes in either direction from this point. While freezing it expands one-tenth of its volume. Water at 4° C. is taken as a standard for determining specific gravity of solids and liquids, specific gravity being defined as the weight of a given solid or liquid as compared with the weight of an equal volume of water at 4° C.

Solution. — A solution is a liquid in which the molecules of some dissolved solid or gas are moving freely. To do this the molecules of a solid must take up energy; hence, heat aids solution of solids and prevents solution of gases.

For the same reason pressure aids solution of gases. The substance dissolved is called the solute, the liquid is the solvent, the quantity of solute in a given volume determines the concentration of the solution. A solution is saturated when its concentration cannot be increased without increase of temperature, unsaturated when its concentration can be increased without increase of temperature. A supersaturated solution is one which will deposit crystals on the addition of a crystal of the solute.

Drinking water may contain mineral matter dissolved from the ground, organic matter from decaying animal and vegetable substances, and. bacteria which are the cause of decay and disease. Drinking water is purified by boiling, by filtering through sand or charcoal, and by distillation, which consists of changing water into steam and then condensing the steam again into water.

Crystals. — Crystals are regular geometrical solids formed either by the cooling or evaporation of a solution, the cooling of a molten mass, or by the sudden cooling of a vapor.

Water of Crystallization. — rWater of crystallization is water that has lost its ordinary properties by becoming a part of the molecules of a crystal.

Efflorescence. — Efflorescence is the process of giving off water of crystallization when exposed to the air.

Deliquescence. — Deliquescence is the process of absorbing water from the air and dissolving in it.

Electrolysis. — Electrolysis and other experiments show that water is composed of two volumes of hydrogen and one volume of oxygen, or about eight parts by weight of oxygen to one part of hydrogen.

Compounds. — Compounds are unlike mixtures in that they have definite, constant composition, are not decomposed by mechanical means, and do not have the properties of their constituents.

1. Make a summary of familiar facts concerning water which are not given in the text.

2. Why does a cloth dry after being wet?

3. Why does the ocean never become full to overflowing?

4. Explain the formation of snow.

5. Describe the appearance of snow flakes.

6. Why does freezing often cause water pipes to leak?

7. Which is larger, and how much, a pound of water at 0° C. or a pound of ice at 0° C?

8. Explain what changes occur in the volume of water during cooling.

9. At what temperature does water reach its maximum density?

10. Compare the Centigrade and Fahrenheit thermometers.

11. Calculate at what temperature the reading of these thermometers is the same.

12. What is meant by the expression, "The specific gravity of iron is 7"?

13. What becomes of sugar when put into hot coffee?

14. What is meant by molecules?

15. How can you show that the molecules of a dissolved substance move through the liquid?

16. Will water dissolve more air in summer or in winter?

17. Why has the concentration of the Dead Sea increased with time?

18. When a solution of camphor in alcohol is poured into water solid camphor is formed. Explain why.

19. How could this camphor be again brought into solution?

20. Why does soda water effervesce when drawn into a glass?

21. Why does it often cool water to dissolve a substance in it ?

22. How could you tell whether sulphur is soluble in water?

23. Why is drinking water apt to be impure?

24. Name the most dangerous impurities that often occur in drinking water.

25. Compare the various methods of purifying drinking water, and state under what conditions each is desirable.

26. How would you make crystals of a metal?

27. What is water of crystallization?

28. What kind of substances make good drying agents?

29. Describe one experiment by which the composition of water may be determined.

30. Explain how you can tell whether candy is a compound or a mixture.

Chapter III Oxygen

Discovery. — About the time of our American Revolution Joseph Priestley, an English chemist, found that on heating a red powder, which had been made by heating mercury in the air, he could obtain a gas. If you will heat some of the same red powder, which we now call mercuric oxide, you may obtain the same gas which Priestley discovered. Priestley did not name his new gas oxygen, but it received this name a little later from the French chemist, Lavoisier. The discovery of oxygen is an important event in the history of chemistry, since it led to the true explanation of combustion and to the overthrow of the false theory held at that time.

Occurrence. — Since the red powder from which Priestley obtained oxygen was made by heating mercury in the air, the oxygen which he obtained must have come originally from the air. Hence, air contains oxygen. We have seen (see p. 13) that it is an important part of water. It is also a constituent of nearly everything that grows. In the solid earth no other element is found in such large quantities or in so many compounds. You see from this that it is a very important substance.

Properties. — From our knowledge of the air we know some of the properties of oxygen. It is colorless and has no taste or odor. In the air it is mixed with other gases lighter than itself, so that air is not quite so heavy as oxygen. One liter of oxygen at 0° C. and normal atmospheric pressure weighs 1.43 grams. If you will thrust a splinter with a spark on it into a jar of oxygen, the splinter will at once burst into flame. From this you see that oxygen makes substances burn. It does this by uniting with the elements of the burning substance, producing new compounds called oxides.

Combustion. — The oxygen of the air acts in the same way, but the other substances in the air get in the way and thus make the burning less rapid than in pure oxygen. A fire burns faster when supplied with plenty of air, but will go out if the air is entirely shut off. The drafts of our stoves and furnaces regulate the flow of air through the fire. The fuel (the coal or wood) unites with the oxygen of the air and produces, besides the ash, certain gaseous oxides which pass up the chimney. If there is not enough air, some of the fuel may also pass up the chimney. This partly explains the great clouds of smoke given off in burning soft coal. There is evidently much loss when a fire is giving off much smoke, but fuel may also go up the flue in the form of invisible unburned gases. Most fires require some heat to start them ; the substances must be brought to the kindling temperature before they will burn. This is usually done with a match flame. The kindling temperature is not the same for all substances ; thus wood burns at a much lower temperature than iron, and phosphorus at a lower temperature than wood. Again, if fuel is cut into small pieces it will burn more freely. This is because it presents more surface- to the oxygen of the air, and requires less heat to bring the pieces to the kindling temperature. A few substances burn spontaneously, that is, they start to burn without the aid of man. Many cases of so-called spontaneous combustion may, no doubt, be traced to other causes. If a substance burns spontaneously, it is because slow chemical action has been going on for some time and the heat thus produced has accumulated uutil the substance being acted upon is heated to the kindling temperature. The substance then bursts into flames. Green hay stored in barns is said sometimes to take fire in this way.