UC-NRLF B M DSfi Tbb MEMCAL SCnOOh uiEMAmir CHEMISTRY FOR NURSES ^^^ THE MACMILLAN COMPANY NKW YORK • BOSTON • CHICAGO - DALLAS ATLANTA • SAN FRANCISCO MACMILLAN & CO., Limited LONDON • BOMBAY • CALCUTTA MELBOURNE THE MACMILLAN CO. OF CANADA, Ltd. TORONTO CHEMISTRY FOU NURSES BY REUBEN QTTENBERG, A.M., M.D. LECTURER TO THE NURSES' TRAINING SCHOOL, MT. SINAI HOSPITAL ; INSTRUCTOR IN BACTERIOLOGY, COLLEGE OP PHYSICIANS AND SURGEONS, COLUMBIA UNI- VERSITY ; AND ASSISTANT IN CLINICAL MICROSCOPY, MT. SINAI HOSPITAL THE MACMILLAN COMPANY AU riffhUi reserved OOPTBIGHT, 1914, bt the maomillan company. Set up and elcctrotyped. Published September, 19x4. Reprinted December, 1914; June, October, 191S; January, November, 1916 ; August, October, 1917* J. S. Cashing Co. — Berwick & Smith C*. * '. NkffwoodjM^SQ,, U'.S.A. r r ,, ^D33 j83 1917 PKEFACE The teaching of chemistry to nurses is a new thing, but the development of medicine has made it inevitable. Some knowledge of the fundamental conceptions of chemistry is as indispensable as anatomy in the modern treatment of disease. In- deed it puzzles one to understand how, in the past, without chemical instruction, nurses have made any sense at all of much that was taught them in materia medica, physiology, and diet cooking. How rapidly this opinion is gaining adherents is shown by the responses to an inquiry sent by the author to the examining boards of thirty-two states of the Union; in twenty-two states, the answers showed, questions in chemistry formed part of the examination for registered nurses. This book has been written therefore in response to what the author believes to be a real need. There is no simple yet modern textbook on this subject. In the past nurses anxious for informa- tion were compelled to refer either to textbooks written for medical students and much too difficult for the average nurse, or to school textbooks, VI PREPACK written from a different point of view and paying no attention at all to many subjects of peculiar importance to nurses. Chemistry to the nurse is an accessory study, — an aid to the understanding of other studies of undoubtedly greater practical importance. For this reason the present book is to be used by the nurse not as a catalogue of facts to be learned outright, but as a reasoned explanation of things which otherwise would remain obscure. The author therefore has avoided all technicalities and has attempted to make the text as readable and enter- taining as possible. The subject of chemistry with its innumerable and important bearings on everyday life usually fascinates every beginner who overcomes the initial fear of its technical difficulty. This book is the development of a short course of lectures given to the undergraduate nurses of the Mount Sinai Hospital at various times in the past two years. The experiments described have all been demonstrated to classes of nurses and can easily be performed with the simplest equipment. It is suggested that teachers perform these experi- ments before their classes. The author wishes to thank several of his friends for their kindly criticism: Dr. Samuel Bookman, PREFACE VU Chemist to Mount Sinai Hospital; Miss George, Dietitian to Mount Sinai Hospital ; Dr. A. S. Blum- garten, author of "Materia Medica for Nurses"; and particularly Mr. Joseph Loew of the De Witt Clinton High School, New York. TABLE OF CONTENTS OHAFTEB PAGB I. Elements and Compounds 1 II. Atoms and Molecules 12 III. Chemical Names and Formulas. Chemical Affinity 18 IV. Energy and Oxidation 27 V. Acids 36 VI. Bases 44 Vn. Salts 50 Vin. Organic Chemistry 56 IX. Carbohydrates 65 X. Fats 79 XI. Proteids 90 XII. Digestion 101 XIII. Urine 113 XIV. Stomach Contents^ and Feces .... 130 INDEX 135 CHEMISTRY FOR NURSES CHAPTER I Elements and Compounds Two Kinds of Changes in Matter. — Since the earliest times men have been studying the different kinds of matter or substances of which the world is composed, and have noticed that these substances frequently undergo changes in appearance and form. Gradually they have come to see that these changes are of two different kinds : the one kind is more or less transient. Thus the change from solid to liquid, as in the melting of ice, or from liquid to gas, as in the boiling of water, the glowing of metal when it is heated, are temporary changes. The water can freeze again, the steam can condense, the metal can cool. Changes of this kind in matter are known as physical changes, and their study constitutes the science of physics. Chemical Changes. — But substances also undergo another kind of change much more permanent in 2 CHEMISTRY FOR NURSES nature. Wood may burn in the flame until nothing is left but ashes, or metals rust and lose their origi- nal appearance entirely. Such deep-going changes in matter are known as chemical changes, and their study is the science of chemistry. Elements. — Chemical changes often result in breaking up one substance into several products. Thus, for instance, when a piece of wood burns there are fumes given off ; at a certain stage there is a black residue composed of carbon (charcoal) ; and if the burning is continued for a long while, white ashes. The products of such chemical changes can often be further decomposed into still other substances.^ When this process is continued, certain forms of matter are finally obtained which cannot by any means whatever be resolved into other substances. These are called elementary substances or chemical elements. Compounds. — By studying all sorts of substances in this way the chemists have discovered that all the matter in the universe is composed of about ^ When the combined weights of all the substances''produced by such decompositions are compared with the weight of the original substance, they are found to be exactly equal. Matter has been changed into different states, but no matter has been made or destroyed. This is the law of the indestructibility of matter. ELEMENTS AND COMPOUNDS 3 eighty of these elements ; everything from a microbe to a star is composed of these same eighty elements in different forms and combinations. It took centuries to find this out because it is not easy to discover what elements are present in a sub- stance. The reason for this difficulty is that when different elements combine to form a new substance they lose their properties entirely. The properties of the compound are nothing like the properties of the elements themselves. Who would think from the properties of water, for instance, that it was composed of two light gases which explode when they are mixed together and ignited? Or, consider common salt, sodium chloride. It is a combination of two ele- ments, — sodium and chlorine. Chlorine is a yellow poisonous vapor. Sodium is a soft metal, very hard to keep in its natural state because it corrodes and combines with almost everything it touches. These two violent things with their peculiar properties combine and the result is our harmless table salt. Difference between Mixtures and Compounds. — But it must not be supposed that just because ele- ments are mixed together the result is necessarily a new compound. Substances may be simply mixed together very intimately without undergoing any combination (in such a way as to form new 4 CHEMISTRY FOR NURSES properties) at all. If you mix particles of carbon (lamp black) and of sulphur, the mixture has no new properties and the two kinds of particles can be recognized separately under a microscope. But if, instead of this, sulphur in the form of vapor is made to pass over the red-hot charcoal, an entirely new substance (carbon disulphide) is formed. It is a volatile and strong-smelling liquid which is of enor- mous importance in the manufacture of rubber and various perfumes. This, then, is the difference be- tween a mixture and a compound: in a mixture the substances are present in their original form and are easily separated; in a compound the elements have undergone a peculiar change by which they not only acquire entirely new properties, but are firmly united so that it takes some powerful force like great heat or a strong electric current or power- ful chemical action to separate them. All sorts of elements combine with each other and thus there are thousands of compounds: the number of possible compounds is almost infinite. Constant Composition of Compounds. — In spite of the great number of compounds formed by the union of different elements each compound always has precisely the same composition, no matter under what conditions you find it. If you take specimens ELEMENTS AND COMPOUNDS 5 of water from a dozen different sources and separate the elements in them, you will always find exactly the same proportion of hydrogen and oxygen. You never find more oxygen in one specimen of water than in another. This fact, that the ele- ments in any chemical compound are always present in precisely the same proportions, is another one of the things that distinguish compoxmds from mixtures. It is obvious, of course, that in a mixture any amoimt of the one or the other ingredient may be present. There is a fundamental reason for this constancy of composition of compounds. This reason will be made clear in the next chapter. How can Different Compounds be composed of the Same Elements? — But the same elements may combine in several different proportions to form several different compounds. For instance, take an- other compound f 2 hydrogen and oxygen, namely, peroxide of hydrogen. It is a combination of exactly the same elements as water, but the elements are combined in a different ratio. The amount of oxygen present in any given amoimt of peroxide of hydrogen is twice as large as in the same amount of water. On this account peroxide of hydrogen is entirely different in its properties from water. It attacks and de- composes organic things such as blood or bacteria. 6 CHEMISTRY FOR NURSES But peroxide of hydrogen in its turn does not vary. The proportions of hydrogen and oxygen obtained from different specimens of it are always the same. There are two general methods of studying com- pounds, — analysis and synthesis. Analysis means decomposition of a compound into the elements which compose it: synthesis means combining different elements to form new compounds. The Most Important Elements. — Below is a partial list of elements. The list includes all the elements which are met in everyday life and which play an important part in medicine or nursing. Many of them, such as carbon and sulphur and the metals, are already familiar in their natural state. The student will easily be able to think of compounds of most of the others. A short description of most of these is found in the appendix W Chapter I. 1 — Aluminium 11 — Hydrogen 2 — Arsenic 12 — Iodine 3 — Bismuth 13 — Iron 4 — Boron 14 — Lead 5 — Bromine 15 — Lithium 6 — Calcium 16 — Magnesium 7 — Carbon 17 — Mercury 8 — Chlorine 18 — Nickel 9 — Copper 19 — Nitrogen 10 — Gold 20 — Oxygen ELEMENTS AND COMPOUNDS 21 — Phosphorus 27 — Sodium 22 — Platinum 28 — Strontium 23 — - Potassium 29 — Sulphur 24 — Radium 30 — Tin 25 — Silicon 31 — Zinc 26 — Silver IMPORTANT ELEMENTS AND COMPOUNDS 1. Aluminium — a light, white metal, which, combined with other elements in the form of clay or of va- rious minerals, forms a large part of the earth's crust. Its compound, aluminium acetate, is used in surgical dressings. 2. Arsenic — a metal. Its compounds are exceedingly- poisonous, and some are used as drugs. 3. Bismuth — a heavy, lustrous metal whose compounds are mostly insoluble (do not dissolve in water) and are used in medicine. On account of being in- soluble, bismuth compounds when given as medicines are found in the stools again. They undergo a chemical change which gives the black color to bis- muth stools. 4. Boron — mentioned because of its compounds, horic add and borax. 5. Bromine j;::^-a heavy, very poisonous, and irritating brown liquid, the compounds of which, known as bromides, are of great use in medicine. 6. Calcium — a yelldw metal which when heated takes fire and burns. One of its compounds (calcium carbonate) forms marble and limestone; another (calcium phosphate) is the chief constituent of 8 CHEMISTRY FOR NURSES bones. We could not live without calcium and it is present in all animals and vegetables. 7. Carbon — occurs as diamond, graphite, charcoal. Its compounds are very numerous and all living things are composed chiefly of them. 8. Chlorine — a poisonous, greenish gas, similar in many ways to bromine and iodine. Its compounds, such as hydrochloric add, sodium chloride, potassium chlorate, are of great importance in physiology and medicine. 9. Copper — a common metal, one of whose compounds, copper sulphate, is a valuable cauterizing agent. 10. Gold — a metal which has very few compounds. This is what makes it a ^* precious '' metal. It does not combine readily with other elements, and therefore does not corrode or rust, but remains un- changed for centuries. 11. Hydrogen — a non-poisonous gas, the lightest sub- stance known, and one of the most abundant and important of all the elements, — present in all living matter. Not present as such in the air. 12. Iodine — shining black crystals, which, dissolved in alcohol, are the familiar tincture of iodine. Traces of it are found in the thyroid gland, and it is im- portant for health, though exactly why is unknown. Some of its compounds {iodides) are valuable drugs. 13. Iron — important not only as a metal, but also in its compounds, some of which are valuable drugs, and one of which, hoemoglobin, is the red coloring matter of our blood and is essential for respiration. ELEMENTS AND COMPOUNDS 9 14. Lead — is a heavy metal ; it can easily be fluidified by heat. Its soluble compounds are poisonous. Some of them are used in medicine {lead acetate). 15. Lithium — is a light metal, some of whose compounds are used in medicine. 16. Magnesium — a silvery white metal which burns brilliantly in the air. (The flash light of photog- raphers is powdered magnesium.) Some of its compounds, such as magnesium sulphate^ citrate^ carbonate J and hydroxide (milk of magnesia), are important drugs. 17. Mercury — a heavy fluid metal whose compounds are familiar and important drugs (calomely bichloride of mercury). 18. Nickel — a metal. 19. Nitrogen — an inert (not chemically active) gas which forms about | of the air. Its compounds, am- wjonia, nitrous oxide or laughing gas, nitric add, niter or saltpeter (potassium nitrate), are used in medicine. In the form of proteid it is not only an essential food, but is present in every living cell. 20. Oxygen — is a colorless, odorless gas which forms about J of the air and | of the earth. It can com- bine with almost every other element, and the pro- cess x)f its combination is known as oxidation or combustion. It is essential to respiration and is present in every living cell. Of its innumerable compounds, water is the most important. - Oxygen gas can be readily recognized by the fact that glow- ing substances are set on fire or burning substances burn much more brightly when introduced into it. 10 CHEMISTRY FOR NURSES 21. Phosphorus — is a waxlike solid which shines in the dark and readily catches fire. It is used in making matches and is very poisonous. Some of its com- pounds, such as sodium phosphate, are used as drugs. Another of its compounds, calcium phosphate, forms about 60 % of hone, and is also present in all fertile soils, — is, in fact, essential to the growth of plants. Other important compounds are present in the blood, in the brain, and in the urine. 22. Platinum — a metal used in jewelry ; does not enter into the body. It is a " precious '' metal for the same reason as gold. 23. Potassium — a. shining white metal which readily forms compounds with a great many other ele- ments. When thrown into water it unites with it so actively as to take fire. It has a great number of important compounds, such as caustic potash (potassium hydrate), potassium chlorate, potassium iodide, potassium bromide, potassium nitrate, and many others. 24. Radium — a peculiar metal which gives off very pene- trating rays like X rays. It was recently discovered by Mme. Curie. It is used in the treatment of cer- tain skin diseases and cancers. 25. Silicon — is not of much importance to our bodies, but it is, nevertheless, next to oxygen the most abundant element. It forms a large part of rock, sand, clay, and soil. Glassware contains much silicon, as do a great many other things that we handle; it is contained in the many forms of stone and crockery dishware. It is very insoluble. ELEMENTS AND COMPOUNDS 11 26. Silver — a metal, some of whose compounds such as silver nitrate, argyrol, protargol, are used in medicine. 27. Sodium — a very widespread element, the sister metal of potassium, and with very similar proper- ties. Some of its compounds, as sodium chloride, carbonate, bicarbonate, are important constituents of the blood. 28. Strontium — is somewhat similar to sodium and potassium. Some of its compounds, such as stron- tium bromide, are used as drugs. 29. Sulphur — is a yellow, inflammable solid used directly as a drug. Its compounds, such as sulphuric acid, magnesium sulphate, etc., are important in medicine. It is found in many parts of the body, such as skin and hair. 30. Tin — a metal. 31. Zinc — a metal some of whose insoluble compounds, such as zinc oxide, are used in medicine for their soothing properties. Its soluble compounds, like zinc sulphate and zinc sulphocarbolate, are poison- ous and are used as disinfectants and cauterizing agents. CHAPTER II Atoms and Molecules The Use of Hypotheses in Science. — In order to explain the phenomena of nature, scientists often use hypotheses, or working theories. These are sup- positions which explain the facts, but which for the time being cannot be proved completely. One of the most useful and important of these hypotheses is the atomic theory, — the idea that all substances are composed of extremely minute particles. Although nobody has ever seen one of these particles, the theory is so useful and explains so much that the whole development of modern chemistry has been built up on it. Molecules. — Suppose that we commenced to divide any substance, say a drop of water, into smaller and smaller particles, and suppose that we had instruments fine enough so that we could con- tinue this division as long as we wanted to, we would finally reach a particle so small that if it could be further divided, it would no longer be water. These 12 ATOMS AND MOLECULES 13 particles of which water is believed to be formed are called molecules. If the molecule of water were broken up into its constituents, hydrogen and oxygen, it would no longer have the properties of water. Hence a mole- cule is defined as the smallest weight of any kind of matter in which the original properties of the matter are retained. ^ Atoms. — But suppose that instead of starting with a compound we were to start with an element itself. Here, too, an overwhelming array of evidence has led scientists to make the hypothesis that ulti- mately particles would be reached which could not be further subdivided. These particles are known as atoms} Molecules Composed of Atoms. — In the first chapter it was stated that the composition of any compound is absolutely constant; when any com- pound is analyzed into its elements we always get exactly the same amount of each of the elements from any given amount of the compound. We see now why this is so. Each molecule of a compound * Recent discoveries in connection with radium and X rays have brought to our knowledge particles far smaller even than atoms, — particles out of which probably atoms themselves are made. 14 CHEMISTKY FOR NURSES has the same number of atoms as every other molecule of the compound. Every molecule of water has one atom of oxygen and two atoms of hydrogen. Therefore, water, no matter where or how obtained, always has constant amounts of oxygen and hydro- gen. Size of Molecules. — The absolute size of atoms and molecules is not known. All atoms and even the largest molecules, those known to contain thou- sands of atoms, are far too small to be seen through the most powerful microscope. It will give you some idea of their size to know that it has been calculated that if a single drop of water were magnified to the size of the earth, the molecules would be something like the size of baseballs. Weight of Atoms. — The atoms of the different elements have different weights ; the atoms of the same elements are all alike. Though their absolute weights are not known, the relative weights of all the atoms are known with great accuracy. Hydro- gen is the lightest and is taken as the standard, while the atom of radium, one of the heaviest, weighs two hundred and twenty-five times as much as the atom of hydrogen. ATOMS AND MOLECULES 15 PHYSICAL STATE OF MATTER Changes in Physical State of Molecules. — A sub- stance remains unchanged no matter what vicissi- tudes it goes through as long as the atoms in its molecules remain together. The molecule of water is the same whether in steam, or water, or ice. But the positions and motions of the molecules may vary. In steam and in all gases the molecules are separated by space and vibrate to and fro. This causes the pressure of gas on the containing wall (for instance, the pressure of the gas in a balloon). In fluids the molecules are packed much more closely together, but they still move to and fro. In solids their rela- tive positions are fixed. The physical state of a sub- stance, whether it is solid, fluid, or gas, depends on temperature and pressure, and it is possible to ob- tain most substances in any one of these three forms. Thus we have reached a different idea of chemical and physical changes from that given in the first chapter. Chemical changes are those in which the atoms in the molecule are changed. Physical changes are changes in which the molecules retain all their atoms intact. This is the reason that chemical changes are generally more permanent and deep-going than physi- cal changes. 16 CHEMISTRY FOR NURSES Many physical processes, such as dissolving, boil- ing, distilling, crystallizing, are made use of in chemi- cal operations. Solution. — When sugar or salt is put into water it disappears (dissolves), but its presence in the water can be recognized by the taste. If more salt or sugar is gradually added, a limit is finally reached, at which no more can be dissolved. The solution is saturated} If now the water is warmed, it will dis- solve some more sugar or salt, for the solubility of most solid substances increases as the temperature rises. If the solution is saturated while very hot and then allowed very slowly to cool, some of the sugar or salt will appear again in the form of crystals. Crystallization. — Likewise, if some of the water evaporates from a saturated solution, crystals form. Crystallization is one of the methods used in chemis- try and in manufacturing to obtain substances in a pure state. For a crystal contains not a mixture of two substances, but only one substance. The Right Solvent must be Chosen. — Substances may dissolve not only in water, but in other fluids. Moreover, the solubility of different substances in different fluids varies greatly. Thus water will dis- 1 See Chapter on Solutions in Blumgarten's " Materia Medica for Nurses." ATOMS AND MOLECULES 17 solve large amounts of salt, but will not dissolve fat at all . Ether, on the other hand, dissolves fat readily, but not salt. Solubility of Gases. — A fluid may dissolve not only solids, but also various fluids and gases. For instance, ether or chloroform will dissolve to some extent in water. A familiar example of a dissolved gas is carbonated water (carbonic acid gas dissolved in water). Oxygen is soluble in water and fish live by breathing dissolved oxygen gas. Dissolved gases do not follow the same rule with regard to tempera- ture as dissolved solids, but on the contrary as the temperature rises, less, not more, of the gas will dissolve.^ Distillation. — When fluids change to gases at ordinary temperature we speak of the process as evaporation. When heat is applied so that the change from fluid to gas is violent we call it hoiling. When gases are cooled sufficiently, or are subjected to pressure, or both, they condense and form liquids again. Distilling consists of first boiling a fluid and then cooling the escaping vapor to a liquid. Distillation is very useful in purifjdng and separating various substances. * The reason that boiled water tastes " flat " is that the heat drives all the dissolved air out of it. c CHAPTER III Chemical Names and Formulas. Chemical Affinity Chelnical Names tell the Composition of Sub- stances. — To make the study of their science as easy as possible, the chemists have tried to give every compound a name that in general tells what its composition is. This can be done, however, only with relatively simple compounds. When we get molecules that contain two or three hundred atoms it is impossible to name them all, but in the more simple compounds the name indicates the atoms, and even in the very complicated ones the name often tells a great part of the story. Thus, combinations of oxygen are known as oxides. Some of the simple oxides already familiar to you are carbon dioxide (the bubbles of carbonated beverages), nitrous oxide (laughing gas), zinc oxide. Most of the elements can form oxides. Many (but not all) compounds of chlorine are called chlorides. It is evident from the name that sodium chloride is a compound in which two ele- ments, sodium and chlorine, have combined. The 18 CHEMICAL NAMES AND FORMULAS 19 same is true of bromides and iodides and sulphides. Each of them is a simple combination of some ele- ment with bromine or iodine or sulphur. In general, the name suggests the combination. Reason fox Using Formulas. — In the more com- plicated parts of chemistry short names have to be used for long compounds. But in order to state simply and completely what atoms form each mole- cule a system of abhreviations has been invented. In addition to the name for each compound there is a formula that tells in a few letters the chemical structure. In writing formulas each element has a letter or pair of letters which represents it. These abbreviations are used by all chemists of the world ; a chemist speaks a sort of international language. It helps greatly in understanding chemistry to know the most important of these abbreviations : — Bromine . . Br. Iron . . , , Fe. (for the Latin Calcium . . Ca. Iodine . . I. ferrum) Carbon . . C. Oxygen . , , 0. Chlorine . . CI. Nitrogen . . . N. Hydrogen . H. Phosphorus . Potassium . P. K. Because phosphorus was known before potassium it has the initial letter ^^ P ^' for its abbreviation ; potassium must have some other initial and so the 20 CHEMISTRY FOR NURSES Latin name of kalium is used for potassium and its abbreviation is simply K. Sodium . . . Na. (natrium) Silver . . . Ag. (argentum, Latin for silver) Sulphur . . . S. How to interpret Formulas of Compounds. — Each initial written alone stands for a single atom. When two or more initials are written together they stand for a compound whose molecule contains as many of each kind of atom as is represented by the little figure written to the right of the letter. For example : KI means a molecule containing one atom of potassium and one of iodine (potassium iodide). The abbreviation CO2 means that each molecule of carbon dioxide contains two atoms of oxygen and one of carbon. The formula for silver nitrate is AgNUg, which means that ona. atom of silve^ is united with ^e of nitrogei^ and Jjiree of oxygen^ Thus there are §ye-..aion^ in the one molecule of silver nitrate. The formula tells at once exactly what silver nitrate is. CHEMICAL AFFINITY What makes Atoms Unite ? — Why do the atoms unite to form molecules? In order to explain this CHEMICAL NAMES AND FORMULAS 21 scientists have had to assume the existence of a powerful and Httle understood force known as chemical affinity. This tendency to unite with other atoms is very strong in some instances, very- weak in others. Thus the tendency of the elements sodium and potassium to combine with other ele- ments is so great that it is extremely hard to keep these substances, even when one gets them, in a pure state. They combine with anything they touch. When thrown into water potassium combines with it so violently as to burst into flames. On the other hand some elements, such as gold and plati- num, have very little tendency to unite with other elements and they form only a few compounds and these with difficulty. This is why they are used as jewelry. Special Affinities of Different Atoms. — Not only do the atoms^ vary in the strength of their unions, but they show peculiar preferences; each atom shows a greater tendency to unite with certain atoms than with others. The laws that govern this chemical affinity are quite definite, so that when a number of different kinds of atoms are mixed together in such a way as to be free to combine, one can always predict with certainty which ones will unite with each other. 22 CHEMISTRY FOU NURSES Thus if any metal such as iron, or even silver or gold, is put into a watery solution of the element chlorine, the metal will always unite with the chlorine, — never with oxygen or hydrogen, because the chemical affinity of chlorine for all metals is very- great. On the other hand, most metals, if kept in water containing no chlorine, or even in moist air, will gradually rust or tarnish, due to union with oxygen. THE USE OF CHEMICAL FORMULAS To show how we can express chemical reactions or changes and think out how they will happen, let us take the action of h ydrochlor ic acid on ^Iver ^itrate,_ Hydr o Qhlor ic acid contains to each mole- cule one atonuiLhydrQgeji and one of chlorine. Its formula is HCl. Mix a drop of clear silver nit rate solution in a test tube with a little .h^dxachlQiLc acid ; a thick, white substance or precipitate forms at once.^ Silver has a very powerful affinity for chlorine and when they unite they form silver ch loride. This is what the precipitate is composed of. ^ A precipitate is a solid formed in a solution as a'result of chemical action. The fluid can be separated from the solid by filtering through filter paper. When this is done, the clear fluid that comes through the paper is known as the filtrate, the solid powder left on the paper is called the residue. CHEMICAL NAMES AND FORMULAS 23 The formula which expresses what happens is : — AgNQa + HCl = AgCl + HNO3 (Silver Nitrate + Hydrochloric Acid = Silver Chloride + Nitric Acid) This says that on adding silve r nitrate to hydro- (chloric add the ^Iver leaves its combination with nitrogen a nd oxyge n and joins the chlorine, for which it has a greater affinity. The hydr ogen which was originally present in the hydroch loric acid being torn from its chlor ine unites with the equally deserted nitr ogen-oxvgen _combination to form another new substance which will later be recognized as nitric ^cid. Thus formulas show the exchange of atoms between molecules. Any compound which contains chlorine united with on e other elem ent will act in the same way as hydrochloric acid when brought into contact with a ^ilver /^, ompound. Thus let the student find out what happens if sodium chloride (NaCl) instead of hydrochloric acid (HCl) is mixed with silver nitrate. Write the formula. i^#y4y'Na^/'=: A^C/ ^//g^/^ff^ • VALENCE The reader will have already noticed that in forming compoands some atoms combine only with one other atom, others join with two, three, or more. Thus the atom of oxygen unites with two atoms of 24 CHEMISTRY FOR NURSES hydrogen to form the molecule of water (H2O), while the atom of clilorine unites with only one atom of hydrogen to form hydrochloric acid (HCl). This is subject to a definite rule which explains why the molecule of any substance contains always fixed numbers of the same atoms grouped in the same way. This explanation is really another working hypothesis, known as the theory of valence. Valences are Imaginary Points of Attachment for Other Atoms. — Imagine the atom as having arms by which it can hold on to the arms of other atoms. Each kind of atom has always a f,xpd number of arms, or valences, as they are more properly called. Thus, for instance, the atoms of hydrogen, spdium, j)otas- sium, chlorine, bromine, iodine, have each only one arm to hold on to the other atoms with. When they unite with each other these atoms therefore form compounds having two atoms only to each molecule. Picture to yourself the atoms with their imaginary arms: — Hydrochloric Acid @ (§) Potassium Iodide @ (J) Sodiiun Bromide ® @ The atom of oxygen as well as that of sulphur has two such arms or valences. The atom of nitrogen CHEMICAL NAMES AND FORMULAS 25 has three (and in reality it has two extra arms which it occasionally can use, but which in its ordinary combinations it does not use). The atom of carbon has four arms. A knowledge of these facts will enable you to understand much more clearly the reason for the structure of some of the molecules that we have been considering. Thus take, for instance, the molecule of water (H2O). In water we have the two-armed atom, oxygen, holding with each of its arms to the arm of a one-armed atom, hydrogen, thus : — ® — @ — ® Or consider ammonia (NH3). Here we have the nitrogen atom holding by its three arms to the arms of three atoms of hydrogen : — Y Two atoms may often be joined by more than one arm or valence. Thus, for instance, in CO2 (carbon dioxide) the Garhoiuiom divides its four arms, giving two to each of the oxv^en ato ms with which it is united : — (Q)=