II. DEFINITIONS 14. A chemical element is a substance that has never been separated into two or more different kinds of matter.1 Over seventy of these elements are known at the present time, and of these seventy, twelve are found constantly in the living substance of plants and animals. The most common of these twelve elements are carbon (symbol, C), hydrogen (H), oxygen (O), nitrogen (N), sulphur (S), phosphorus (P), iron (Fe), and calcium (Ca), which is found in lime. [In addition to the elements already studied (C, O, H, N), the others mentioned should be shown to students; and if time permits, some of these elements may be burned or oxidized in oxygen and the characteristics of the oxids thereby formed may be discussed.] 15. A chemical compound is a substance formed by the union of two or more chemical elements. Two of the important compounds considered in biology are carbon dioxid (formula CO2), which means that it is composed of one part of carbon and two parts of oxygen, and water (formula H2O), which means that it is composed of two parts of hydrogen and one part of oxygen. 16. A mixture differs from a compound in the fact that the elements or compounds of which the former is composed are not chemically united. In air, for instance, the oxygen and nitrogen are not chemically combined, but are simply put together as one might mix pepper and salt. Again, when sugar is dissolved or mixed with water, the two compounds are mingled so closely that the sugar disappears; it may easily be obtained unchanged in its composition by evaporating the water. 1 There are, however, exceptions to this statement, but they are too technical for discussion in an elementary text-book. 17. Oxidation is the chemical union of oxygen with some other substance. It may take place slowly, as when carbon is made to glow in the air; or it may take place rapidly, as when carbon burns in oxygen. But whenever oxidation takes place, (1) an oxid is formed, (2) a certain amount of heat is liberated, and (3) if the process is sufficiently rapid, light is seen. III. A STUDY OF THE FOOD SUBSTANCES 18. Introduction. The food substances needed by plants and animals may be divided into five classes, namely: (1) carbohydrates (i.e. starches and sugars); (2) fats and oils; (3) proteins, which are also known as albuminous or nitrogenous substances (e.g., white of egg, lean meat, gluten of wheat); (4) minerals (e.g. common salt, saltpeter, phosphate of lime); (5) water. 19. To determine the chemical composition of starch. Laboratory Study No. 6. Suggested as home work. Warm some starch in an old cooking spoon in order to drive off any water that may be in it, but do not allow it to burn. To determine when the starch is free from water, hold the heated starch under a dry, cold tumbler, and if no moisture collects upon the tumbler, the starch contains no water. Now set the starch on fire, and hold a cold, dry glass over the burning starch. 1. Tell what you have done and state what is formed on the inside of the tumbler by the burning of the starch. 2. What is the only chemical element that could possibly form water by burning (i.e. by uniting with oxygen)? 3. What chemical element, therefore, must have been present in the starch in order to have produced water when dry starch is burned? 1 The term protein is used throughout this book instead of proteid, because of the unanimous recommendation in favor of the former term by the American Society of Biological Chemists and the American Physiological Society. See Science, April 3, 1908. 4. What substance is left in the cooking spoon after the flame goes out? 5. Name two chemical elements proved to be present in starch. 6. Starch also contains oxygen. Name now the three chemical elements of which this nutrient is composed. 20. To determine the chemical composition of sugar, fat, and protein. — Laboratory Study No. 7. (Optional.) 1. Test sugar in the same way as directed in Laboratory Study No. 6, 1-5 (above). a. Describe each of the experiments, giving results and conclusions. b. Sugar, like starch, has oxygen also in its composition. Name now all the chemical elements of which sugar is composed. 2. In a similar way test a fat (e.g. lard, or the fat of meat). a. State what you do, what you see, and what you conclude. b. Fat, like starch and sugar, has oxygen in its composition, but in a different proportion. State, therefore, the three elements present in fat. 3. (Demonstration.) Secure a vegetable protein (e.g. gluten) and test it as directed above. a. Describe your experiments and give your results and conclusions. b. Besides the two elements you have shown to be present, protein also contains oxygen, nitrogen, sulphur, phosphorus, and often other elements. State, now, the chemical elements of which this food substance is composed. 21. Summary. The carbohydrates, as we have learned and as their name implies, are composed of the chemical elements carbon, hydrogen, and oxygen. The same three chemical elements are likewise present in fats and proteins, but in different proportions. Proteins, however, in addition to the carbon, hydrogen, and oxygen, contain at least three other chemical elements, namely, nitrogen, sulphur, and phosphorus; in fact, proteins are the most complex of all chemical substances known. Following is the composition of the various nutrients studied thus far: Starch, composed of C, H, O (in the proportion of C6H1005). Protein, composed of C, H, O, N, S, P (and sometimes of other elements). Having demon 22. Tests for the food substances. strated that the various food substances are chemical compounds, each composed of several chemical elements, we are now to carry on experiments by which it will be possible to test for each of these food substances. By this means we shall be able to prove the presence or absence of starch, grape sugar, protein, fat, mineral matters, and water in the foods used by plants, animals, and man. 23. To test foods for starch. Laboratory Study No. 8. Materials: Corn starch, grape sugar, white of egg, fat or oil, salt, water; various foods in the home kitchen; iodine solution (see below); test tubes; gas burner or alcohol lamp. Preparation of iodine solution: A quart (1000 cc.) of iodine solution is made by dissolving in 5 teaspoonfuls (40 cc.) of water, onehalf teaspoonful (4 grams) of potassium iodide, and one-fourth this amount of iodine (1 gram). This solution, when thoroughly mixed, should be diluted to make one quart (1000 cc.). In a clean bottle this mixture will keep indefinitely.1 1. Put a small amount (size of a pinhead) of corn starch in a test tube, add water, shake the mixture, and boil it over a gas flame. Pour into the starch mixture 1 From Peabody's "Laboratory Exercises." Henry Holt & Co. thus formed a few drops of iodine. Describe the experiment, and state what color is produced. 2. Try the effect of iodine on each of the other food substances as follows: Put a small amount of grape sugar into a test tube; into a second tube put some white of egg (protein); into a third some fat or oil; into a fourth some mineral matter (salt); and into a fifth some water. Add a little water to each and boil as in 1 above to cook each nutrient. Add a drop or two of iodine solution to each test tube. Tell what happens. Do any of the colors thus produced resemble at all the color resulting from the addition of iodine to starch? 3. From the preceding, state how you can determine whether or not a substance contains starch. 4. (Optional home work.) Test as many foods as you can (e.g. oatmeal, flour, raw meat, milk, parsnip, potato, onions, apples, beans, rice, pepper) in the following way: Put a small amount of a given food into a test tube or in a sauce pan, add a little water, and boil to cook each food, then add a few drops of iodine. Before making each test make sure that the test tube or saucer is clean. Prepare in your note-book a table like the following, and fill in under each head the names of the foods you have proved to contain or to be without starch. STARCH PRESENT STARCH ABSENT 24. To test foods for grape sugar. Laboratory Study No. 9. Materials: Grape sugar, corn starch, white of egg, fat or oil, salt, water; various food substances common in home kitchen; Fehling's solution (see below); test tubes, gas burner or alcohol lamp. |