~Uses.~ Solutions of hydrogen dioxide are used largely as oxidizing agents. The solution sold by druggists contains 3% of the dioxide and is used in medicine as an antiseptic. Its use as an antiseptic depends upon its oxidizing properties. EXERCISES 1. Why does the chemist use distilled water in making solutions, rather than filtered water? 2. How could you determine the total amount of solid matter dissolved in a sample of water? 3. How could you determine whether a given sample of water is distilled water? 4. How could the presence of air dissolved in water be detected? 5. How could the amount of water in a food such as bread or potato be determined? 6. Would ice frozen from impure water necessarily be free from disease germs? 7. Suppose that the maximum density of water were at 0 deg. in place of 4 deg.; what effect would this have on the formation of ice on bodies of water? 8. Is it possible for a substance to contain both mechanically inclosed water and water of crystallization? 9. If steam is heated to 2000 deg. and again cooled, has any chemical change taken place in the steam? 10. Why is cold water passed into C instead of D (Fig. 24)? 11. Mention at least two advantages that a metal condenser has over a glass condenser. 12. Draw a diagram of the apparatus used in your laboratory for supplying distilled water. 13. 20 cc. of hydrogen and 7 cc. of oxygen are placed in a eudiometer and the mixture exploded. (a) How many cubic centimeters of aqueous vapor are formed? (b) What gas and how much of it remains in excess? 14. (a) What weight of water can be formed by the combustion of 100 L of hydrogen, measured under standard conditions? (b)What volume of oxygen would be required in (a)? (c)What weight of potassium chlorate is necessary to prepare this amount of oxygen? 15. What weight of oxygen is present in 1 kg. of the ordinary hydrogen dioxide solution? In the decomposition of this weight of the dioxide into water and oxygen, what volume of oxygen (measured under standard conditions) is evolved? CHAPTER V THE ATOMIC THEORY ~Three fundamental laws of matter.~ Before we can gain any very definite idea in regard to the structure of matter, and the way in which different kinds of substances act chemically upon each other, it is necessary to have clearly in view three fundamental laws of matter. These laws have been established by experiment, and any conception which may be formed concerning matter must therefore be in harmony with them. The laws are as follows: ~Law of conservation of matter.~ This law has already been touched upon in the introductory chapter, and needs no further discussion. It will be recalled that it may be stated thus: _Matter can neither be created nor destroyed, though it can be changed from one form into another._