Читать книгу Physiology and Hygiene for Secondary Schools - Francis M. Walters - Страница 19

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To show the Difference between Free Oxygen and Oxygen in Combination.—Examine some crystals of potassium chlorate (KClO3). They contain oxygen in combination with potassium and chlorine. Place a few of these in a small test tube and heat strongly in a gas or alcohol flame. The crystals first melt, and the liquid which they form soon appears to boil. If a splinter, having a spark on the end, is now inserted in the tube, it is kindled into a flame. This shows the presence of free oxygen, the heat having caused the potassium chlorate to decompose. The difference between free and combined oxygen may also be shown by decomposing other compounds of oxygen, such as water and mercuric oxide.

Preparation and Properties of Oxygen.—Intimately mix 3 grams (½ teaspoonful) of potassium chlorate with half its bulk of manganese dioxide, and place the mixture in a large test tube. Close the test tube with a tight-fitting stopper which bears a glass tube of sufficient[pg 114] length and of the right shape to convey the escaping gas to a small trough or pan partly filled with water, on the table. Fill four large-mouthed bottles with water and, by covering with cardboard, invert each in the trough of water. Arrange the test tube conveniently for heating, letting the end of the glass tube terminate under the mouth of one of the bottles (Fig. 58). Using an alcohol lamp or a Bunsen burner, heat over the greater portion of the tube at first, but gradually concentrate the flame upon the mixture. Do not heat too strongly, and when the gas is coming off rapidly, remove the flame entirely, putting it back as the action slows down. After all the bottles have been filled, remove the end of the glass tube from the water, but leave the bottles of oxygen inverted in the trough until they are to be used. On removing the bottles from the trough, keep the tops covered with wet cardboard.

Fig. 58—Apparatus for generating oxygen.

1. Examine a bottle of oxygen, noting its lack of color. Insert a small burning splinter in the upper part of the bottle and observe the change in the rate of burning. The air contains free oxygen, but it is diluted with nitrogen. Compare this with the undiluted oxygen in the bottle as to effect in causing the splinter to burn.

2. In a second bottle of oxygen insert a splinter without the flame, but having a small spark on the end. As soon as the oxygen kindles the spark into a flame, withdraw from the bottle and blow out the flame, but again insert the spark. Repeat the experiment as long as the spark is kindled by the oxygen into a flame. This experiment is usually performed as a test for undiluted oxygen.

3. Make a hollow cavity in the end of a short piece of crayon. Fasten a wire to the crayon, and fill the cavity with powdered sulphur.[pg 115] Ignite the sulphur in the flame of an alcohol lamp or Bunsen burner, and lower it into a bottle of oxygen. Observe the change in the rate of burning, the color of the flame, and the material formed in the bottle by the burning. The gas remaining in the bottle is sulphur dioxide (SO2), formed by the uniting of the sulphur and the oxygen.

4. Bend a small loop on the end of a piece of picture wire. Heat the loop in a flame and insert it in some powdered sulphur. Ignite the melted sulphur which adheres, and insert it quickly in a bottle of oxygen. Observe the dark, brittle material which is formed by the burning of the iron. It is a compound of the iron with oxygen, similar to iron rust, and formed by their uniting.

Preparation and Properties of Carbon Dioxide.—1. (a) Attach a piece of carbon (charcoal) no larger than the end of the thumb to a piece of wire. Ignite the charcoal in a hot flame and lower it into a vessel of oxygen. Observe its combustion, letting it remain in the bottle until it ceases to burn. Note that the burning has consumed a part of the carbon and has used up the free oxygen. Has anything been formed in their stead?

(b) Remove the charcoal and add a little limewater. Cover the bottle with a piece of cardboard, and bring the gas and the limewater in contact by shaking. Note any change in the color of the limewater. If it turns white, the presence of carbon dioxide is proved.

2. Burn a splinter in a large vessel of air, keeping the top covered. Add limewater and shake. Note and account for the result.

3. Place several pieces of marble (limestone) in a jar holding at least half a gallon. Barely cover the marble with water, and then add hydrochloric acid until a gas is rapidly evolved. This gas is carbon dioxide.

(a) Does it possess color?

(b) Insert a burning splinter to see if it supports combustion.

(c) Place a bottle of oxygen by the side of the vessel of carbon dioxide. Light a splinter and extinguish the flame by lowering it into the vessel of carbon dioxide. Withdraw immediately, and if a spark remains on the splinter, thrust it into the bottle of oxygen. Then insert the relighted splinter into the carbon dioxide. Repeat several times, kindling the flame in one gas and extinguishing it in the other. Finally show that the spark also may be extinguished by holding the splinter a little longer in the carbon dioxide.

(d) Tip the jar containing the carbon dioxide over the mouth of a tumbler, as in pouring water, though not far enough to spill the acid, and[pg 116] then insert a burning splinter in the tumbler. Account for the result. Inference as to the weight of carbon dioxide.

Fig. 59—Simple apparatus for illustrating passage of oxygen through the body.

(e) Review experiments (page 101) showing the presence of carbon dioxide in the breath.

To illustrate the General Movement of Oxygen through the Body.—Into a glass tube, six inches in length and open at both ends, place several small lumps of charcoal (Fig. 59). Fit into one end of this tube, by means of a stopper, a smaller glass tube which is bent at right angles and which is made to pass through a close-fitting stopper to the bottom of a small bottle. Another small tube is fitted into a second hole in this stopper, but terminating near the top of the bottle, and to this is connected a rubber tube about eighteen inches in length. The arrangement is now such that by sucking air from the top of the bottle, it is made to enter at the distant end of the tube containing the charcoal. After filling the bottle one third full of limewater, heat the tube containing the charcoal until it begins to glow. Then suck the air through the apparatus (as in smoking, without drawing it into the lungs), observing what happens both in the tube and in the bottle. What are the proofs that the oxygen, in passing through the tube, unites with the carbon, forms carbon dioxide, and liberates energy? Compare the changes which the oxygen undergoes while passing through the tube with the changes which it undergoes in passing through the body.