Common Science Part 39

Bend the platinum electrodes up so that they will stick up into the test tubes from below. Bubbles should immediately begin to gather on the platinum wire and to rise in the test tubes. As the test tubes fill with gas, the water is forced out; so you can tell how much gas has collected at any time by seeing how much water is left in each tube.

One tube should fill with gas twice as fast as the other. The gas in this tube is hydrogen; there is twice as much hydrogen as there is oxygen in water. The tube that fills more slowly contains oxygen.

When the faster-filling tube is full of hydrogen--that is, when all of the water has been forced out of it--take the electrode out and let it hang loose in the gla.s.s. Put a piece of cardboard about 1 inch square over the mouth of the test tube; take the test tube out of the water and turn it right side up, keeping it covered with the cardboard. Light a match, remove the cardboard cover, and hold the match over the open test tube. Does the hydrogen in it burn?

When the tube containing the oxygen is full, take it out, covered, just as you did the hydrogen test tube. But in this case make the end of a stick of charcoal glow, remove the cardboard from the tube, and then plunge the glowing charcoal into the test tube full of oxygen.

[Ill.u.s.tration: FIG. 160. The electrodes are made of loops of platinum wire sealed in gla.s.s tubes.]

Only oxygen will make charcoal burst into flame like this.

When people found that they could take water apart in this way and turn it into hydrogen and oxygen, and when they found that whenever they combined hydrogen with oxygen they got water, they knew, of course, that water was not an element. Maybe some day they will find that some of the eighty-five or so substances that we now consider elements can really be divided into two or more elements; but so far the elements we know show no signs of being made of anything except themselves.

The last section of this book will explain something about the way the chemist goes to work to find out what elements are hidden in compounds.

[Ill.u.s.tration: FIG. 161. Water can be separated into two gases by a current of electricity.]

THE QUICK WAY CHEMISTS WRITE ABOUT ELEMENTS. Since everything in the world is made of a combination or a mixture of elements, chemists have found it very convenient to make abbreviations for the names of the elements so that they can quickly write what a thing is made of.

They indicate hydrogen by the letter H. O always means oxygen to the chemist; C means carbon; and Cl means chlorine, the poison gas so much used in the World War. The abbreviation stands for the Latin name of the element instead of for the English name, but they are often almost alike. The Latin name for the metal sodium, however, is _natrum_, and chemists always write Na when they mean sodium; this is fortunate, because S already stands for the element sulfur. Fe means iron (Latin, _ferrum_). But I stands for the element iodine. (The iodine you use when you get scratched is the element iodine dissolved in alcohol.) It is not necessary for you to remember the chemical symbols unless you mean to become a chemist or unless you read a good deal about chemistry. But almost every one knows at least that H means hydrogen, O means oxygen, and C means carbon.

When a chemist wants to show that water is made of hydrogen two parts and oxygen one part, he writes it very quickly like this: H_2O (p.r.o.nounced "H two O"). "H_2O" means to a chemist just as much as "w-a-t-e-r" means to you; and it means even more, because it tells that water is made of two parts hydrogen and one part oxygen. If a chemist wanted to write, "You can take water apart and it will give you two parts of hydrogen and also one part of oxygen," this is what he would put down:

H_2O -> 2H+O.

If he wanted to show that you could combine two parts of hydrogen and one part of oxygen to form water, he would write it quickly like this:

2H+O -> H_2O.

These are called _chemical equations_. You do not need to remember them; they are put here merely so that you will know what they look like. Some of them are much longer and more complicated, like this:

HC_2H_3O_2+NaHCO_3 -> H_2O+CO_2+NaC_2H_3O_2.

This is the chemist's way of saying, "Vinegar is made of one part of hydrogen gas that will come off easily and that gives it its sour taste, two parts of carbon, three parts of hydrogen that does not come off so easily, and two parts of oxygen. When you put this with baking soda, which is made of one part of the metal sodium, one part of hydrogen, one part of carbon, and three parts of oxygen, you get water and carbon dioxid gas and a kind of salt called sodium acetate." Or, more briefly, "If you put baking soda with vinegar, you get water, a gas called carbon dioxid, and a salt." You can see how much shorter the chemist's way of writing it is.

SOME ELEMENTS YOU ALREADY KNOW. Here is a list of some elements that you are already pretty well acquainted with. The abbreviation is put after the name for each. This list is only for reference and need not be learned.

Aluminum (Al) Carbon (C) Charcoal, diamonds, graphite (the lead in a pencil is graphite), hard coal, and soot are all made of carbon.

Chlorine (Cl) A poison gas that was used in the war.

Copper (Cu) Gold (Au) Hydrogen (H) The lightest gas in the world; you got it from water in the last experiment and will get it from an acid in the next.

Iodine (I) It is a solid; what you use is iodine dissolved in alcohol.

Iron (Fe) Lead (Pb) Mercury (Hg) This is another name for quicksilver.

Nickel (Ni) Nitrogen (N) About four fifths of the air is pure nitrogen.

Oxygen (O) This is the part of the air we use in breathing.

You got some out of water, and you will have it to deal with in another experiment.

Phosphorus (P) Phosphorus makes matches glow in the dark, and it makes them strike easily.

Platinum (Pt) Radium (Ra) Silver (Ag) Sodium (Na) You are not acquainted with sodium by itself, but when it is combined with the poison gas, chlorine, it makes ordinary table salt.

Sulfur (S) Tin (Sn) Zinc (Zn)

For the rest of the elements you can refer to any textbook on chemistry.

HOW ELEMENTS HIDE IN COMPOUNDS. One strange thing about an element is that it can hide so completely, by combining with another element, that you would never know it was present unless you took the combination apart. Take the black element carbon, for instance. Sugar is made entirely of carbon and water. You can tell this by making sugar very hot. When it is hot enough, it turns black; the water part is driven off and the carbon is left behind. Yet to look at dry, white sugar, or to taste its sweetness, one would never suspect that it was made of pure black, tasteless carbon and colorless, tasteless water.

Mixing carbon and water would never give you sugar. But combining them in the right proportions into a chemical compound does produce sugar.

Not only is carbon concealed in sugar, but it is present in all plant and animal matter. That is why burning almost any kind of food makes it black. You drive off most of the other elements and separate the food into its parts by getting it too hot; the water evaporates and so does the nitrogen; what is left is mainly black carbon.

MAKING HYDROGEN COME OUT OF HIDING. The light gas, hydrogen, conceals itself as perfectly as carbon does by combining with other elements.

It is hiding in everything that is sour and in many things that are not sour. And you can get it out of sour things with metals. In some cases it is harder to separate than in others; and some metals separate it better than others do. But one sour compound that you can easily get the hydrogen out of is hydrochloric acid (HCl), which is hydrogen combined with the poison gas, chlorine. One of the best metals to get the hydrogen out with is zinc. Here are the directions for doing it and incidentally for making a toy balloon:

EXPERIMENT 91. _Do this experiment on the side of the laboratory farthest from any flames or fire. Do not let any flame come near the flask in which you are making hydrogen._

In the bottom of a flask put two or three wads of zinc shavings, each about the size of your thumb. Fit a one-hole rubber stopper to the flask. Take the stopper out and put a piece of gla.s.s tubing about 5 inches long through the hole of the stopper, letting half an inch or so stick down into the flask when the stopper is in place (Fig. 162). With a rubber band fasten the mouth of a rubber balloon over the end of the gla.s.s tube that will be uppermost. Fill the balloon by blowing through the gla.s.s tube to see if all connections are tight, and to see how far it may be expanded without danger of breaking. You can tell when the balloon has about all it will hold, by pressing gently with your fingers. If the rubber feels tight, do not blow any more. Let the air out of the balloon again.

Now get some hydrochloric acid (HCl) diluted with three parts of water. Find the bottle marked "HCl, dilute 1-3," in which the acid is already diluted. Before you open the bottle, get some solution of soda, and keep it near you; if in this experiment or any other you spatter acid on your hands or face or clothes, wash it off _immediately_ with soda solution.

_Remember this._ Ammonia will do as well as the soda solution to wash off the acid, but be careful not to get it into your eyes.

Pour the hydrochloric acid (HCl) on the zinc shavings in the bottom of the flask, until the acid stands about an inch deep.

Then quickly put the rubber stopper with its attachments into the flask, so that the gas that bubbles up will blow up the balloon.

If the bubbles do not form rapidly, ask the teacher to pour a little strong hydrochloric acid into the flask; but this will probably not be necessary. Let the balloon keep filling until it is as large as you blew it. But if the bubbles stop coming before it gets as large as that, close the neck of the balloon by pinching it tightly, and take the stopper out. Let some one add more zinc shavings and more acid to the flask; put the stopper back in, and stop pinching the neck of the balloon.

_In this and all other experiments when you use strong acids, pour the used acids into the crockery jar that is provided for such wastes. Do not pour them into the sink, as acids ruin sink drainpipes._

When the balloon is full, close the neck by slipping the rubber band up from the part of the neck that is over the gla.s.s tube on to the upper part of the neck. Pull the balloon off the gla.s.s tube and pinch the neck firmly shut. Take the stopper out and rinse the flask several times with running water. Any zinc that is left should be rinsed thoroughly, dried, and set aside so that it may be used again. Now tie one end of a long thread firmly around the mouth of the balloon and let the balloon go. Does it rise? If it does not, the reason is that you did not get it full enough. In that case make more hydrogen and fill it fuller, as explained above.

[Ill.u.s.tration: FIG. 162. Filling a balloon with hydrogen.]

[Ill.u.s.tration: FIG. 163. Adding more acid without losing the gas.]

Here is another experiment with hydrogen:

EXPERIMENT 92. Put a wad of zinc shavings, about the size of the end of your little finger, into the bottom of a test tube.

Cover it with hydrochloric acid (HCl) diluted one to three, as in the preceding experiment. After the bubbles have been rising for a couple of minutes, take the test tube to the side of the laboratory where the burners are, and hold a lighted match at its mouth. Will hydrogen burn?

Remember that the hydrogen which the zinc is driving out of the acid is exactly the same as the hydrogen you drove out of water with an electric current. There is a metal called _sodium_ (Na) and another called _pota.s.sium_ (K) which are as soft as stiff putty and as shiny as silver; if you put a tiny piece of sodium (Na) or pota.s.sium (K) on water, it will drive the hydrogen out of the water just as zinc drove it out of the acid. The action is so swift and violent and releases so much heat that the hydrogen which is set free catches fire. This makes it look as if the metal were burning as it sputters around on top of the water. There is so much sputtering that the experiment is dangerous; people have been blinded by the hot alkaline water spattering into their eyes. So you cannot try this until sometime when you take a regular course in chemistry.

[Ill.u.s.tration: FIG. 164. Trying to see if hydrogen will burn.]

GETTING OXYGEN, A GAS, FROM TWO SOLIDS. Oxygen (O) can hide just as successfully as hydrogen. Practically all elements can do the same by combining with others. Here is an experiment in which you can get the gas, oxygen, out of a couple of solids. If you went to the moon or some other place where there is no air, you could carry oxygen very conveniently locked up in these solid substances. Oxygen, you remember, is the part of the air that keeps us alive when we breathe it.

EXPERIMENT 93. In a test tube mix about one half teaspoonful each of white pota.s.sium chlorate crystals and black grains of manganese dioxid. Put a piece of gla.s.s tubing through a cork so that the tubing will stick down a little way into the test tube. _Do not put the gla.s.s tubing through the cork while the cork is in the test tube: insert the gla.s.s tubing first, then put the cork into the test tube._ Put one end of a 2-foot piece of rubber tubing over the gla.s.s tube and put the other end into a pan of water.

Fill a flask or bottle to the brim with water, letting it overflow a little; hold a piece of cardboard firmly over the mouth of the bottle; turn the bottle upside down quickly, putting the mouth of it under water in the pan; take the cardboard away. The water should all stay in the bottle.

Now shove the rubber tube into the neck of the bottle until it sticks up an inch or two. During this experiment, be careful not to let the neck of the bottle or flask pinch the rubber tubing; small pieces of wood or gla.s.s tubing laid beside the rubber tubing where it goes under the run of the neck will prevent this.

Hold, the test tube, tightly corked, over the flame of a burner, keeping the tube at a slant and moving it slightly back and forth so that all the material in it will be thoroughly heated. If you stop heating the test tube even for a couple of seconds, take the cork out; if you do not remove the cork, the cooling gas in the test tube will shrink and allow the water from the pan to be forced through the rubber tube into the test tube, breaking it into pieces.

When enough gas has bubbled up into the bottle to force all the water out, and when bubbles begin to come up outside the bottle, uncork the test tube and lay it aside where it will not burn anything; then slide the cardboard under the mouth of the bottle and turn it right side up; leave the cardboard on the bottle.