Experiment 99.--(1) Put a crystal of I in the palm of the hand and watch it for a minute. (2) Put 2 or 3 crystals into a t.t., and warm it, meanwhile holding a stirring-rod half-way down the tube. Notice the vapor, also a sublimate on the sides of the t.t.

and rod. (3) Add to 2 or 3 crystals in a t.t. 5 cc. of alcohol, C2H5OH; warm it, and see whether a solution is formed. If so, add 5 cc. H2O and look for a ppt. of I. Does this show that I is not at all soluble in H2O, or not so soluble as in alcohol?

163. Starch Solution and Iodine Test.

Experiment 100.--Pulverize a gram or two of starch, put it into an evaporating-dish, add 4 or 5 drops of water, and mix; then heat to the boiling-point 10 cc. H2O in a t.t., and pour it over the starch, stirring it meanwhile.

(1) Dip into this starch paste a piece of paper, hold it in the vapor of I, and look for a change of color. (2) Pour a drop of the starch paste into a clean t.t., and add a drop or two of the solution of I in alcohol. Add 5 cc. H2O, note the color, then boil, and finally cool. (3) The presence of starch in a potato or apple can be shown by putting a drop of I solution in alcohol on a slice of either, and observing the color. (4) Try to dissolve a few crystals of I in 5 cc. H2O by boiling. If it does not disappear, see whether any has dissolved, by touching a drop of the water to starch paste. This should show that I is slightly soluble in water.

164. Iodo-Starch Paper.

Experiment 101.--Add to some starch paste that contains no I 5 cc. of a solution of KI, and stir the mixture. Why is it not colored blue? Dip into this several strips of paper, dry them, and save for use. This paper is called iodo-starch paper, and is used as a test for ozone, chlorine, etc. Bring a piece of it in contact with the vapor of chlorine, bromine, or ozone, and notice the blue color.

Experiment 102.--Add a few drops of chlorine water to 2cc. of the starch and KI solution in 10 cc. H2O. This should show the same effect as the previous experiment.

165. Explanation.--Only free I, not compounds of it, will color starch blue. It must first be set free from KI. Ozone, chlorine, etc., have a strong affinity for K, and when brought in contact with KI they unite with K and set free I, which then acts on the starch present. Com- plete the equation: KI + Cl = ?

166. Occurrence.--The ultimate source of I is sea water, of which it const.i.tutes far too small a percentage to be separated artificially. Sea-weeds, or algae, especially those growing in the deep sea, absorb its salts--NaI, KI, etc.--from the water. It thus forms a part of the plant, and from this much of the I of commerce is obtained. Algae are collected in the spring, on the coasts of Ireland, Scotland, and Normandy, where rough weather throws them up. They are dried, and finally burned or distilled; the ashes are leached to dissolve I salts; the water is nearly evaporated, and the residue is treated with H2SO4, and MnO2, as in the case of Br and Cl. I also occurs in Chili, as NaI and NaIO3, mixed with NaNO3. This is an important source of the I supply.

167. Uses.--I is much used in medicine, and was formerly employed in taking daguerreotypes and photographs. Its solution in alcohol or in ether is known as tincture of iodine.

168. Fluorine.--F, Cl, Br, I, are called halogens or haloids, and exist in compounds--salts--in sea water. F is the most active of all elements, combining with every element except O. Until recently it has never been isolated, for as soon as set free from one compound it attacks the nearest substance, and seems to be as much averse to combining with itself, or to existing in the elementary state, as to uniting with O. It is supposed to be a gas, and, as is claimed, has lately been isolated by electrolysis from HF in a Pt U-tube. Fluorite (CaF2) and cryolite (Al2F6 + 6 NaF) are its two princ.i.p.al mineral sources. The enamel of the teeth contains F in composition.

CHAPTER x.x.xIII.

THE HALOGENS.

169. Halogens Compared.--The elements F, Cl, Br, I, form a natural group. Their properties, as well as those of their compounds, vary in a step-by-step way, as seen below. F is sometimes an exception. They are best remembered by comparing them with one another. Notice:

1. Similarity of name-ending. Each name ends in ine.

2. Similarity of origin. Salt water is the ultimate source of all, except F.

3. Similarity of valence. Each is usually a monad.

4. Similarity of preparation. Cl, Br, I, are obtained from their salts by means of MnO2 end H2SO4.

5. Variation in occurrence. Cl occurs in sea-salt, Br in sea- water, I in sea-weed.

6. Variation in color; F being colorless, Cl green, Br red, I violet.

7. Gradation in sp. gr.; F 19, Cl 35.5, Br 80, I 127.

8. Gradation in state, corresponding to sp. gr.; F being a light gas, Cl a heavy gas, Br a liquid, I a solid.

9. Corresponding gradation in their usual chemical activity; F being most active, then Cl, Br, and I.

10. Corresponding gradation in the strength of the H acids; the strongest being HF, the next, HCl, etc.

11. Corresponding gradation in the explosibility of their N compounds; the strongest NCl3, the next, NBr3, etc.

12. Corresponding gradation in the number of H and O acids; Cl 4, Br 3, I 2.

170. Compounds.--The following are some of the oxides, acids, and salts of the halogens. Name them.

CI2O (+H2O=) 2 HClO. The salts are hypochlorites, as Ca(ClO)2.

Cl2O3 (+H20=) 2 HClO2. The salts are chlorites, as KClO2.

Cl2O4 -- HClO3 The salts are chlorates, as KClO3.

-- HClO4 The salts are perchlorates, as KClO4, -- HBrOThe salts are ? KBrO, -- -- The salts are wanting.

-- HBrO3.The salts are ? KBrO3, -- HBrO4.The salts are ? KBrO4, -- -- The salts are wanting.

-- -- The salts are wanting.

I2O5 (+H2O=) 2 HIO3. The salts are ? KIO3.

-- HIO4. The salts are ? KIO4.

F forms no oxides, and no acids except HF. HF, HCl, HBr, HI, are striking ill.u.s.trations of acids with no O. HClO4 is a very strong oxidizing agent. A drop of it will set paper on fire, or with powdered charcoal explode violently. This is owing to the ease with which it gives up 0. Notice why its molecule is broken up more readily than HC103. The higher the molecular tower, or the more atoms it contains, the greater its liability to fall. Some organic compounds contain hundreds of atoms, and hence are easily broken down, or, as we say, are unstable. Inorganic compounds are, as a rule, much more stable than organic ones. It is not always true, however, that the compound with the least number of atoms is the most stable. SO2 is more stable than SO3, but H2SO3 is less so than H2SO4.

Chapter x.x.xIV.

VAPOR DENSITY AND MOLECULAR WEIGHT.

Examine a liter measure, in the form of a cube,--cubic decimeter, --and a cubic centimeter.

171. Gaseous Weights and Volumes.--A liter of H, at 0 degrees and 760 mm., weighs nearly 0.09 g. This weight is called a crith.

Find the weight of H in the following, in criths and in grams: 15 1., 0.07 1., 50.3 1., 0.035 1., 0.6 1..

It has been estimated that there are (10) 24. molecules of H in a liter. Does the number vary for different gases? The weight of a molecule of H in parts of a crith is 1/(10) 24.; in parts of a gram .09/(10) 24.. If the H molecule is composed of 2 atoms, what is the weight of its atom in fractions of a crith? What in fractions of a gram? The weight of the H atom is a microcrith.

What part of a crith is a microcrith?

172. Vapor Density.--Vapor density, or specific gravity referred to H as the standard, (Physics) is the ratio of the weight of a given volume of a gas or vapor to the weight of the same volume of H. A liter of steam weighs nine times as much as a liter of H.

Its vapor density is therefore nine. For convenience, a definite volume of H is usually taken as the standard, viz., the H atom.

The volume of the H atom and that of the half-molecule of H2O, or of any gas are identical, each being represented by one square.

If, then, the standard of vapor density is the H atom, half the molecular weight of a gas must be its vapor density, since it is evident that we thus compare the weights of equal volumes. The vapor density of H2O, steam, is found from the symbol as follows: (2 + 16) / 2 = 9. To obtain the vapor density of any compound from the formula, we have only to divide its molecular weight by two. Find the vapor density of HCl, N2O, NO, C12H22O11, Cl, CO2, HF, SO2. Explain each case.

The half-molecule, instead of the whole, is taken; because our standard is the hydrogen atom, the smallest portion of matter, by weight, known to science.

How many criths in a liter of HCl? How many grams? Compute the number of criths and of grams in one liter of the compounds whose symbols appear above.

PROBLEMS.

(1) A certain volume of H weighs 0.36 g. at standard temperature and pressure. How many liters does it contain? If one liter weighs 0.09 g., to weigh 0.36 g. it will take 0.36 / 0.09 = 4 liters.

(2) How many liters, or criths, of H in 63 g.? 2.7 g.? 1 g.? 5 g.? 250 g.? Explain each.

(3) Suppose the gas to be twice as heavy as H, how many liters in 0.36 g.? A liter of the gas will weigh 0.18 g. (0.09 X 2). In 0.36 g. there will be 0.36 / 0.18 = 2. Answer the question for 63 g., 2.7 g., etc.

(4) How many liters of Cl in each of the above numbers of grams?

(5) How many of HCl? H2O (steam)? CO2? Explain fully every case.