An Introduction to Chemical Science Part 8

Alkali Metals are those which form alkalies. Name three.

An Alkaline Reaction is the turning of red litmus blue.

An Acid Reaction is the turning of blue litmus red.

Experiment 43.--Pour 5 cc. of a solution of litmus in water, into a clean t.t. or small beaker. Pour 2 or 3 cc. of HCl into an evaporating-dish, and the same quant.i.ty of NH4OH into another dish. Take a drop of the HCl on a stirring-rod and stir the litmus solution with it. Note the acid reaction. Clean the rod, and with it take a drop (or more if necessary) of NH4OH, and add this to the red litmus solution, noting the alkaline reaction.

Experiment in the same way with the two other princ.i.p.al acids and the two other alkalies.

Litmus paper is commonly used to test these reactions, and hereafter whenever the term LITMUS is employed in that sense, the test-paper should be understood. This paper can be prepared by dipping unglazed paper into a strong aqueous solution of litmus.

CHAPTER XVI.

SALTS.

71. Acids and Bases are usually Opposite in Character.--When two forces act in opposition they tend to neutralize each other. We may see an a.n.a.logy to this in the union of the two opposite cla.s.ses of compounds, acids and bases, to form salts.

72. Neutralization.

Experiment 44.--Put into an evaporating-dish 5 cc. of NaOH solution. Add HCl to this from a t.t., a few drops at a time, stirring the mixture with a gla.s.s rod (Fig. 20), and testing it with litmus paper, until the liquid is neutral, i.e. will not turn the test paper from blue to red, or red to blue. Test with both colors. If it turns blue to red, too much acid has been added; if red to blue, too much base. When it is very nearly neutral, add the reagent, HCl or NaOH, a drop at a time with the stirring-rod. It must be absolutely neutral to both colors.

Evaporate the water by heating the dish over asbestus paper, wire gauze, or sand, in an iron plate (Fig. 21) till the residue becomes dry and white. Cool the residue, taste, and name it. The equation is: HCl + NaOH = NaCl + HOH or H2O. Note which elements, positive or negative, change places. Why was the liquid boiled?

The residue is a type of a large cla.s.s of compounds, called salts.

(Fig. 20) (Fig. 21)

Experiment 45. -- Experiment in the same way with KOH solution and H2SO4, applying the same tests. H2SO4 + 2 KOH = K2SO4 + 2 HOH. What is the solid product?

Experiment 46.--Neutralize NH4OH with HNO3, evaporate, apply the tests, and write the equation. Write equations for the combination of NaOH and H2SO4; NaOH and HNO3; KOH and HCl; KOH and HNO3; NH4OH and HCl; NH4OH and H2SO4. Describe the experiment represented by each equation, and be sure you can perform it if asked to do so. What is the usual action of a salt on litmus? How is a salt made? What else is formed at the same time? Have all salts a saline taste? Does every salt contain a positive element or radical? A negative?

73. A Salt is the product of the union of a positive and a negative element or radical; it may be made by mixing a base and an acid.

The salt KI represents what acid? What base, or hydrate? Write the equation for making KI from its acid and base. Describe the experiment in full. Cla.s.sify, as to acids, bases, or salts: KBr, Fe(OH)2, HI, NaBr, HNO2, Al2(OH)6, KClO3, HClO3, H2S, K2S, H2S03, K2SO4, Ca(OH)2, CaCO3, NaBr03, CaSO4, H2CO3, K2CO3, Cu(OH)2, Cu(NO3)2, PbSO4, H3P04, Na2P04. In the SALTS above, draw a light vertical line, separating the positive from the negative part of the symbol. Now state what acid each represents. What base. Write the reaction in the preparation of each salt above from its acid and base; then state the experiment for producing it.

74. Naming Salts.--(NO3) is the nitrate radical; KNO3 is pota.s.sium nitrate. From what acid? (NO2) is the nitrite radical; KN02 is pota.s.sium nitrite. From what acid? Note that the endings of the acids are OUS and IC; also that the names of their salts end in ITE and ATE. From which acid--IC or OUS--is the salt ending in ATE derived? That ending in ITE?

Name these salts, the acids from which they are derived, and the endings of both acids and salts: NaNO3, NaNO2, K2SO4, K2SO3, CaSO4, CaSO3, KClO3, KClO2, KClO, KClO4 (use prefixes HYPO and PER, as with acids), Ca3(PO4)2, Ca3(P03)2, CuSO4, CuSO3, AgNO3, Cu(NO3)2. FeS, FeS2, are respectively FERROUS SULPHIDE and FERRIC SULPHIDE. Name: HgCl, HgCl2, FeCl2, Fe2Cl6, FeSO4, Fe2(SO4)3.75.

Acid Salts.--Write symbols for nitric, sulphuric, phosphoric acids. How many H atoms in each? Replace all the H in the symbol of each with Na, and name the products. Again, in sulphuric acid replace one atom of H with Na; then in phosphoric replace first one, then two, and finally three H atoms with Na. HNaSO4 is hydrogen sodium sulphate; HNa2P04 is hydrogen di-sodium phosphate. Name the other salts symbolized. Name HNaNH4P04.

Though these products are all salts, some contain replaceable H, and are called acid salts. Those which have all the H replaced by a metal are normal salts. Name and cla.s.sify, as to normal or acid salts: Na2CO3, HNaCO3, K2SO4, HKSO4, (NH4)2SO4, HNH4SO4, Na3P04, HNa2P04, H2NaP04.

The BASICITY of an acid is determined by the number of replaceable H atoms in its molecule. It is called MOn.o.bASIC if it has one; DIBASIC if two; TRI- if three, etc. Note the basicity of each acid named above. How many possible salts of H2SO4 with Na?

Of H3P04 with Na? Which are normal and which acid? What is the basicity of H4Si04?

Some normal, as well as acid, salts change litmus. Na2CO3, representing a strong base and a weak acid, turns it blue. There are other modes of obtaining salts, but this is the only one which we sball consider.

76. Salts Occur Abundantly in Nature, such as NaCl, MgSO4, CaCO3.

Acids and bases are found in small quant.i.ties only. Why is this?

Why are there not springs of H2SO4 and NH4OH? We have seen that acids and bases are extremely active, have opposite characters, and combine to form relatively inactive salts. If they existed in the free state, they would soon combine by reason of their strong affinities. This is what in all ages of the world has taken place, and this is why salts are common, acids and bases rare.

Active agents rarely exist in the free state in large quant.i.ties.

Oxygen seems to be an exception, but this is because there is a superabundance of it. While vast quant.i.ties are locked up in compounds in rocks, water, and salts of the earth, much remains with which there is nothing to combine.

CHAPTER XVII.

CHLORHYDRIC ACID.

77. We have seen that salts are made by the union of acids and bases. Can these last be obtained from salts?

78. Preparation of HCl.

Experiment 47.--Into a flask put 10 g. coa.r.s.e NaCl, and add 20 cc. H2SO4. Connect with Woulff bottles [Woulff bottles may be made by fitting to wide-mouthed bottles corks with three holes, through which pa.s.s two delivery tubes, and a central safety tube dipping into the liquid, as in Figures 22 and 23.] partly filled with water, as in Figure 22. One bottle is enough to collect the HCl; but in that case it is less pure, since some H2SO4 and other impurities are carried over. Several may be connected, as in Figure 23. The water in the first bottle must be nearly saturated before much gas will pa.s.s into the second. Heat the mixture 15 or 20 minutes, not very strongly, to prevent too much foaming.

Notice any current in the first bottle. NaCl + H2SO4 = HNaSO4 + HCl. Intense heat would have given: 2NaCl + H2SO4 = Na2SO4 + 2HCl. Compare these equations with those for HNO3. In which equation above is H2SO4 used most economically? Both reactions take place when HCl is made on the large scale.

(Fig. 22)

79. Tests. Experiment 48.--(1) Test with litmus the liquid in each Woulffbottle. (2) Put a piece of Zn into a t.t. and cover it with liquid from the first bottle. Write the reaction, and test the gas. (3) To 2 cc.solution AgNO3 in a t.t. add 2 cc.of the acid. Describe, and write the reaction. Is AgCl soluble in water?

(4) Into a t.t. pour 5 cc.Pb(NO3)2 solution, and add the same amount of prepared acid. Give the description and the reaction.

(5) In the same way test the acid with Hg2(NO3)2 solution, giving the reaction. (6) Drake a little HCl in a t.t., and bring the gas escaping from the d.t. in contact with a burning stick. Does it support the combustion of C? (7) Hold a piece of dry litmus paper against it. [figure 23] (8) Hold it over 2 cc.of NH4OH in an evaporating-dish. Describe, name the product, and write the reaction. (3), (4), (5), (8), are characteristic tests for this acid.

80. Chlorhydric, Hydrochloric or Muriatic, Acid is a Gas.--As used, it is dissolved, in water, for which it has great affinity.

Water will hold, according to temperature, from 400 to 500 times its volume of HCl. Hundreds of thousands of tons of the acid are annually made, mostly in Europe, as a bye-product in Na2CO3 manufacture. The gas is pa.s.sed into towers through which a spray of water falls; this absorbs it. The yellow color in most commercial HCl indicates impurities, some of which are Fe, S, As, and organic matter. As, S, etc., come from the pyrites used in making H2SO4. Chemically pure (C.P.) acid is freed from these, and is without color. The gas may be dried by pa.s.sing it through a gla.s.s tube holding CaCl2 (Fig. 16) and collecting it over mercury.

The muriatic acid of commerce consists of about two- thirds water by weight. HCl can also be made by direct union of its const.i.tuents.81. Uses.--HCl is used to make Cl, and also bleaching- powder. Its use as a reagent in the laboratory is ill.u.s.trated by the following experiment:-- Experiment 49.--Put into a t.t. 2 cc. AgNO3 solution, add 5 cc. H2O, then add slowly HCl so long as a ppt. (precipitate) is formed. This ppt. is AgCl.

Now in another t.t. put 2 cc. Cu(NO3)2, solution, add 5 cc. H2O, then a little HCl. No ppt. is formed. Now if a solution of AgNO3 and a solution of Cu(NO3)2 were mixed, and HCl added, it is evident that the silver would be precipitated as chloride of silver, while the copper would remain in solution. If now this be filtered, the silver will remain on the filter paper, while in the filtrate will be the copper. Thus we shall have performed an a.n.a.lysis, or separated one metal from another. Perform it. Note, however, that any soluble chloride, as NaCl, would produce the same result as HCl.

BROMHYDRIC AND IODIHYDRIC ACIDS.

82. NaCl, being the most abundant compound of Cl, is the source of commercial HCl. KCl treated in the same way would give a like product. Theoretically HBr and HI might be made in the same way from NaBr and NaI, but the affinity of H for Br and I is weak, and the acids separate into their elements, when thus prepared.

83. To make HI.

Experiment 50.--Drop into a t.t. three or four crystals of I, and add 10 cc. H2O. Hold in the water the end of a d.t. from which H2S gas is escaping. Observe any deposit, and write the reaction.

FLUORHYDRIC ACID.

84. Preparation and Action.

Experiment 51.--Put 3 or 4 g. powdered CaF2, i.e. fluor spar or fluorite, into a shallow lead tray, e.g. 4x5 cm, and pour over it 4 or 5 cc. H2SO4. A piece of gla.s.s large enough to cover this should previously be warmed and covered on one side with a very thin coat of beeswax. To distribute itevenly, warm the other side of the gla.s.s over a flame. When cool, scratch a design (Fig. 24) through the wax with a sharp metallic point. Lay the gla.s.s, film side down, over the lead tray. Warm this five minutes or more by placing it high over a small flame (Fig. 25) to avoid melting the wax. Do not inhale the fumes. Take away the lamp, and leave the tray and gla.s.s where it is not cold, for half an hour or more.

Then remove the wax and clean the gla.s.s with naphtha or benzine.

Look for the etching.

Two things should have occurred: (1) the generation of HF. Write the equation for it. (2) Its etching action on gla.s.s. In this last process HF acts on SiO2 of the gla.s.s, forming H2O and SiF4.

Why cannot HF be kept in gla.s.s bottles?

A dilute solution of HF, which is a gas, may be kept in gutta percha bottles, the anhydrous acid in platinum only; but for the most part, it is used as soon as made, its chief use being to etch designs on gla.s.s-ware. Gla.s.s is also often etched by a blast of sand (SiO2).

Notice the absence of O in the acids HF, HCI, HBr, HI, and that each is a gas. HF is the only acid that will dissolve or act appreciably on gla.s.s.

Chapter XVIII.

NITRIC ACID.