Part 9 (1/2)

85. Preparation. Experiment 52.--To 10 g. KNO3 or NaNO3, in a flask, add 15 cc. H2SO4. Securely fasten the cork of the d.t., as HNO3 is likely to loosen it, and pa.s.s the other end to the bottom of a t.t. held deep in a bottle of water (Fig. 26). Apply heat, and collect 4 or 5 cc.of the liquid. The usual reaction is: KNO3 + H2SO4 = HKSO4 + HNO3. With greater heat, 2 KNO3 + H2SO4 = K2SO4 + 2HNO3. Which is most economical of KNO3? Of H2SO4? Instead of a flask, a t.t. may be used if desired (Fig. 27).

86. Properties and Tests.

Experiment 53.--(1) Note the color of the prepared liquid. (2) Put a drop on the finger; then wash it off at once. (3) Dip a quill or piece of white silk into it; then wash off the acid.

What color is imparted to animal substances? (4) Add a little to a few bits of Cu turnings, or to a Cu coin. Write the equation.

(5) To 2 cc.indigo solution, add 2 cc. HNO3. State the leading properties of HNO3, from these tests.

87. Chemically Pure HNO3 is a Colorless Liquid.-- The yellow color of that prepared in Experiment 52 is due to liquid NO2 dissolved in it. It is then called fuming HNO3, and is very strong. NO2 is formed at a high temperature.

Commercial or ordinary HNO3, is made from NaNO3, this being cheaper than KNO3; it is about half water.

88. Uses. HNO3 is the basis of many nitrates, as AgNO3, used for photography, Ba(NO3)2 and Sr(NO3)2 for fire-works, and others for dyeing and printing calico; it is employed in making aqua regia, sulphuric acid, nitro-glycerine, gun-cotton, aniline colors, zylonite, etc.

Enough experiments have been performed to answer the question whether some acids can be prepared from their salts. H2SO4 is not so made, because no acid is strong enough to act on its salts. In making HCl, HNO3, etc., sulphuric acid was used, being the strongest.

AQUA REGIA.

89. Preparation and Action. Experiment 54.--Into a t.t. put 2 cc.

HNO3, and 14 qcm. of either Au leaf or Pt. Warm in a flame. If the metal is pure, no action takes place. Into another tube put 6 cc. HCl and add a similar leaf. Heat this also. There should be no action. Pour the contents of one t.t. into the other. Note the effect. Which is stronger, one of the acids, or the combination of the two? Note the odor. It is that of Cl. 3HCl + HNO3 = NOCl + 2H2O + Cl2. This reaction is approximate only. The strength is owing to nascent chlorine, which unites with Au. Au + 3Cl = AuCl3. If Pt be used, PtCl4 is produced. No other acid except nitro-hydrochloric will dissolve Au or Pt; hence the ancients called it aqua regia, or king of liquids. It must be made as wanted, since it cannot be kept and retain its strength.

CHAPTER XIX.

SULPHURIC ACID.

90. Preparation.

Experiment 55.--Having fitted a cork with four or five perforations to a large t.t., pa.s.s a d.t. from three of these to three smaller t.t., leaving the others open to the air, as in Figure 28. Into one t.t. put 5 cc. H2O, into another 5 g. Cu turnings and 10 cc. H2SO4, into the third 5 g. Cu turnings and 10 cc. dilute HNO3, half water. Hang on a ring stand, and slowly heat the tubes containing H2O and H2SO4. Notice the fumes that pa.s.s into the large t.t.

Trace out and apply to Figure 28 these reactions:--

(1) Cu + 2 H2SO4 = CuSO4 + 2 H2O + SO2.

(2) 3 Cu + 8 HNO3 = 3 Cu(NO3)2+ 4 H2O + 2 NO.

(3) NO + O = NO2.

(4) SO2 + H2O + NO2 =H2SO4 + NO.

(4) comes from combining the gaseous products in (1), (2), (3).

In (3), NO takes an atom of O from the air, becoming NO2, and at once gives it up, to the H2SO3 (H2O + SO2), making H2SO4, and again goes through the same operation of taking up O and pa.s.sing it along. NO is thus called a carrier of O. It is a reducing agent, while NO2 is an oxidizing agent. This is a continuous process, and very important, since it changes useless H2SO3 into valuable H2SO4. If exposed to the air, H2SO3 would very slowly take up O and become H2SO4.

Instead of the last experiment, this may be employed if preferred: Burn a little S in a receiver. Put into an evaporating-dish, 5 cc. HNO3, and dip a paper or piece of cloth into it. Hang the paper in the receiver of SO2, letting no HNO3 drop from it. Continue this operation till a small quant.i.ty of liquid is found in the bottle. The fumes show that HNO3 has lost O. 2 HNO3 + SO2 = H2SO4 + 2 NO2.

91. Tests for H2SO4.

Experiment 56.--(1) Test the liquid with litmus. (2) Transfer it to a t.t., and add an equal volume of BaCl2 solution. H2SO4 + BaCl2 = ? Is BaSO4 soluble? (3) Put one drop H2SO4 from the reagent bottle in 10 cc. H2O in a clean t.t., and add 1 cc. BaCl2 solution. Look for any cloudiness. This is the characteristic test for H2SO4 and soluble sulphates, and so delicate that one drop in a liter of H2O can be detected. (4) Instead of H2SO4, try a little Na2SO4 solution. (5) Put two or three drops of strong H2SO4 on writing-paper, and evaporate, high over a flame, so as not to burn the paper. Examine it when dry. (6) Put a stick into a t.t. containing 2 cc. H2SO4, and note the effect. (7) Review Experiment 5. (8) Into an e.d. pour 5 cc. H2O, and then 15 cc.

H2SO4. Stir it meantime with a small t.t. containing 2 or 3 cc.

NH4OH, and notice what takes place in the latter; also note the heat of the e.d.

The effects of (5), (6), (7), and (8) are due to the intense affinity which H2SO4 has for H2O. So thirsty is it that it even abstracts H and O from oxalic acid in the right proportion to form H2O, combines them, and then absorbs the water.

92. Affinity for Water.--This acid is a desiccator or dryer, and is used to take moisture from the air and prevent metallic substances from rusting. In this way it dilutes itself, and may increase its weight threefold. In diluting, the acid must always be poured into the water slowly and with stirring, not water into the acid, since, as H2O is lighter than H2SO4, heat enough may be set free at the surface of contact to cause an explosion.