Chapter 54

ACIDS AND BASES.

66. What Acids Are.

Experiment 39.--Pour a few drops of chlorhydric acid, HCl, into a clean evaporating-dish. Add 5 cc. H2O, and stir. Touch a drop to the tongue, noting the taste. Dip into it the end of a piece of blue litmus paper, and record the result. Thoroughly wash the dish, then pour in a few drops of nitric acid, HNO3, and 5 cc. H2O, and stir. Taste, and test with blue litmus. Test in the same way sulphuric acid, H2SO4. Name two characteristics of an acid. In a vertical line write the formulae of the acids above. What element is common to them all? Is the rest of the formula positive or negative?

67. An Acid is a substance composed of H and a negative element or radical. It has usually a sour taste, and turns blue litmus red. Litmus is a vegetable extract obtained from lichens in Southern Europe. Acids have the same action on many other vegetable pigments. Are the following acid formulae, and why? H2SO3, HBr, HNO2, H3PO3, H4SiO4. Most acids have O as well as H. Complete the symbols for acids in the following list, and name them, from the type given:--

HCl, chlorhydric acid. HN03, nitric acid. ?Br, ? ?Cl? ? ?I, ? ?Br? ? ?F, ? ?I? ? H3PO4, phosphoric acid. H3PO3, phosphorous acid. ?As? ? ?As? ?

Complete these equations:--

H2SO3 - H2O = ? | 2 HN03 - H2O = ? H2SO4 - H2O = ? | 2 HNO2 - H2O = ? H2CO3 - H2O = ? | 2 H3AsO4 - 3 H2O = ?

Are the products in each case metallic or non-metallic oxides? They are called anhydrides. Notice that each is formed by the withdrawal of water from an acid. Reverse the equations; as, SO3 + H2O = ?

68. An Anhydride is what remains after water has been removed from an acid; or, it is the oxide of a non- metallic element, which, united with water, forms an acid. SO2 is sulphurous anhydride, SO2 sulphuric anhydride, the ending ic meaning more O, or negative element, than ous. Name the others above.

Anhydrides were formerly called acids,--anhydrous acids, in distinction from hydrated ones, as CO2 even now is often called carbonic acid.

Experiment 40.--Hold a piece of wet blue litmus paper in the fumes of SO2, and note the acid test. Try the same with dry litmus paper.

Experiment 41.--Burn a little S in a receiver of air containing 10 cc. H2O, and loosely covered, as in the O experiment. Then shake to dissolve the SO2. H2O + SO2 = H2SO3. Apply test paper.

69. Naming Acids.--Compare formulae H2SO3 and H2SO4. Of two acids having the same elements, the name of the one with least O, or negative element, ends in ous, the other in ic. H2SO3 is sulphurous acid, H2SO4, sulphuric acid. Name H3PO4 and H3PO3; H3AsO3 and H3ASO4; HNO2 and HNO3.

If there are more than two acids in a series, the prefixes hypo, less, and per, more, are used. The following is such a series: HClO, HClO2, HClO3, HClO4.

HClO3 is chloric acid; HClO2, chlorous; HClO, hypochlorous; HClO4 perchloric. Hypo means less of the negative element than ous; per means more of the negative element than ic. Name: H3PO4 (ic), H3PO3, H3PO2. Also HBrO (HBrO2 does not exist), HBrO3 (ic), HBrO4.

What are the three most negative elements? Note their occurrence in the three strongest and most common acids. Hereafter note the names and symbols of all the acids you see.

70. What Bases Are.

Experiment 42.--Put a few drops of NH4OH into an evaporating- dish. Add 5 cc. H2O, and stir. Taste a drop. Dip into it a piece of red litmus paper, noting the effect. Cleanse the dish, and treat in the same way a few drops NaOH solution, recording the result. Do the same with KOH. Acid stains on the clothing, with the exception of those made by HNO3, maybe removed by NH4OH. H2SO4, however, rapidly destroys the fiber of the cloth.

Name two characteristics of a base. In the formulae of those bases, what two common elements? Name the radical. Compare those symbols with the symbol for water, HOH. Is (OH) positive or negative? Is the other part of each formula positive or negative? What are two constituents, then, of a base? Bases are called hydrates. Write in a vertical line five positive elements. Note the valence of each, and complete the formula for its base. Affix the names. Can you see any reason why the three bases above given are the strongest?

Taking the valences of Cr and Fe, write symbols for two sets of hydrates, and name them. Try to recognize and name every base hereafter met with.

A Base is a substance which is composed of a metal, or positive radical, and OH. It generally turns red litmus blue, and often has an acrid taste.

An Alkali is a base which is readily soluble in water. The three principal alkalies are NH4OH, KOH, and NaOH.

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 quantity 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 principal 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.