CHAPTER XXX.

THE ALKALI METALS.

408. General Properties. The metals, like the non-metals, are generally studied in groups or natural families based upon similarity of properties. The Alkali group consists of the five metals named below and the radical ammonium, NH4, the compounds of which resemble those of sodium and potassium (cf. § 210). These metals are called " alkali " metals because the two most important members of the group are contained in the alkalies, i. e., in sodium and potassium hydroxides.

All of these metals have a silvery white luster and are easily cut. In air they become coated with a layer

of the oxide and the hydroxide; if carbon dioxide is present, these pass into the corresponding carbonates. The alkali metals burn when heated in air, and decompose water at ordinary temperatures; therefore none of them is found free in nature. The salts of these metals are practically all soluble in water.

The properties of the alkali metals change in the order of the atomic weights, e. g., the higher the atomic weight the lower the melting-point. The chemical activity and the electro-positive character increase from lithium to cœsium. Cæsium is the most electro-positive element known.

409. Lithium.

Lithium is widely distributed in nature, but no mineral known contains a large proportion of it. It is found in minute quantities in most mineral waters, in many plants, and in the blood. Lithium is the lightest of the metals. Its salts color the Bunsen flame crimson.

410. Sodium. Sodium occurs widely distributed and in large quantities, especially as sodium chloride, NaCl. This exists as rock-salt and sea-salt, and in many mineral springs. Sodium silicate is found in many rocks; the nitrate is Chili saltpeter. Large deposits of the sulphate and carbonate exist. The ashes of plants growing in or near the sea contain sodium carbonate, and were formerly the source of many sodium compounds.

B

Sodium is prepared by the electrolysis of melted sodium hydroxide (Fig. 81). This is Castner's process. The resistance of the hydroxide generates the heat that keeps it melted, after the operation has begun. The vessel containing the hydroxide is of iron, and the anode (BB) is either a cylindrical iron or nickel vessel, or a number of pieces of gas carbon. The cathode (A), extending up into the hydroxide, is of iron or carbon. When the current passes, globules of sodium collect upon the cathode, and rise to the top at Na. They accumulate in the collecting cylinder (Na, H) which floats on the melted hydroxide, just over the cathode. The lower part of the collecting cylinder consists of wire gauze (shown by dots), which prevents the globules of sodium from going to either side. The anion (OH) gives water and oxygen at the anode:

FIG. 81.

4 OH+4(+)→→→→→→ 2 H2O+O2 ↑ .

Electrolysis of some of the water gives hydrogen at the cathode and more oxygen at the anode. The hydrogen accumulates with the sodium in the collecting cylinder (Na, H), and protects the sodium from the air (cf. § 54). The melted sodium is removed by means of ladles, while the hydrogen escapes under the cover (L). The oxygen (O) is conducted away with great care, to prevent its coming in contact with the other products.

411. Properties of Sodium. - Sodium is a white metal, so soft at ordinary temperatures that it can

PROPERTIES OF SODIUM.

377

be molded between the fingers, or pressed into wire. At -20° C. it is quite hard. It has the characteristic properties of the metals, including conductance for heat and electricity (cf. §§ 74, 401, and 408). The molecule is monatomic (cf. § 142); so is that of potassium.

Sodium burns in air or oxygen with a bright, yellow flame. The same characteristic color is imparted to a non-luminous flame by any sodium compound, and serves as a test for the element.

Sodium burns energetically in chlorine (cf. § 118) and in sulphur vapor. It acts upon alcohol much as upon water, forming sodium ethylate and hydrogen (cf. § 220).

2 C2H5OH+2 Na →2 C2H5ONа+H2↑.

It does not react with ether, nor with the liquid hydrocarbons, and is usually kept under ligroin or kerosene (cf. § 525). When exposed to ordinary air it is soon converted into the carbonate.

An alloy of sodium and mercury, called sodium amalgam (cf. § 403), is an important reducing agent; it is diluted sodium.

When sodium is burned in air or oxygen, the product is a mixture of the monoxide (Na2O) and the peroxide (Na2O2). The peroxide is the sodium salt of hydrogen peroxide (cf. § 339). It is a powerful oxidizing and bleaching agent.

(1) Na2O2+2 HOH 2 NaOH+H2O2.

(2) 2 H2O2· →→→→→ 2 H2O+O2 (nascent; cf. § 340).

The monoxide is made by beating sodium nitrate or hydroxide with sodium, in the absence of air.

412. Sodium Hydroxide. Sodium hydroxide, or caustic soda, is formed when sodium or its oxides react with water. The commercial methods of preparing it are:

(1) Boiling a solution of sodium carbonate with slaked lime (calcium hydroxide).

Na2CO3+Ca(OH)2 2 NaOH+CaCO3↓.

The

The calcium carbonate is precipitated. sodium hydroxide solution is drawn off and evaporated.

(2) Electrolysis of aqueous sodium chloride (Castner's process).