Experiment shows this is actually the case:

Sodium+water-sodium hydroxide+hydrogen.

It is plain that in both cases the hydroxide contains half the hydrogen of the water used, the other half being set free. We may thus define a hydroxide as water with half of its hydrogen replaced by a metal.

By proper methods sodium may be made to react with sodium hydroxide; 23 grams of sodium again liberate 1 gram of hydrogen; this time from the sodium hydroxide,

Sodium hydroxide+sodium →→→→→ sodium oxide+hydrogen.

40 g.

23 g.

62 g.

1 g.

That the reaction is a replacement of hydrogen by sodium is more evident if we give details:

+sodium-sodium, 23 g.+hydrogen.

23 g.

sodium, 23 g.

76. Equivalent Weights. In §§ 64 and 65 we learned that the Law of Definite Proportions applies to combination of elements; § 75 shows that it applies

EQUIVALENT WEIGHTS.

65

also to the replacement of one element by another. Chemists use two sets of numbers to represent equivalent amounts of the elements, that is, to represent the proportions by weight in which the elements combine with, or replace, one another. One of these sets is called the equivalent weights; the other, the atomic weights. The atomic weights will be studied later (§ 152). They depend upon the equivalent weights, but are not identical with them.

The equivalent weight of an element is the weight of it, in grams, that combines with, or replaces, 8 grams of oxygen or 1 gram (more exactly, 1.008 g.) of hydrogen. The weights taken as standards are equivalent to each other, for they represent the proportions in which the two elements unite to form water.

The following table gives some of these weight relations in an easily scanned form:

The equivalent weights are determined solely by accurate quantitative experiments. They depend upon no theory. To

get the equivalent weight of zinc, we treat a known weight of zinc with some acid, collect and get the weight of the hydrogen, and calculate the weight of zinc needed to produce 1.008 g. of hydrogen. Or we may convert the zinc into zinc oxide, and calculate how much zinc would have united with 8 g. of oxygen.

77. Exercises and Problems.

1. If 10 c.c. of hydrogen are mixed with 10 c.c. of oxygen, and the mixture is exploded in a eudiometer, which gas is left? How much of it? How much steam is formed?

2. When hydrogen was passed over some heated cupric oxide 1.89 g. water were formed, and the cupric oxide lost 1.678 g. Calculate the weight of oxygen combined with 1 gram of hydrogen.

3. In an experiment, 13.98 g. iron were dissolved in dilute sulphuric acid, and the hydrogen formed measured 5.6 liters under standard conditions. What is the equivalent weight of iron?

4. The equivalent weight of potassium is 39. How many grams of hydrogen are formed when 5 grams of it act upon water? How many grams of water are used up? How much potassium hydroxide is formed?

5. When 1.336 g. calcium reacted with water, 808 c.c. hydrogen were collected over water at 15° C. and 753 mm. Calculate the equivalent weight of calcium.

6. How would you determine the per cent of water in a potato?

7. What evidence is there that the hydrogen of water is more divisible than the oxygen?

8. For what units of measurement is water taken as a standard?

CHAPTER VI.

SOLUTION AND CRYSTALLIZATION.

78. Nature of Solution. By solution, or dissolving, we generally mean the uniform mixing of a solid or gas with some liquid, called the solvent. The resulting mixture is called a solution. The dissolved substance is called the solute. Water, alcohol, and ether are common solvents. The solute imparts some of its properties, such as taste and color, to the solution, but its particles cannot be seen, and they will not settle when the solution stands. All true solutions are clear, whether colored or not. Particles that settle out on standing are said to be suspended in the liquid. While suspended they make the liquid turbid. Suspended materials can usually be filtered out; dissolved substances cannot.

The volume of a solution is greater than that of the solvent alone, but usually less than that of solvent and solute together. A considerable amount of powdered sugar may be added to a vessel apparently full of water without causing an overflow, while a much smaller quantity of an insoluble substance, such as sand, cannot.

79. Solubility. By the solubility of a solute we

mean the maximum amount of it that can be taken up by a given quantity of solvent, in the presence of an excess of the solute. In the case of solids, solubility is expressed in the number of grams of solute for each 100 grams of solvent, at some definite temperature. Of course, any amount less than this maximum can be dissolved. When no solvent is

named, water is understood.

By the concentration of a solution we mean the weight of solute actually present in a given quantity of solvent. A solution having a small concentration of solute is said to be dilute; one having a large concentration is said to be concentrated; one at maximum concentration, in the presence of an excess of the solute, is saturated.

ditions that affect solubility are called solubility factors. The chief solubility factors for solids and liquids are: