Solutions

SOLUTIONS. When sugar is put in contact with water it dissolves and the liquid so obtained has the following properties which are characteristic of solutions:— (I ) It is homogeneous, i.e., it is not possible to distinguish by any means, as for example by the most powerful microscope, parts which are sugar and parts which are water; the smallest amount which can be distinguished contains both sugar and water. In this a solution differs from a mixture which is heterogeneous, i.e., in which it is possible to distinguish parts which are different from others. It may be supposed that in solutions the ultimate particles or molecules of which substances are made are intimately mingled together. If the single molecules of bodies could be made visible, the distinction between solutions and mixtures would fail. But except in some very special circumstances the smallest quantity of matter which can be distinguished contains some thousands of molecules, so that the distinction is of practical value.

(2) The composition of a solution can be varied : either water or sugar can be added to a sugat solution (within certain limits) and the solution still remains homogeneous. In this a solution differs from a chemical compound, which is homogeneous but has a definite composition which cannot be varied.

Types of Solutions.

The property of forming solutions is a very general one. All gases mix with each other in all propor tions forming homogeneous gaseous "mixtures," which are tech nically solutions. Liquids may dissolve not only solids, but also other liquids and gases. It is also possible to obtain "solid solu tions." Thus if two solid substances are melted together and then allowed to solidify, we may get a homogeneous solid substance of variable composition, containing the two substances, i.e., a solid solution (see CHEMISTRY : Physical).

There is a limit to the amount of a solid substance which can dissolve in a given quantity of a liquid at any particular tem perature. When this limiting amount is reached the solution is said to be saturated. The concentration of the dissolved substance in the saturated solution, expressed in a proper way, is known as its solubility. In some cases two liquids are completely miscible, i.e., they form homogeneous solutions when mixed in all pro portions. In others they are only partially miscible. Thus, when ether is added to water it dissolves at first, but ultimately a sat urated solution is obtained and if more ether is added it forms a separate layer in contact with the water solution. This ether

dissolves water from the aqueous solution until it is saturated with water. Thus two liquid layers are obtained, a saturated solution of ether in water and a saturated solution of water in ether.

In solutions of gases in liquids it is necessary to take into ac count another factor, the pressure of the gas. It was found by William Henry in 1803 that the solubility of a gas at a given temperature is proportional to its pressure (Henry's law). A familiar application of this is the soda-water syphon, which con tains an aqueous solution of carbon dioxide, saturated at a pres sure somewhat greater than the pressure of the atmosphere. Since this solution contains more carbon dioxide than corresponds to saturation at atmospheric pressure, gas is liberated when the excess pressure is released, causing effervescence.

Expression of Composition.

The substances of which a solution is composed are termed its components. Frequently the substance which dissolves is distinguished as the solute, and the liquid or medium into which it dissolves is called the solvent. But this is a matter of convenience, for there is no fundamental dis tinction between the two, and in many cases it is not clear which component is to be regarded as solute, and which as solvent. The composition of a solution is best expressed in terms of the rela tive numbers of the molecules of the components. Thus if we have a binary solution containing molecules of a substance A, and molecules of a substance B, the fraction 14/ (74-Ent,)=N., i.e., the ratio of the number of molecules of A to the total number of molecules in the solution, is known as the molar fraction of A. Similarly the ratio is the molar fraction, of B. The concentration of any component may be expressed as the amount of it in a given volume (volume concentration) or in a given weight (weight concentration) of the solution. If the amount of a substance be expressed in terms of the number of molecules, the corresponding molecular concentration is obtained.

A convenient unit for stating molecular concentration is the gram-molecule. This is the amount of a substance which contains as many molecules as there are atoms in one gram of hydrogen. The number of gram-molecules of a substance in a litre of solu tion is often termed the molar concentration; whilst the same quantity in i,000 grams of solvent is distinguished as the molal concentration.