A special case of dissociation (the process of decay of larger particles of a substance - molecules of ions or radicals - into smaller particles) is electrolytic dissociation, in which neutral molecules of a substance called an electrolyte in a solution (as a result of exposure to molecules of a polar solvent) decay into charged particles: cations and anions. This explains the ability of electrolyte solutions to conduct current.
It is customary to divide all electrolytes into two groups: weak and strong. Water belongs to weak electrolytes, the dissociation of water is characterized by a small number of dissociated molecules, since they are quite stable and practically do not decay into ions. Pure (without impurities) water weakly conducts electric current. This is due to the chemical nature of the molecule itself, when positively polarized hydrogen atoms are introduced into the electron shell of a relatively small oxygen atom, which is negatively polarized.
The strength and weakness of electrolytes is characterized by the degree of dissociation (denoted by α, often this value is expressed in% from 0 to 100 or in fractions of a unit from 0 to 1) - the ability to decay into ions, that is, the ratio of the number of decayed particles to the number of particles before decay. Substances such as acids, salts and bases under the action of polar molecules of water decay into ions completely. The dissociation of water is accompanied by the decomposition of 2 molecules into the proton + and the hydroxyl group -. If we represent the electrolyte dissociation equation in the form: M = K ++ A-, then the water dissociation can be expressed by the equation: 2↔ ++ -, and the equation by which the degree of water dissociation is calculated can be represented in two forms (through concentration of protons formed or concentration of formed hydroxyl groups): α = [ +] / [2] or α = [ -] / [2]. Since the value of α is affected not only by the chemical nature of the substance, but also by the concentration of the solution or its temperature, it is customary to talk about the apparent (imaginary) degree of dissociation.
The tendency of weak electrolyte molecules, including water, to decompose into ions is more characterized by a dissociation constant (a special case of the equilibrium constant), which is usually denoted as CD. To calculate this quantity, the law of effective masses is applied, which establishes the ratio between the masses of the obtained and starting materials. The electrolytic dissociation of water is the decomposition of the original water molecules into hydrogen protons and a hydroxyl group, therefore, the dissociation constant is expressed by the equation: Kd = [H +] • [OH -] / [H2O]. This value for water is constant and depends only on temperature, at a temperature equal to 25 ° C, Cd = 1.86 • 10-16.
Knowing the molar mass of water (18 grams / mol), as well as neglecting the concentration of dissociated molecules and taking the mass of 1 dm3 of water per 1000 g, we can calculate the concentration of undissociated molecules in 1 dm3 of water: [2] = 1000 / 18.0153 = 55.51 mol / dm3. Then from the equation of dissociation constant you can find the product of the concentrations of protons and hydroxyl groups: [H +] • [OH -] = 1.86 • 10-16 • 55.51 = 1 • 10-14. When extracting the square root of the obtained value, the concentration of protons (hydrogen ions) is obtained, which determines the acidity of the solution and is equal to the concentration of hydroxyl groups: [H +] = [OH -] = 1 • 10-7.
But in nature, water of such purity does not exist due to the presence of dissolved gases in it or contamination of water by other substances (in fact, water is a solution of various electrolytes), therefore at 25 ° C the concentration of hydrogen protons or the concentration of hydroxyl groups differs from 1 • 10-7. That is, the acidity of water is due to the course of not only such a process as the dissociation of water. The hydrogen index is the negative logarithm of the concentration of hydrogen ions (pH), it was introduced to assess the acidity or alkalinity of water and aqueous solutions, since it is difficult to use numbers with negative degrees. For pure water, pH = 7, but since there is no pure water in nature, and the dissociation of water proceeds along with the decomposition of other dissolved electrolytes, the pH can be less or more than 7, that is, for water, almost pH ≠ 7.