Neutral, acidic and basic means. Neutral, acidic and basic systems

Neutral medium:

A medium is considered neutral if it has the same concentration, in mol/L, of the hydronium ions (H₃O⁺) and hydroxide (OH^-).

An example of a neutral medium that even serves as a standard for other solutions is pure, distilled water at a temperature of 25ºC. At this temperature, it has exactly 1. 10^-7 mol/L of both ions. Hence, your ionic product (K_w) is equal to 10^-14 (mol/L)²:

K_w = [H₃O⁺]. [oh^-]
K_w = (1. 10^-7 mol/L). (1. 10^-7 mol/L) = 10^-14 (mol/L)²

The pH and pOH are given below:

pH= - log [H₃O⁺] pOH= -log[OH^-]
pH= - log 1. 10^-7 pOH= - log 1. 10^-7
pH = 7pOH = 7

This shows us that, in a neutral solution, the pH is equal to pOH.

• Acid medium:

In an acidic medium, the concentration of H ions₃O⁺ is larger than that of OH ions^-.

Such a solution can be achieved by adding a small part of the H ions₃O⁺, for example, by means of an acid.

According to Le Chatelier's principle, when a disturbance is caused to a system in equilibrium, it tends to readjust itself in order to reduce the effects of this force. This means that if an acid is added to water, the H ions

₃O⁺ they will be in excess and the balance will shift in the reverse reaction direction, to the left. So these excess ions will react with the OH ions^-. Thus, the concentration of OH ions^- will decrease and the solution will become acidic.

The ionic product (Kw) is always equal to 10^-14, but the concentration of H ions₃O⁺ is greater than the concentration of OH ions^-1. Therefore, the pH is greater than the pOH in an acidic medium, but its sum always equals 14.

Also, the greater the concentration of H ions₃O⁺, the greater the pH value. At room temperature (25°C), pH < 7 and pOH > 7.

• Basic means:

In the basic medium, the concentration of OH ions^- is larger than that of H ions₃O⁺.

If we add a base to water, it means we are adding OH ions.^- and, as explained in the previous item, by Le Chatelier's principle, the equilibrium of the reaction of self-ionization of water will move in the opposite direction, with excess ions reacting with H ions₃O⁺, decreasing your concentration and making the solution basic.

In that case, the pOH will be greater than the pH. At room temperature (25°C), pH > 7 and pOH < 7.

Briefly:

By Jennifer Fogaça
Graduated in Chemistry