Strength of Acids and Alkalis

Acids are substances that donate protons or hydrogen ions (H+), while alkalis (bases that are soluble in water) accept protons or donate hydroxide ions (OH-).
The strength of an acid or alkali refers to the degree of ionisation in water. A strong acid or alkali is fully ionised, while a weak one is only partially ionised.
The pH scale is used to measure the strength of acids and alkalis. It runs from 0 to 14 with 7 being neutral.
A pH below 7 indicates an acidic substance, while a pH above 7 indicates an alkaline substance. The further away from 7, the stronger the acid or alkali is.
Dilution doesn’t change the pH by the same amount as it does not change the strength of an acid or alkali but reduces concentration.
Neutralisation reactions occur when an acid reacts with an alkali, producing a salt and water. The pH moves towards 7 in these reactions.

Acid-Base Indicators

Indicators such as litmus paper, phenolphthalein, and universal indicator can be used to estimate the pH of a substance.
Litmus paper turns red in acidic conditions and blue in alkaline conditions.
Universal indicator shows a range of colours and can give a more precise pH.

In water, a small number of water molecules break apart into ions in a process called self-ionisation or autoprotolysis: 2H2O(l) ⇌ H3O+(aq) + OH-(aq)
An ionic product, Kw, exists for water and at 25°C its value is 1.0 x 10^-14 mol2 dm^-6.
The pH of a solution can be calculated using the expression: pH = -log[H+] and pOH = -log[OH-], where H+ is the concentration of hydrogen ions and OH- is the concentration of hydroxide ions.
The relationship between pH, pOH and pKw is as follows: pH + pOH = pKw. This relationship is helpful in calculating the pH or pOH of a solution when given one of these values.