Strong and Weak Acids and Alkalis Explained

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What are Strong and Weak Acids?

Strong acids completely ionise or dissociate in water, whereas weak acids partially ionise in water.

Hydrochloric acid is a commonly used strong acid which completely dissociates in water as shown by the below equation:

HCl _(aq) → H⁺_(aq) + Cl⁻_(aq)

The equation demonstrates that hydrochloric acid breaks down (dissociates/ionises) into its component ions in the presence of water.

An example of a weak acid is carbonic acid and as shown by the equation below, it only partially dissociates in water:

H₂CO₃ _(aq) ⇌ 2H⁺_(aq) + CO₃²⁻_(aq)

The equation shows a partial dissociation of carbonic acid to its ions, this is demonstrated by the reversible reaction symbol that tells us the reaction is continuously going back and forth.

Note the symbols after each compound, these are state symbols which represent the state of matter that the compound is currently in.

What is the difference between pH and strength?

Acidity and alkalinity are measured by the pH scale which is logarithmic, so each step up or down the scale means a difference of 10x. Acidic solutions have a pH of less than 7, neutral solutions have a pH of 7 and alkaline solutions have a pH of more than 7.

The concentration of an acid or alkaline solution is related to the number of hydrogen or hydroxide ions per unit volume, this means that even weak acids such as citric acid can be the same pH as strong acids such as hydrochloric acid.

Strength is a measure of how much acids or alkalis ionise or dissociate in water.

What are Strong and Weak Alkalis?

Strong alkalis, just like strong acids, completely ionise or dissociate in water, whereas weak alkalis partially ionise in water. Check below to see the formula for sodium hydroxide which is a strong alkali.

Ammonia is an example of a weak alkali, as shown by the equation below:

ammonia + water ⇌ ammonium + hydroxide

NH_3(g)+ H₂O_(l)⇌ NH₄⁺_(aq)+ OH^–_(aq)