Difference Between Strength & Concentration

Strength and concentration are two words that are used in the English language to describe the potency of a solution. The words are used interchangeably in regular speech and can be considered synonymous. In chemistry, however, strength and concentration are two different things and are used to specify certain characteristics of acids.

The strength of an acid is a measure of the degree of ionization in an aqueous solution. The greater the number of ions dissociated, or the number of cations and anions released in solution, the stronger the acid. For example, hydrochloric acid (HCL) dissociates completely into H+ and Cl- ions in solution, so it is very strong. Acetic acid (CH3COOH), which is found in household white vinegar, releases few ions in solution, so it is considered to be a weak acid.

The concentration of an acid is a measure of the amount of available acid ions dissolved in a solvent. Concentration can be measured in moles, parts per million or percentage. The concentration is a ratio of the solute to solvent content of a solution. Acidic solutions with low numbers of available ions in solution are called dilute solutions whereas those with high numbers of available ions are called concentrated solutions.

Strong acids are those that are fully dissociated in solution. If the percentage dissociation of ions is less than one hundred, the acid is considered weak. A one-directional arrow in a chemical equation represents dissolution of a strong acid in water. Weak acids only partially dissociate in solution. Two arrows pointing in opposite directions in a chemical equation represent weak acids.

The concentration of an acid is a measure of the amount of available acid ions dissolved in a solvent. Concentration can be measured in moles, parts per million or percentage. The concentration is a ratio of the solute to solvent content of a solution. Acidic solutions with low numbers of available ions in solution are called dilute solutions whereas those with high numbers of available ions are called concentrated solutions.

You might also come across the colloquial use of "strong" to describe a concentrated solution containing any solute — but this informal use is not precise.