Concentration and Strength Are Different

What students typically write

“Concentrated hydrochloric acid is a stronger acid than dilute hydrochloric acid.” Or: “Weak acids are not very concentrated.”

Students treat “strong” and “concentrated” as synonyms, and “weak” and “dilute” as synonyms. In everyday English this works. In chemistry it does not.

Why it is wrong

Concentration and strength measure completely different things:

Concentration is the amount of solute per unit volume of solution. A concentrated acid has many moles of acid dissolved per dm3. A dilute acid has few. Concentration can be changed by adding or removing water.

Strength describes the degree of ionisation. A strong acid (HCl, H2SO4, HNO3) fully ionises in water: every molecule releases its H+ ion. A weak acid (ethanoic acid CH3COOH, citric acid, carbonic acid) only partially ionises: most molecules remain intact, and only a small fraction produce H+ ions.

These two properties are independent. You can have:

• Concentrated strong acid (many moles of fully ionised acid per dm3)
• Dilute strong acid (few moles of fully ionised acid per dm3)
• Concentrated weak acid (many moles of partially ionised acid per dm3)
• Dilute weak acid (few moles of partially ionised acid per dm3)

What the mark scheme actually wants

A typical question: “Explain the difference between a strong acid and a concentrated acid.” [2]

Marking points:

1. A strong acid is completely/fully ionised in water (1 mark)
2. A concentrated acid has a large amount of solute / many moles of acid per unit volume of solution (1 mark)

An answer that says “a strong acid is one with a high concentration” scores 0/2 because it conflates the two concepts.

Worked example: wrong vs right

Question: Ethanoic acid (CH3COOH) is a weak acid. A student has a 2 mol/dm3 solution of ethanoic acid. Explain why this is a concentrated weak acid. [2]

Wrong answer: “It is weak because it is not very concentrated. 2 mol/dm3 is not that much.” This scores 0/2. It confuses weakness with dilution and 2 mol/dm3 is actually a reasonably high concentration.

Right answer: “It is concentrated because there are 2 moles of ethanoic acid per dm3 of solution, which is a large amount per unit volume (1). It is weak because ethanoic acid only partially ionises in water, so only some of the molecules release H+ ions (1).” This scores 2/2. Each term is defined correctly and independently.

How to avoid this mistake

Use this mental framework:

• Concentration answers: how much acid is in the solution? (mol/dm3)
• Strength answers: how much of that acid has ionised? (fully or partially)

When you see “strong” in a chemistry question, think ionisation. When you see “concentrated”, think amount per volume. Never swap them.

A practical tip: vinegar is a concentrated-ish solution of a weak acid (ethanoic acid). Battery acid is a relatively dilute solution of a strong acid (sulfuric acid). You can drink vinegar on salad; you would not drink battery acid. That is not about concentration. It is about the degree of ionisation and the nature of the acid.

For calculations involving concentration, see concentration of solutions. The mole and Avogadro constant page explains the mol/dm3 unit in detail.