Strong and Weak Acids

Two different ways to describe an acid — Higher Tier

An acid solution can be described by both its concentration and its strength. These describe different things: concentration tells us how much acid is dissolved in a given volume of solution; strength tells us how completely that acid forms ions in water.

An aqueous solution is a solution in which water is the solvent. When acids dissolve in water, they ionise to produce hydrogen ions, $\mathrm{H^+}$. These hydrogen ions are responsible for the solution’s acidity.

Dilute and concentrated describe the amount of acid

A dilute acid solution contains a relatively small amount of acid per unit volume of solution. A concentrated acid solution contains a relatively large amount per unit volume.

Imagine two equal-sized samples of hydrochloric acid solution. If one contains more dissolved acid than the other, it is more concentrated. Comparing equal volumes matters: a large container of dilute acid could contain more acid overall than a small container of concentrated acid.

Adding water increases the volume of a solution without adding more acid, so it makes the solution more dilute.

Strong and weak describe ionisation

A strong acid is completely ionised in aqueous solution. Hydrochloric acid, nitric acid and sulfuric acid are strong acids.

For example, hydrochloric acid forms hydrogen ions and chloride ions in water:

$$\mathrm{HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)}$$

Here, $(aq)$ means aqueous: dissolved in water. The equation shows the formation of ions, rather than intact hydrochloric acid molecules remaining in solution.

A weak acid is only partially ionised in aqueous solution. Some acid molecules form ions, while others remain as un-ionised molecules. Ethanoic acid, citric acid and carbonic acid are weak acids.

Strength and concentration are therefore independent descriptions. Hydrochloric acid can be dilute but still strong: adding water does not change it into a weak acid. Ethanoic acid can be concentrated but still weak: dissolving more of it does not make its ionisation complete.

How ionisation affects pH

The pH of a solution depends on its hydrogen ion concentration, not simply on the total amount of acid added. A higher hydrogen ion concentration means a lower pH.

At the same acid concentration, a stronger acid produces a higher hydrogen ion concentration and therefore has a lower pH. For example, hydrochloric acid has a lower pH than ethanoic acid of the same concentration: hydrochloric acid is completely ionised, whereas ethanoic acid is only partially ionised.

A neutral solution has pH 7, with equal concentrations of hydrogen ions and hydroxide ions. Acidic solutions have pH values below 7 and higher hydrogen ion concentrations than a neutral solution. Alkaline solutions have pH values above 7 and lower hydrogen ion concentrations than a neutral solution. Moving towards lower pH means increasing acidity; moving towards higher pH means increasing alkalinity.

Each whole pH step represents a tenfold change

If hydrogen ion concentration increases by a factor of 10, pH decreases by one whole number. If hydrogen ion concentration decreases by a factor of 10, pH increases by one whole number.

For example, a solution at pH 3 has ten times the hydrogen ion concentration of a solution at pH 4. A solution at pH 2 has $10 \times 10 = 100$ times the hydrogen ion concentration of one at pH 4, because there are two pH steps between them.

The same relationship applies across neutral and alkaline solutions. A solution at pH 7 has ten times the hydrogen ion concentration of one at pH 8. Alkaline solutions still contain hydrogen ions, but at a lower concentration.

Strength versus concentration — Higher Tier

• Strong: completely ionised in aqueous solution — hydrochloric, nitric and sulfuric acids.
• Weak: partially ionised — ethanoic, citric and carbonic acids.
• Dilute/concentrated: relatively little/much acid per unit volume of solution.
• A strong acid can be dilute; a weak acid can be concentrated.

Strength and pH

At the same acid concentration:

Greater ionisation → higher concentration → lower .