Acids and Bases

La méthode, étape par étape

1. 1

   Read the pH scale

   0–6 acidic · 7 neutral · 8–14 alkaline

   Lower pH means more acidic. The scale is logarithmic, so each step of one represents a tenfold change in hydrogen ion concentration — pH 3 is ten times more acidic than pH 4, not slightly more.

2. 2

   Identify the ions responsible

   acids release H⁺ · alkalis release OH⁻

   Acidity comes from hydrogen ions in solution and alkalinity from hydroxide ions. This is why an acid must be dissolved to behave as one — the ions have to be free to move.

3. 3

   Know the neutralisation equation

   acid + base → salt + water

   The hydrogen ions and hydroxide ions combine to form water, and what remains forms the salt. This general equation covers every neutralisation and is worth memorising in exactly this form.

4. 4

   Name the salt from the acid

   hydrochloric → chloride · sulfuric → sulfate · nitric → nitrate

   The acid determines the second half of the salt's name and the metal determines the first. Sodium hydroxide with hydrochloric acid gives sodium chloride, every time.

5. 5

   Choose the right indicator

   universal indicator for a value; litmus for acid or alkali

   Universal indicator gives an approximate pH through a range of colours. Litmus only distinguishes acid from alkali. A pH meter gives a precise numerical value, which is what quantitative work requires.

The pH scale is not linear

Each whole number on the pH scale represents a tenfold difference in hydrogen ion concentration. So a solution at pH 2 is a hundred times more acidic than one at pH 4, not twice as acidic. This surprises students and is examined directly.

It also explains why small pH changes matter so much in biology and in the environment. A drop of one pH unit in a lake or in blood is an enormous chemical change, not a marginal one.

• pH below 7 acidic, 7 neutral, above 7 alkaline
• Acids release H⁺; alkalis release OH⁻
• acid + base → salt + water
• The acid names the salt's second half
• Each pH unit is a tenfold change

Every alkali is a base; not every base is an alkali

A base is any substance that neutralises an acid. An alkali is a base that dissolves in water. Copper oxide is a base and not an alkali because it does not dissolve; sodium hydroxide is both.

The distinction is examined and the two words are used loosely outside the classroom, which is why students mix them up. Remembering that alkali is the narrower category — the soluble ones — settles it.

Naming the salt

The metal from the base gives the first part of the salt's name and the acid gives the second. Hydrochloric acid produces chlorides, sulfuric acid sulfates, nitric acid nitrates, and phosphoric acid phosphates.

So magnesium oxide plus sulfuric acid gives magnesium sulfate and water. Once the pattern is established, naming the product of any neutralisation is immediate — which is what the question usually asks for.

How we teach acids and bases

We establish the logarithmic nature of the pH scale early, because students otherwise carry a linear intuition through the whole topic and misjudge every comparison question.

We drill salt naming as a pattern rather than a list to memorise. Three acid-to-salt-name pairings cover most of what appears, and a student who knows the pattern can name the product of a reaction they have never seen.