GCSE Chemistry

GCSE Chemistry acids and alkalis, sorted for the exam

Last updated September 2026

Acids and alkalis is the GCSE Chemistry topic where the marks hinge on precise wording — strong versus concentrated, the right salt name, the right test for the right gas. This revision game guide sorts out those distinctions properly, then points you at a game that covers acids and alkalis alongside the rest of the syllabus.

Strong, weak, concentrated or dilute

The pH scale runs from 0 to 14: acids sit below 7, alkalis above it, and pure water sits at 7. Universal indicator turns red through orange and yellow as pH drops, and blue through purple as it rises — litmus is cruder, just red for acid and blue for alkali. Each whole step down the scale means ten times more hydrogen ions in solution, which is why a pH of 1 is far more acidic than a pH of 4, not just "a bit more".

The pair examiners really test is strong versus concentrated, because students mix them up constantly. Strong and weak describe how fully an acid ionises in water — hydrochloric, sulfuric and nitric acid ionise completely, ethanoic and citric acid only partially. Concentrated and dilute describe how much acid is dissolved in a given volume of water, and the two are independent: a concentrated weak acid can still have a higher pH than a dilute strong one.

• Strong acids — hydrochloric, sulfuric, nitric; fully ionise, so more H⁺ ions for a given concentration
• Weak acids — ethanoic, citric, carbonic; only partially ionise, so fewer H⁺ ions even when concentrated
• Concentration — how much acid is dissolved in the water, separate from how strong the acid is

Neutralisation and naming salts

Three reactions cover most of the marks here. Acid plus base gives a salt and water. Acid plus a reactive metal gives a salt and hydrogen gas, tested with a lit splint — a squeaky pop confirms it. Acid plus a metal carbonate gives a salt, water and carbon dioxide, tested by bubbling the gas through limewater, which turns cloudy. The salt's name comes straight from the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates, nitric acid makes nitrates — magnesium plus hydrochloric acid gives magnesium chloride, every time. The required practical for making a soluble salt — react the acid with excess insoluble base, filter, then evaporate and crystallise — is worth being able to describe step by step, and that's the kind of sequence that only sticks with repeated practice, spaced out, not one read-through.

Questions students actually ask

What's the actual difference between strong and concentrated? Strong or weak is fixed for a given acid — how completely it ionises in water. Concentrated or dilute is adjustable — how much of that acid is dissolved in the water. Dilute it and the concentration changes, not the strength.

How do you test for carbon dioxide? Bubble the gas through limewater. If it turns cloudy, carbon dioxide is present — a white precipitate of calcium carbonate is forming in solution.

Do I need to memorise pH numbers for specific acids? No — exam questions usually give you the pH value. What you need is the scale itself: 0–14, acids below 7, alkalis above 7, and each step representing a tenfold change in hydrogen ion concentration.