When an acid dissolves in water, it releases hydrogen ions (H⁺) into the solution. These tiny, positively charged particles are what make acids behave the way they do — they are responsible for the sour taste of citric acid, the corrosive nature of concentrated acids, and the reactions with metals and carbonates. The more H⁺ ions in a solution, the more acidic it is and the lower the pH.
Think of an acid as a "hydrogen ion donor". When you dissolve it in water, it breaks apart and releases H⁺. The water molecules play an essential role — without water, acids cannot fully ionise or show their acidic properties. That is why pure (anhydrous) acids are far less reactive than their aqueous solutions.
The key equations:
Hydrochloric acid: HCl → H⁺ + Cl⁻
Sulfuric acid: H₂SO₄ → 2H⁺ + SO₄²⁻
Nitric acid: HNO₃ → H⁺ + NO₃⁻
Alkalis are the opposite: they release hydroxide ions (OH⁻) when dissolved in water.
Sodium hydroxide: NaOH → Na⁺ + OH⁻
Potassium hydroxide: KOH → K⁺ + OH⁻
Remember: An alkali is a base that dissolves in water. All alkalis are bases, but not all bases are alkalis (e.g., copper oxide is a base but doesn't dissolve).
This deep dive covers Deep Dive: What Actually Makes an Acid? within Acids and Alkalis for GCSE Chemistry. Revise Acids and Alkalis in Chemical Changes for GCSE Chemistry with 25 exam-style questions and 21 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 2 of 12 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Acids and Alkalis
Which ion do acids produce when dissolved in water?
Explain the difference between a strong acid and a concentrated acid.