Direct comparison
Acid vs base
An acid donates hydrogen ions and sits below pH 7; a base accepts hydrogen ions, often releasing hydroxide ions, and sits above pH 7.
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Side-by-side comparison
| Dimension | Acid | Base |
|---|---|---|
| Defining behaviour | Donates hydrogen ions (H⁺) — a proton donor. | Accepts hydrogen ions; an alkali releases OH⁻. |
| pH range | Below 7. | Above 7. |
| Ions in water | Increases H⁺ (H₃O⁺) concentration. | Increases OH⁻ concentration (for alkalis). |
| Taste (do not test!) | Sour. | Bitter, often soapy to the touch. |
| Litmus indicator | Turns blue litmus red. | Turns red litmus blue. |
| Reaction with the other | Neutralised by a base to give salt and water. | Neutralised by an acid to give salt and water. |
| Reaction with metals | Many react, releasing hydrogen gas. | Generally do not react with metals the same way. |
| Everyday example | Vinegar, lemon juice, stomach acid. | Baking soda, soap, oven cleaner, bleach. |
| Strong vs weak | Strong acids ionise fully (e.g. HCl); weak partly (e.g. ethanoic acid). | Strong bases dissociate fully (e.g. NaOH); weak partly (e.g. ammonia). |
Strength and concentration are not the same thing
Two ideas are easily confused. Strength describes how fully an acid or base ionises in water: a strong acid such as hydrochloric acid ionises almost completely, while a weak acid such as ethanoic acid (in vinegar) only partly ionises. Concentration describes how much acid or base is dissolved in a given volume. A weak acid can be concentrated, and a strong acid can be dilute. Both affect the pH, but they answer different questions — strength is about the substance, concentration is about how much of it is present.
Common questions
FAQ
What happens when an acid and a base mix?+
They neutralise each other. The acid’s hydrogen ions react with the base’s hydroxide ions to form water, while the remaining ions form a salt. This is called a neutralisation reaction, and it usually moves the pH toward 7. Mixing equal amounts of a strong acid and strong base gives a neutral solution of salt and water, often releasing heat.
What is the difference between a base and an alkali?+
All alkalis are bases, but not all bases are alkalis. A base is any substance that accepts hydrogen ions or neutralises an acid. An alkali is specifically a base that dissolves in water to release hydroxide ions, giving an alkaline solution above pH 7. Sodium hydroxide is an alkali; an insoluble base like copper oxide is a base but not an alkali.
Does a low pH mean a strong acid?+
A low pH means the solution is strongly acidic in effect, but it does not by itself prove the acid is "strong" in the chemical sense. pH reflects the concentration of hydrogen ions actually present, which depends on both the acid’s strength and how concentrated it is. A concentrated weak acid and a dilute strong acid could share a similar pH.
Going deeper
