Acids and bases
An acid gives protons away and a base accepts them; that is the Brønsted definition and it is the one you need. pH is −log[H⁺], so it falls as acidity rises and each unit is a factor of ten: pH 3 is a hundred times more acidic than pH 5. Pure water has pH 7 at 25 °C because [H⁺]·[OH⁻] = 10⁻¹⁴. The difference between strong and weak is not how much it burns but how much it dissociates: hydrochloric dissociates completely, acetic barely one per cent. That shows up in a titration: a strong acid gives a clean jump through pH 7, while a weak one first makes a plateau — the buffer, where the pH barely moves — and its equivalence point lands above 7, because the salt left behind is basic. At half-titration the pH is exactly the pKa, which is the quickest way to measure it.
Where this sits
- Comes after
- Solutions and gases
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According to Brønsted, an acid is something that…
- Donates protons
- Dissolves in water
- Accepts protons
- Donates electrons
Why An acid donates protons (H⁺) and a base accepts them; they always come in pairs, because a proton leaving one place has to arrive somewhere else. Donating electrons is a different matter: that is oxidation. Confusing the two pairings — protons for acid-base, electrons for redox — is among the hardest habits to undo.
easierWhat distinguishes a strong acid from a weak one?
- How much it dissociates in water
- That one is liquid and the other solid
- How badly it burns
- Its concentration
Why Strong means it dissociates completely: every bit of hydrochloric you add releases its proton. Weak means it dissociates barely: acetic releases about one per cent. It is independent of concentration, hence the surprise: concentrated vinegar can have a lower pH than very dilute hydrochloric. Strong is not the same as concentrated, and not being clear on that is dangerous in a laboratory.
in betweenWhy does a weak acid's equivalence point with sodium hydroxide land above pH 7?
- Because the salt left behind is a base
- Because the indicator is wrong
- Because there is excess hydroxide
- Because the acid is not fully used up
Why At the equivalence point exactly all the acid has been consumed, but what remains is not neutral: it is acetate, acetic acid's conjugate base, and a weak acid's conjugate base is strong enough to take a proton from water and release OH⁻. That is why the pH ends at 8 or 9, and why the indicator must fit the case: phenolphthalein here, not methyl orange.
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