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ChemistryDifficulty 2-4

Acids and Bases

Acids and bases are two complementary families of substances. In the Brønsted–Lowry picture (1923), an acid donates a proton (H⁺) and a base accepts one, so every acid–base reaction is a proton transfer. Strong acids such as hydrochloric acid ionise essentially completely in water, while weak acids such as acetic acid ionise only slightly. When an acid and a base react in the right proportions they neutralise each other, typically forming a salt and — for hydroxide bases — water.

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Acid strength and solvent

Strong acids such as HCl ionise essentially completely in water; weak acids such as acetic acid establish an equilibrium lying far to the left (about 1.3% ionised at 0.100 M). In water every acid stronger than H₃O⁺ is levelled to H₃O⁺, and strong bases are levelled to OH⁻, so water cannot rank them.

HA+H2O⇌H3O++A−\mathrm{HA + H_2O \rightleftharpoons H_3O^+ + A^-}

General acid ionisation; the position of equilibrium distinguishes strong from weak acids.

Modelling assumption in the simulation

ScienceVerse's neutralisation model treats both reagents as fully dissociated, monoprotic/monobasic species reacting 1:1 at 25 °C; it does not model weak-acid equilibria, buffers or polyprotic acids.

Common misconception: The Arrhenius definition is not wrong but limited: it cannot describe proton transfer outside water or bases such as NH₃ that produce OH⁻ by accepting a proton rather than containing hydroxide.
Full explanation — the complete reference version every reading depth is based on

Two definitions

  • Arrhenius (1884): an acid dissolves in water to give hydrogen ions (really hydronium ions, H₃O⁺); a base dissolves to give hydroxide ions (OH⁻).
  • Brønsted–Lowry (1923): an acid is a proton donor and a base is a proton acceptor — a broader definition that also covers reactions such as ammonia accepting a proton from water.

After an acid gives away its proton, what is left is its conjugate base; after a base accepts a proton it becomes its conjugate acid. Water can play either role: it accepts a proton from hydrogen fluoride but donates one to ammonia, so it is called amphiprotic.

HF+H2O⇌H3O++F−\mathrm{HF + H_2O \rightleftharpoons H_3O^+ + F^-}

Hydrogen fluoride (acid) donates a proton to water (base); F⁻ is the conjugate base of HF.

Strong and weak

'Strong' describes how completely an acid ionises, not how concentrated it is. Hydrochloric acid ionises essentially completely in water, while in a 0.100 M solution of acetic acid (the acid in vinegar) only about 1.3% of the acid molecules are ionised.

Common misconception: A common mistake is to treat 'strong' and 'concentrated' as the same thing. Strength describes what fraction of the acid ionises in water; concentration describes how much acid is dissolved in a given volume.

Neutralisation (the pH simulation's model)

Acids react with hydroxide bases to form a salt and water, in the mole ratio set by the balanced equation. For hydrochloric acid and sodium hydroxide the ratio is 1:1 — HCl + NaOH → NaCl + H₂O — so at the equivalence point moles of acid equal moles of base. Moles are concentration (in mol/L) times volume (in L).

caVa=cbVb⇒Vb=caVacbc_a V_a = c_b V_b \quad\Rightarrow\quad V_b = \frac{c_a V_a}{c_b}

Volume of base needed to neutralise a monoprotic acid with a monobasic base (1:1 ratio).

Worked example (ScienceVerse calculation, the same one the pH & acid–base simulation performs): 100 mL (0.100 L) of 0.1 M HCl contains 0.0100 mol of acid. With 0.2 M NaOH, V(base) = (0.1 × 0.100) ÷ 0.2 = 0.050 L, so 50 mL of base reaches the equivalence point.

How we know, and staying safe

Svante Arrhenius's theory, proposed in 1883, that dissolved substances split into charged ions — which is why their solutions conduct electricity — earned the 1903 Nobel Prize in Chemistry and laid the groundwork for describing acids and bases by the ions they produce.

Warning: Hydrochloric acid and sodium hydroxide are both classified as causing severe skin burns and eye damage (GHS hazard statement H314 in PubChem's classification). Explore acids and bases with the simulation; real ones are handled only in properly supervised laboratories with eye protection.

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Content status: published 1 October 2026.

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