Chemistry

Acids Bases and Salts

5,996 words, 11 sections. The first two are free.

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The opening sections

Definitions first, then the reasoning. Each section is short enough to read in one sitting.

Acids, bases and salts is where the ionic equilibrium of the standard account becomes a working method. The official syllabus does not name the chapter, so a student searching the contents page will not find the heading, but the papers do not share that omission: 119 distinct questions on this material have appeared in 55 official papers between 2000 and 2026. Every equilibrium here is an application of the work in Chemical Equilibrium, which is where the material belongs on a revision plan: the ionisation of weak acids, the ionic product of water, buffers and the hydrolysis of salts are all equilibrium calculations wearing a different set of words. A student who has mastered the chapter can place any substance within the three definitions of an acid, move between pH, pOH and concentration without slipping a sign, predict whether a dissolved salt will turn its solution acidic or basic, and read a titration through to an unknown concentration.

For most of the nineteenth century an acid was a substance that tasted sour and turned litmus red, and a base was its chemical opposite. Arrhenius gave that instinct a mechanical form: an acid is a substance that releases hydrogen ions when it dissolves in water, and a base is one that releases hydroxide ions. The definition covers the common mineral acids and alkalis, and it is still the wording at the start of many questions, but it has a narrow edge. It needs water as the solvent, and it cannot describe ammonia, which contains no hydroxide at all and yet neutralises acid perfectly well.

Bronsted and Lowry, working independently in 1923, took the water out of the definition. They watched the reaction rather than the substance, and called an acid a proton donor and a base a proton acceptor. Hydrogen chloride dissolved in water is an acid because it hands its proton to a water molecule; ammonia is a base because the lone pair on its nitrogen takes a proton. This reading is wider than the Arrhenius one and matches the way the paper usually talks about acid strength, which is why it carries the largest share of the questions.

Lewis went further and gave up the proton altogether. A Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair donor. That reading contains the proton as the simplest Lewis acid, since a bare $H^+$ has no electrons of its own and will take a lone pair from anything willing to give one, and it also reaches neutral molecules with an incomplete octet. Boron trifluoride, $BF_3$ , and aluminium chloride, $AlCl_3$ , each have a central atom short of a full octet, so each accepts a lone pair and acts as a Lewis acid; water, with two lone pairs and no electron deficiency, is a Lewis base and not an acid. The three definitions are not rivals to be chosen between. They are layers, and a species that is an Arrhenius acid is a Bronsted acid and a Lewis acid too.

Worked example

Decide whether the hydrogen carbonate ion, $HCO_3^-$ , can act as both a Bronsted acid and a Bronsted base in water. Write it as an acid first, $HCO_3^- \rightleftharpoons H^+ + CO_3^{2-}$ , and then as a base, $HCO_3^- + H^+ \rightleftharpoons H_2CO_3$ . Both equations are legitimate in water, so the ion both donates and accepts a proton and is amphoteric. Run the same test on the carbonate ion, $CO_3^{2-}$ , and the acid direction fails, because there is no stable species one proton lighter that forms readily, so carbonate is a base only. The step students get wrong is judging the ion by a rule of thumb such as the number of hydrogen atoms it carries; the test is whether a written half-reaction actually releases a proton.

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Acids Bases and Salts

The opening sections

Introduction

1. Three definitions of an acid and a baseFor most of the nineteenth century an acid was a substance that tasted sour and turned litmus red, and a base was its chemical opposite. Arrhenius gave that ins

9 more sections in this chapter