Acids, Bases and Salts

Fig. 6.10 is a molecular-level cartoon, not a graph. It shows a small cluster of the sodium chloride crystal lattice at the centre — alternating Na⁺ and Cl⁻ ions packed together. Surrounding this solid chunk are individual Na⁺ and Cl⁻ ions that have left the lattice and are now floating freely in water. Each free ion is surrounded by several water molecules, oriented in a specific way: the oxygen end (partially negative) of water points toward the Na⁺ ion, and the hydrogen ends (partially positive) point toward the Cl⁻ ion. The rest of the space is filled with unbound water molecules, drawn as simple V-shaped molecules.

The physical idea is that the dissolution of an ionic solid is not just a simple mixing. The polar water molecules pull ions off the crystal surface by surrounding each ion with the oppositely charged end of the water dipole. This process — called hydration — stabilises the separated ions in solution. Without hydration, the electrostatic attraction between Na⁺ and Cl⁻ would keep them locked in the crystal. The figure makes clear that the ions do not exist as bare charged spheres in water; they are always surrounded by a shell of oriented water molecules.

This figure directly supports the textbook's distinction between strong and weak electrolytes. Sodium chloride is a strong electrolyte because, as the figure shows, nearly every NaCl unit that dissolves breaks completely into hydrated Na⁺ and Cl⁻ ions. There is no equilibrium between NaCl molecules and ions — the dissociation is essentially 100%. In contrast, a weak electrolyte like acetic acid would be shown with most molecules remaining intact and only a few ions present.

The key formula that the textbook develops from this idea is the equilibrium constant for ionic dissociation of a weak electrolyte. For a general weak acid HA:

where $K_a$ is the acid dissociation constant, $[\text{H}_3\text{O}^+]$ is the concentration of hydronium ions, $[\text{A}^-]$ is the concentration of the conjugate base, and $[\text{HA}]$ is the concentration of unionised acid. For a weak base B:

where $K_b$ is the base dissociation constant, $[\text{BH}^+]$ is the concentration of the conjugate acid, $[\text{OH}^-]$ is the hydroxide ion concentration, and $[\text{B}]$ is the concentration of unionised base. …

The textbook's biographical box on Michael Faraday sits alongside the discussion of strong and weak electrolytes because it was Faraday's own experiments — testing which aqueous solutions conduct electricity, and how strongly — that first established the distinction this chapter relies on.

Faraday (1791–1867) was born near London into a family of very limited means. He began as an apprentice bookbinder, and it was through reading the books he was binding that he developed an interest in science. He became a laboratory assistant to Humphry Davy, succeeded him as Director of the Royal Institution laboratories in 1825, and became its first Fullerian Professor of Chemistry in 1833. …