Acid-Base Chemistry

An acid produces H⁺ ions in aqueous solution. An alkali is a soluble base that produces OH^- ions in water. A base neutralises an acid; insoluble copper(II) oxide is a base but is not an alkali. Acidic and alkaline solutions both contain H⁺ and OH^-; their relative concentrations differ.

Compare colours with the supplied indicator chart. Universal Indicator estimates a pH range; a suitable calibrated pH probe gives a numerical reading. We use qualitative comparisons here, not calculations of pH from hydrogen-ion concentration.

Soil pH affects nutrient availability and plant growth. Calcium hydroxide can treat excessively acidic soil because it neutralises acid. Adding too much creates a different problem: overly alkaline soil. Determine the starting pH and add an appropriate amount rather than assuming more is better.

The acid supplies the salt anion: hydrochloric acid gives chlorides, sulfuric acid sulfates, and nitric acid nitrates. The metal or base supplies the cation. Not every metal reacts with dilute acid: copper, for example, does not release hydrogen from dilute hydrochloric acid.

For an acid reacting with an alkali, the reacting ions are H⁺(aq) + OH^-(aq) -> H₂O(l). This is neutralisation. The other dissolved ions are spectators. For an ammonium salt and alkali, NH₄⁺(aq) + OH^-(aq) -> NH₃(g) + H₂O(l); warming helps ammonia escape.

For example, CO₂(g) + 2NaOH(aq) -> Na₂CO₃(aq) + H₂O(l) with excess alkali shows acidic-oxide behaviour. CuO reacting with an acid shows basic behaviour. An insoluble oxide can still be acidic or basic: classification is not determined by whether it dissolves in water.

Acids supply H⁺; alkalis supply OH^-. Acid + base gives salt and water; carbonate also gives CO₂; suitable metal gives H₂. Alkali and ammonium salt release NH₃ on warming.