Acids and Bases

Acid-base chemistry explains digestion, industrial processes, soil management, and dozens of everyday observations. The key is understanding the definitions precisely — particularly the distinction between a base and an alkali — and being able to write balanced equations for every type of acid reaction.

Definitions¶

An acid is a proton donor — a substance that releases hydrogen ions (H⁺) in aqueous solution. Acids have replaceable hydrogen atoms that can be displaced by metals or neutralised by bases.

A base is a proton acceptor — a substance that can neutralise an acid. Bases are usually metal oxides or metal hydroxides.

An alkali is a soluble base. When dissolved in water, it produces hydroxide ions (OH⁻). All alkalis are bases, but not all bases are alkalis — only the soluble ones.

An acid anhydride is a non-metal oxide that reacts with water to produce an acid:

$\text{SO}_3\text{(g)} + \text{H}_2\text{O(l)} \rightarrow \text{H}_2\text{SO}_4\text{(aq)}$

Types of Oxides¶

Whether something is acidic or basic is closely linked to the type of oxide it forms. Many common acids and bases are simply non-metal or metal oxides that have reacted with water.

The pH Scale¶

Knowing whether something is an acid or a base is qualitative. The pH scale gives a number.

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is.

Indicators change colour with pH. Litmus turns red in acid and blue in alkali. Universal indicator shows a spectrum of colours across the full range. Phenolphthalein is colourless in acid and pink in alkali. Methyl orange is red in acid and yellow in alkali.

Strength of Acids and Alkalis¶

pH tells you how acidic a solution is. Strength explains why: two acids at the same concentration can have quite different pH values depending on how much they ionise.

Strength depends on the degree of ionisation — how completely the substance breaks into ions in water.

Strong acids ionise completely in water:

$\text{HCl(aq)} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$

Weak acids ionise only partially — most molecules remain intact:

$\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{H}^+\text{(aq)} + \text{CH}_3\text{COO}^-\text{(aq)}$

NaOH and KOH are strong alkalis (fully ionise); ammonia solution (NH₃(aq)) is a weak alkali.

Reactions of Acids¶

With the definitions in place, the next question is what acids actually do. There are five reaction types to know.

With Metals¶

Dilute acids react with metals above hydrogen in the reactivity series, producing a salt and hydrogen:

$\text{acid} + \text{metal} \rightarrow \text{salt} + \text{hydrogen}$

$\text{Mg(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)}$

$\text{Zn(s)} + 2\text{HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}$

Test for hydrogen: a burning splint produces a squeaky pop.

With Carbonates and Hydrogen Carbonates¶

$\text{acid} + \text{carbonate} \rightarrow \text{salt} + \text{water} + \text{carbon dioxide}$

$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

$\text{NaHCO}_3\text{(s)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

Test for CO₂: bubble through limewater — turns milky white.

Applications: antacid tablets contain NaHCO₃ or CaCO₃ to neutralise excess stomach acid; baking powder uses this reaction to release CO₂ in baking.

With Bases (Neutralisation)¶

$\text{acid} + \text{base} \rightarrow \text{salt} + \text{water}$

$\text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}$

$\text{H}_2\text{SO}_4\text{(aq)} + \text{CuO(s)} \rightarrow \text{CuSO}_4\text{(aq)} + \text{H}_2\text{O(l)}$

The net ionic equation for all strong acid-strong alkali neutralisations is:

$\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$

Reaction of Bases with Ammonium Salts¶

Bases also react with ammonium salts, not just acids. This is important in agriculture: lime and ammonium fertiliser should never be applied to soil together, because the reaction releases ammonia gas and wastes the nitrogen.

When a base is heated with an ammonium salt, ammonia gas is released:

$\text{Ca(OH)}_2\text{(s)} + 2\text{NH}_4\text{Cl(s)} \xrightarrow{\Delta} \text{CaCl}_2\text{(s)} + 2\text{H}_2\text{O(l)} + 2\text{NH}_3\text{(g)}$

Test for ammonia: turns damp red litmus paper blue.

Acids in Living Systems¶

Acids show up well outside the laboratory, in foods, organisms, and animal defence.

Applications of neutralisation:

• Toothpaste is mildly alkaline and neutralises acids produced by bacteria in the mouth, protecting tooth enamel.
• Lime (calcium hydroxide) is added to acidic soil to raise the pH and improve conditions for plant growth.
• Antacid tablets neutralise excess hydrochloric acid in the stomach to relieve indigestion.