Properties of Acids and Bases

Acids and bases are two important groups of chemicals. They are identified by how they behave in water, how they change indicator colours, and how they react with certain substances.

What are acids, bases, and alkalis?

• Acids release hydrogen ions, $H^+$, in water. They are proton donors.
• Bases are metal oxides or hydroxides. They are proton acceptors.
• Alkalis are bases that dissolve in water to release hydroxide ions, $OH^-$.

Ions in solution

Aqueous acids contain $H^+$ ions. Aqueous alkalis contain $OH^-$ ions. When an acid and an alkali react, their ions make water:

$$H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$$

Indicator colours

• Litmus: red in acid, blue in alkali.
• Methyl orange: red in acid, yellow in alkali, orange near neutral.
• Thymolphthalein: colourless in acid/neutral, blue in alkali.
• Universal indicator: shows pH. Acidic (pH < 7) = red–yellow; neutral (pH 7) = green; alkaline (pH > 7) = blue–purple.

How acids react

• With metals: acid + metal → salt + hydrogen gas (test: burning splint makes a pop). Example: $\mathrm{H_2SO_4 + Mg \rightarrow MgSO_4 + H_2}$.
• With bases/alkalis (neutralisation): acid + base → salt + water. Example: $\mathrm{NaOH + HNO_3 \rightarrow NaNO_3 + H_2O}$.
• With carbonates: acid + carbonate → salt + water + carbon dioxide (CO$_2$ turns limewater milky). Example: $\mathrm{2HCl + Na_2CO_3 \rightarrow 2NaCl + H_2O + CO_2}$.

How bases and alkalis react

• With acids: form salt and water (neutralisation).
• With ammonium salts (on warming): make ammonia gas, water, and a salt. Example: $\mathrm{NaOH + NH_4Cl \rightarrow NH_3 + H_2O + NaCl}$ (ammonia turns damp red litmus blue).

Strong and weak acids

• Strong acid: completely dissociates in water. Example: $\mathrm{HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)}$.
• Weak acid: partially dissociates (reversible). Example: $\mathrm{CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)}$.

Comparing acidity and pH

Lower pH means higher $H^+$ concentration (more acidic). Higher pH means higher $OH^-$ concentration (more alkaline). Universal indicator colours help compare this quickly.