Acids and bases are two of the most significant classes of chemical compounds, found in everything from household cleaners to biological digestive juices. In chemistry, we define them based on their characteristic reactions and their impact on indicators. Understanding these properties is essential for performing neutralisation reactions and synthesizing various salts. The IGCSE 0654 Core syllabus requires a detailed description of acid-base reactivity and the use of the pH scale for classification.

• Acids: Substances that produce hydrogen ions ($H^{+}$) in aqueous solution. They have a pH of less than 7.
• Bases: Substances that react with acids to form a salt and water only. Bases are typically metal oxides (e.g., copper(II) oxide) or metal hydroxides (e.g., magnesium hydroxide).
• Alkalis: These are soluble bases. When dissolved in water, alkalis produce hydroxide ions ($OH^{-}$). All alkalis are bases, but not all bases are alkalis.

Acids exhibit three fundamental types of reactions:

(a) Reaction with Metals

Acids react with reactive metals (above hydrogen in the reactivity series) to produce a salt and hydrogen gas.

• Equation: Acid + Metal $\rightarrow$ Salt + Hydrogen
• Observation: Effervescence (bubbles) is seen, and the metal disappears.

(b) Reaction with Bases

This is a standard neutralisation reaction.

• Equation: Acid + Base $\rightarrow$ Salt + Water
• Observation: No gas is evolved; the mixture often warms up (exothermic).

(c) Reaction with Carbonates

Acids react with metal carbonates to produce a salt, water, and carbon dioxide gas.

• Equation: Acid + Carbonate $\rightarrow$ Salt + Water + Carbon Dioxide
• Observation: Vigorous effervescence. The gas turns limewater milky.

Chemical indicators are substances that change color depending on the acidity or alkalinity of a solution.

The pH Scale

The pH scale (0 to 14) is used to quantify relative acidity:

• pH 1–6: Acidic (red to yellow).
• pH 7: Neutral (green). Pure water is neutral.
• pH 8–14: Alkaline (blue to purple).

Neutralisation is the reaction between an acid and a base (or alkali) to produce a salt and water.

• Ionic Basis: $H^{+}(aq) + OH^{-}(aq) \rightarrow H_2O(l)$.

The hydrogen ions from the acid combine with the hydroxide ions from the alkali to form neutral water, while the remaining ions form the salt.

Acids and bases represent the chemical opposites of the aqueous world. By utilizing indicators and understanding their predictable reaction patterns with metals and carbonates, chemists can safely manipulate these substances for countless applications, from industrial synthesis to maintaining the delicate pH balance within living organisms.