Acids, Bases and Salts

Overview

Acids, Bases and Salts form a foundational chemistry unit in Bihar TET Paper II, bridging everyday experiences (lemon sourness, soap slipperiness) with core chemical concepts. This topic tests your understanding of chemical properties, indicators, neutralisation reactions, and the pH scale—all essential for teaching upper-primary science effectively.

Questions typically appear in two forms: direct factual recall (identifying acids/bases, pH values) and application-based problems (predicting products of neutralisation, interpreting indicator colour changes). Mastery requires understanding both the observable properties and the underlying ionic behaviour of these substances. Since this topic connects to human biology (digestion), agriculture (soil pH), and daily life, expect scenario-based questions linking chemistry to real-world contexts.

Key Concepts

• **Acids** are substances that release hydrogen ions (H⁺) when dissolved in water. They taste sour, turn blue litmus red, and react with metals to produce hydrogen gas.

• **Bases** are substances that release hydroxide ions (OH⁻) in water. They taste bitter, feel soapy/slippery, and turn red litmus blue.

• **Alkalis** are bases that dissolve in water. All alkalis are bases, but not all bases are alkalis (e.g., copper hydroxide is a base but not an alkali as it is insoluble).

• **Salts** are ionic compounds formed by the neutralisation reaction between an acid and a base. They consist of a cation from the base and an anion from the acid.

• **Neutralisation** is the reaction between an acid and a base producing salt and water: Acid + Base → Salt + Water.

• **pH scale** measures the hydrogen ion concentration in a solution, ranging from 0 to 14. pH 7 is neutral; below 7 is acidic; above 7 is basic.

• **Indicators** are substances that show different colours in acidic and basic solutions—used to identify the nature of a substance.

• **Strong vs Weak acids/bases**: Strong acids (HCl, H₂SO₄) ionise completely; weak acids (acetic acid, citric acid) ionise partially. Same logic applies to bases.

Formulas / Key Facts

| Formula/Fact | Context | |--------------|---------| | HCl → H⁺ + Cl⁻ | Hydrochloric acid ionisation in water | | NaOH → Na⁺ + OH⁻ | Sodium hydroxide ionisation in water | | Acid + Base → Salt + Water | General neutralisation equation | | HCl + NaOH → NaCl + H₂O | Specific neutralisation example | | 2HCl + Zn → ZnCl₂ + H₂↑ | Acid + Metal → Salt + Hydrogen gas | | Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑ | Acid + Carbonate → Salt + Water + Carbon dioxide | | pH = 7 (neutral), pH < 7 (acidic), pH > 7 (basic) | pH scale interpretation | | Litmus: Red in acid, Blue in base | Litmus indicator behaviour | | Phenolphthalein: Colourless in acid, Pink in base | Phenolphthalein indicator | | Methyl orange: Red in acid, Yellow in base | Methyl orange indicator |