Acids, Bases and Salts

Overview

Acids, Bases and Salts form a foundational chapter in Chemistry for TS TET Paper II, connecting everyday substances like lemon juice, soap and table salt to core chemical principles. This topic tests both conceptual understanding and practical applications—expect questions on identifying acids/bases using indicators, writing neutralisation reactions, understanding pH scale, and recognising common acids, bases and salts with their uses.

For the TET exam, you must master three areas: (1) definitions and properties that distinguish acids from bases, (2) the indicator colour changes that appear frequently in MCQs, and (3) the formation, types and everyday applications of salts. This chapter also builds the foundation for understanding chemical reactions, so a strong grasp here helps across Chemistry topics.

Key Concepts

• **Acids** are substances that release hydrogen ions (H⁺) in water, taste sour, turn blue litmus red, and react with metals to produce hydrogen gas. Examples: HCl, H₂SO₄, HNO₃, CH₃COOH.

• **Bases** are substances that release hydroxide ions (OH⁻) in water, taste bitter, feel soapy, and turn red litmus blue. Examples: NaOH, KOH, Ca(OH)₂, NH₄OH.

• **Alkalis** are bases that dissolve in water. All alkalis are bases, but not all bases are alkalis (e.g., Cu(OH)₂ is a base but not an alkali as it is insoluble).

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

• **pH Scale** measures acidity or basicity on a scale of 0 to 14. pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic. Lower pH means stronger acid.

• **Indicators** are substances that show different colours in acidic and basic solutions. They help identify the nature of unknown solutions.

• **Salts** are ionic compounds formed by the reaction of acids and bases. The metal/positive ion comes from the base, and the negative ion comes from the acid.

• **Strong vs Weak Acids/Bases**: Strong acids (HCl, H₂SO₄) completely dissociate in water; weak acids (CH₃COOH, H₂CO₃) partially dissociate. Similarly for bases.

Formulas / Key Facts

| Fact | Detail | |------|--------| | Litmus colour change | Blue litmus → Red in acid; Red litmus → Blue in base | | Phenolphthalein | Colourless in acid; Pink in base | | Methyl orange | Red in acid; Yellow in base | | Turmeric indicator | Yellow in acid/neutral; Reddish-brown in base | | pH of pure water | 7 (neutral) | | pH of gastric juice | About 1.2 (strongly acidic) | | pH of blood | 7.35–7.45 (slightly basic) | | Neutralisation equation | HCl + NaOH → NaCl + H₂O | | Baking soda formula | NaHCO₃ (Sodium hydrogen carbonate) | | Washing soda formula | Na₂CO₃·10H₂O (Sodium carbonate decahydrate) | | Plaster of Paris | CaSO₄·½H₂O (Calcium sulphate hemihydrate) |