Acids, Bases and Salts

Overview

Acids, Bases and Salts form a foundational chemistry topic in TN TET Paper II, appearing consistently in the Mathematics and Science section. This topic connects directly to everyday life—from the citric acid in lemons to the sodium hydroxide in soap—making it both practically relevant and conceptually important for upper primary teaching.

For TN TET, expect questions on identification of acids and bases using indicators, properties and chemical reactions, pH scale interpretation, and common salts with their uses. The pedagogy aspect often asks how to demonstrate these concepts through simple classroom experiments. Mastering this topic requires understanding the characteristic properties, memorising key examples, and knowing the neutralisation reaction thoroughly.

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: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), acetic acid (vinegar).

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

• **Alkalis** are bases that dissolve in water. All alkalis are bases, but not all bases are alkalis. Example: NaOH is an alkali; Cu(OH)₂ is a base but not an alkali (insoluble).

• **Indicators** are substances that show different colours in acidic and basic solutions. They help identify whether a substance is acid or base without tasting.

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

• **Salts** are ionic compounds formed from neutralisation reactions. They contain a metal (from base) and a non-metal part (from acid).

• **pH Scale** measures acidity or basicity on a scale of 0 to 14. pH 7 is neutral; below 7 is acidic; above 7 is basic. Lower pH means stronger acid; higher pH means stronger base.

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

Formulas / Key Facts

**Essential Reactions:**

• Acid + Metal → Salt + Hydrogen gas

Example: 2HCl + Zn → ZnCl₂ + H₂↑

• Acid + Metal carbonate → Salt + Water + Carbon dioxide