Chemistry — SSC CHSL Study Notes

Overview

Chemistry in SSC CHSL is a high-yield section within General Science, typically yielding 3–5 direct questions in Tier 1. The syllabus emphasizes fundamental concepts rather than deep theoretical knowledge — you need strong recall of the periodic table structure, acid-base-salt reactions, and basic organic chemistry terminology. Most questions are fact-based or require simple one-step reasoning (e.g., "Which gas turns lime water milky?" or "What is the valency of calcium?"). Mastering this topic means memorizing key facts, chemical formulas, and properties rather than solving numerical problems. Students who systematically cover the periodic table trends, common reactions of acids/bases/salts, and nomenclature of simple organic compounds can secure full marks in this section. Use mnemonic devices for groups and periods, and practice writing common chemical equations from memory.

Key Concepts

• **Periodic Table Organization**: Elements are arranged in order of increasing atomic number in 18 groups (vertical columns) and 7 periods (horizontal rows). Groups share similar valence electron configurations and chemical properties; periods show gradual property changes from metals to non-metals.

• **Metals, Non-metals and Metalloids**: Metals (left and center) are electropositive, lustrous, malleable, good conductors. Non-metals (right side) are electronegative, brittle, poor conductors. Metalloids (B, Si, Ge, As, Sb, Te) show intermediate properties and are semiconductors.

• **Acids and Bases**: Acids donate H⁺ ions in water (sour taste, turn blue litmus red); bases accept H⁺ or donate OH⁻ (bitter taste, slippery, turn red litmus blue). pH scale (0–14) measures acidity/basicity: <7 acidic, =7 neutral, >7 basic.

• **Neutralization and Salts**: Acid + Base → Salt + Water. Salts are ionic compounds formed from the cation of a base and anion of an acid. Common salts include NaCl (table salt), NaHCO₃ (baking soda), CaSO₄·2H₂O (gypsum).

• **Organic Chemistry Basics**: Organic compounds contain carbon-hydrogen bonds. Hydrocarbons (only C and H) are classified as alkanes (single bonds, CₙH₂ₙ₊₂), alkenes (one double bond, CₙH₂ₙ), alkynes (one triple bond, CₙH₂ₙ₋₂). Functional groups (–OH, –COOH, –CHO) determine compound families.

• **Valency and Chemical Bonding**: Valency is the combining capacity of an element (number of electrons lost, gained or shared). Ionic bonds form by electron transfer (metal + non-metal); covalent bonds by electron sharing (non-metal + non-metal).

• **Common Reactions and Indicators**: Litmus, phenolphthalein and methyl orange are acid-base indicators. CO₂ turns lime water milky (forms CaCO₃). Metals react with acids to produce hydrogen gas.