Chemistry
Elements, Compounds, Acids, Bases, Salts and Chemical Reactions
Overview
Chemistry forms a core component of the General Science section in TNPSC Group IV, tested at SSLC standard. Questions typically focus on identification of elements and compounds, properties of acids-bases-salts, types of chemical reactions, and their everyday applications. This is a scoring area because concepts are straightforward and frequently repeated.
Students must grasp the periodic table basics, distinguish between physical and chemical changes, understand the pH scale, and recognize common chemical reactions. Tamil Nadu Board Class 9 and 10 Science textbooks cover these topics adequately.
Mastering this topic also helps in understanding related areas like Botany (photosynthesis), Zoology (digestion, respiration), and environmental science (acid rain, water treatment).
Key Concepts
- Element vs Compound vs Mixture: An element contains only one type of atom (e.g., iron, oxygen). A compound contains two or more elements chemically combined in fixed ratio (e.g., water H₂O). A mixture has components physically combined with no fixed ratio (e.g., air, salt water).
- Atom and Molecule: Atom is the smallest particle of an element that retains its identity. Molecule is the smallest particle of an element or compound that can exist independently (e.g., O₂, H₂O).
- Periodic Table Basics: Elements arranged by increasing atomic number. Rows are periods (7 total), columns are groups (18 total). Metals on left, non-metals on right, metalloids along the zigzag line.
- Acids: Substances that release H⁺ ions in water, taste sour, turn blue litmus red, react with metals to produce hydrogen gas. Examples: HCl (hydrochloric acid), H₂SO₄ (sulphuric acid), CH₃COOH (acetic acid).
- Bases: Substances that release OH⁻ ions in water, taste bitter, feel soapy, turn red litmus blue. Examples: NaOH (sodium hydroxide), Ca(OH)₂ (calcium hydroxide), NH₄OH (ammonium hydroxide).
- Salts: Formed by neutralization of acid and base. Contain a metal cation and an acid anion. Examples: NaCl (common salt), CaSO₄ (gite of Paris precursor), NaHCO₃ (baking soda).
- pH Scale: Measures acidity/basicity from 0 to 14. pH 7 is neutral. Below 7 is acidic, above 7 is basic. Stomach acid has pH ~2, blood has pH ~7.4, soap solution has pH ~9-10.
- Chemical Reaction: Process where reactants transform into products with new chemical properties. Evidence includes colour change, gas evolution, precipitate formation, temperature change.
Formulas / Key Facts
| Concept | Formula / Fact |
|---|---|
| Water formula | H₂O — 2 hydrogen atoms + 1 oxygen atom |
| Common salt | NaCl — sodium chloride |
| Baking soda | NaHCO₃ — sodium bicarbonate |
| Washing soda | Na₂CO₃ — sodium carbonate |
| Plaster of Paris | CaSO₄·½H₂O — calcium sulphate hemihydrate |
| Gypsum | CaSO₄·2H₂O — calcium sulphate dihydrate |
| Quick lime | CaO — calcium oxide |
| Slaked lime | Ca(OH)₂ — calcium hydroxide |
| Neutralization | Acid + Base → Salt + Water |
| pH of pure water | 7 (neutral at 25°C) |
| Universal indicator | Shows colour gradient from red (acidic) to violet (basic) |
| Litmus source | Lichen plant extract |
Types of Chemical Reactions:
- Combination: A + B → AB (e.g., 2H₂ + O₂ → 2H₂O)
- Decomposition: AB → A + B (e.g., 2H₂O → 2H₂ + O₂ by electrolysis)
- Displacement: A + BC → AC + B (e.g., Zn + CuSO₄ → ZnSO₄ + Cu)
- Double Displacement: AB + CD → AD + CB (e.g., NaCl + AgNO₃ → AgCl + NaNO₃)
- Oxidation: Gain of oxygen or loss of electrons
- Reduction: Loss of oxygen or gain of electrons
Worked Examples
Example 1: Identifying Acid or Base
Question: Soap solution turns red litmus blue and has a pH of 9. Is it acidic or basic?
Solution:
- Red litmus turning blue indicates a base
- pH 9 is greater than 7, confirming basic nature
- Answer: Soap solution is basic (alkaline)
Example 2: Neutralization Reaction
Question: What is formed when hydrochloric acid reacts with sodium hydroxide?
Solution:
- HCl + NaOH → NaCl + H₂O
- Acid + Base → Salt + Water
- Answer: Sodium chloride (common salt) and water are formed
Example 3: Displacement Reaction
Question: When an iron nail is dipped in copper sulphate solution, what happens?
Solution:
- Iron is more reactive than copper
- Fe + CuSO₄ → FeSO₄ + Cu
- Iron displaces copper from the solution
- Answer: Blue colour fades, reddish-brown copper deposits on nail, solution turns green (ferrous sulphate)
Common Mistakes
❌ Confusing compounds with mixtures → A compound has a fixed ratio and cannot be separated by physical methods; a mixture has no fixed ratio and can be separated physically (filtration, evaporation).
❌ Reversing litmus test results → Remember: Acid turns Blue litmus Red (ABR), Base turns Red litmus Blue (BRB). Create a mnemonic: "Acid = Blue to Red."
❌ Thinking pH 0 is neutral → pH 7 is neutral, not 0. pH 0 indicates a very strong acid.
❌ Assuming all salts are neutral → Some salts are acidic (NH₄Cl), some are basic (Na₂CO₃), and some are neutral (NaCl). It depends on the parent acid and base strength.
❌ Confusing oxidation with oxygen addition only → Oxidation also means loss of electrons or loss of hydrogen. Reduction is the opposite. Remember: OIL RIG (Oxidation Is Loss, Reduction Is Gain — of electrons).
Quick Reference
- Litmus test: Acid = blue→red; Base = red→blue
- pH scale: 0-6 acidic, 7 neutral, 8-14 basic
- Neutralization: Always produces salt + water
- Reactivity series (decreasing): K > Na > Ca > Mg > Al > Zn > Fe > Cu > Ag > Au
- Baking soda + acid → CO₂ gas released (used in fire extinguishers, cooking)
- Plaster of Paris sets hard by absorbing water — used in casts and moulds