Chemistry Basics – West Bengal Patwari / Lekhpal
Purpose: Quick, clear, exam‑ready notes on the fundamentals of chemistry that you’ll need for the General Awareness & Current Affairs paper.
1. Simple Explanation
| Term | What It Means | Everyday Example |
|---|---|---|
| Chemistry | The science that studies matter, its composition, structure, properties, and the changes it undergoes. | Cooking, cleaning, making soap. |
| Matter | Anything that has mass and occupies space. | Water, air, iron nails. |
| Atom | The smallest unit of a chemical element. | Hydrogen atom in a water molecule. |
| Element | A pure substance made of only one type of atom. | Oxygen, iron, copper. |
| Compound | A substance made from two or more different elements chemically bonded. | Salt (NaCl), water (H₂O). |
| Mixture | A physical combination of two or more substances that can be separated physically. | Salted water, air. |
| Chemical Bond | Force that holds atoms together. | Covalent bond in H₂O, ionic bond in NaCl. |
| Reaction | A process where one or more substances are changed into new substances. | 2H₂ + O₂ → 2H₂O. |
| Molarity (M) | Concentration expressed as moles of solute per litre of solution. | 0.5 M NaOH solution. |
| pH | Measure (0–14) of acidity or basicity of a solution. | 7 = neutral, <7 = acidic, >7 = basic. |
2. Key Points
| Category | Highlights | Notes for Exam |
|---|---|---|
| Atomic Structure | Protons (positive), Electrons (negative), Neutrons (neutral). | Atomic mass ≈ protons + neutrons. |
| Periodic Table | 7 periods (rows), 18 groups (columns). | Trend: atomic radius ↑ down a group, ↓ across a period. |
| Types of Bonds | Ionic (electron transfer), Covalent (electron sharing), Metallic (electron sea). | Ionic: metal + non‑metal; Covalent: non‑metal + non‑metal. |
| Chemical Reactions | Combustion, Synthesis, Decomposition, Single Replacement, Double Replacement. | Balanced equations must obey atom & charge conservation. |
| Stoichiometry | Uses mole ratios from balanced equations. | Useful for calculating yields, masses, volumes. |
| Acids & Bases | Acid donates H⁺, Base accepts H⁺. | Common acids: HCl, H₂SO₄. Common bases: NaOH, KOH. |
| Redox Reactions | Oxidation = loss of electrons, Reduction = gain of electrons. | Identified via changes in oxidation numbers. |
| pH Scale | pH = –log[H⁺]. | 0–3 very acidic, 7 neutral, 11–14 very basic. |
| Energy in Chemistry | Energy is absorbed or released during reactions. | Endothermic (absorbs), Exothermic (releases). |
| Relevance to Daily Life | Food preparation, agriculture, medicine, industry, environment. | Patwari may need to know chemical waste disposal, soil fertility, etc. |
3. Worked Example
Problem
A 0.40 M NaCl solution is prepared by dissolving 8 g of NaCl in 500 mL of water.
Questions:
- Verify if the stated molarity is correct.
- Calculate the mass of NaCl needed to prepare 1 L of the same solution.
Solution
1. Verify the molarity
-
Step 1: Find moles of NaCl in 8 g.
Molar mass NaCl = 22.99 (Na) + 35.45 (Cl) = 58.44 g mol⁻¹.
Moles = mass / molar mass = 8 g / 58.44 g mol⁻¹ = 0.137 mol. -
Step 2: Volume of solution = 500 mL = 0.500 L.
Molarity (M = \frac{n}{V}) = 0.137 mol / 0.500 L = 0.274 M.
The given 0.40 M is incorrect; the actual molarity is 0.274 M.
2. Mass needed for 1 L of 0.40 M solution
-
Desired moles: ( n = M \times V = 0.40 \text{ mol L}^{-1} \times 1 \text{ L} = 0.40 \text{ mol}).
-
Mass = moles × molar mass = 0.40 mol × 58.44 g mol⁻¹ = 23.38 g.
Answer:
- The solution is 0.274 M, not 0.40