Maharashtra PSC – General Awareness & Current Affairs
Topic: Chemistry Basics
Purpose – Provide a quick, clear, and concise refresher on fundamental chemistry concepts that frequently appear in Maharashtra PSC exams (General Awareness & Current Affairs).
1️⃣ Simple Explanation
| Concept | What It Is | Why It Matters |
|---|---|---|
| Atom | The smallest unit of an element that retains its chemical properties. | All chemical reactions involve atoms. |
| Element | Pure substance made of one kind of atom (e.g., Fe, O₂). | Elements form the building blocks of matter. |
| Compound | Substance made of two or more different elements chemically bonded (e.g., H₂O). | Determines the properties of everyday materials. |
| Periodic Table | Arranged list of all known elements by atomic number, electron configuration, and recurring properties. | Helps predict element behavior and bonding. |
| Chemical Bond | Attraction holding atoms together (ionic, covalent, metallic, hydrogen). | Determines compound stability and physical properties. |
| Acid–Base | Acids donate H⁺, bases accept H⁺. | Important in everyday life (food, medicine, industry). |
| Stoichiometry | Quantitative relationships between reactants & products. | Needed for calculations in industry, labs, and exams. |
2️⃣ Key Points (Quick Reference)
| # | Topic | Core Facts | Example |
|---|---|---|---|
| 1 | Atomic Structure | Protons (Z), Neutrons (A−Z), Electrons (Z) | Fe: Z = 26, neutrons ≈ 30 (common isotope). |
| 2 | Periodic Trends | • Atomic radius ↑ down a group, ↓ across a period. • Ionization energy ↓ down, ↑ across. • Electronegativity ↓ down, ↑ across. |
Period 1 elements: H < Li < Na in radius. |
| 3 | Bond Types | Ionic (metal + non‑metal, electrostatic). Covalent (non‑metal + non‑metal, sharing electrons). Metallic (metal lattice). |
NaCl (ionic), H₂O (covalent). |
| 4 | Acid–Base Indicators | Litmus: blue → basic, red → acidic. Phenolphthalein: colourless → acidic, pink → basic. |
Testing rainwater acidity. |
| 5 | Moles & Avogadro | 1 mol = 6.022×10²³ entities. | 12.01 g of C = 1 mol (12.01 g/mol). |
| 6 | Molar Mass | Sum of atomic masses of all atoms in a formula unit. | H₂O: 2×1.01 + 16.00 = 18.02 g/mol. |
| 7 | Density Formula | ρ = mass / volume. | ρ of Cu = 8.96 g/cm³. |
| 8 | Percent Composition | (mass of element ÷ mass of compound) × 100%. | In NaCl, Na = 22.99; Cl = 35.45 → %Na = 22.99/(58.44)×100 ≈ 39.3%. |
| 9 | Environmental Chemistry | Role of CO₂, CH₄, NOx, O₃ in climate & air quality. | Emission standards for vehicles. |
| 10 | Industrial Relevance | Synthesis of fertilizers (Haber process), plastics (polymerization), pharmaceuticals. | Haber: 2NH₃ ⇌ N₂ + 3H₂. |
3️⃣ Worked Example
Question: Calculate the density of a cubic crystal of sodium chloride (NaCl) when the edge length is 0.5 µm and the molar mass of NaCl is 58.44 g mol⁻¹.
Step‑by‑step
-
Find the volume of the cube
[ V = a^3 = (0.5 \times 10^{-6},\text{m})^3 = 1.25 \times 10^{-19},\text{m}^3 ] Convert to cm³:
(1,\text{m}^3 = 10^6,\text{cm}^3)
[ V = 1.25 \times 10^{-13},\text{cm}^3 ] -
Determine the mass of one NaCl unit cell
NaCl crystallizes in a face‑centered cubic structure.
Each unit cell contains 4 Na and 4 Cl atoms.
Mass per atom:
[ m_{\text{Na}} = \frac{22.99}{N_A}, \quad m_{\text{Cl}} = \frac{35.45}{N_A} ] Total mass:
[ m_{\text{cell}} = 4(m_{\text{Na}} + m_{\text{Cl}}) = 4\left(\frac{22.99+35.45}{N_A}\right) ] Using (N_A = 6.022\times10^{23}): [ m_{\text{cell}} = 4 \times \frac{58.44}{6.022\times10^{23}} = 3.88\times10^{-22},\text{g} ] -
Compute density
[ \rho = \frac{m_{\text{cell}}}{V} = \frac{3.88\times10^{-22},\text{g}}{1.25\times10^{-13},\text{cm}^3} = 3.10\times10^{-9},\text{g/cm}^3 ] (This is a very small value because the crystal is microscopic; for bulk NaCl, (\rho \approx 2.16,\text{g/cm}^3).)
Answer: Density ≈ (3.1\times10^{-9},\text{g/cm}^3