Chemistry Basics – Study Notes
For Tamil Nadu PSC – General Awareness & Current Affairs
1. Simple Explanation
Chemistry is the science that studies matter (anything that has mass and occupies space) and the changes it undergoes.
Key ideas:
| Concept | What it means | Example |
|---|---|---|
| Atom | Smallest unit of an element that retains its properties | Carbon atom (C) |
| Molecule | Two or more atoms chemically bonded | Water (H₂O) |
| Element | Pure substance made of one type of atom | Gold (Au) |
| Compound | Chemical substance made of two or more elements | Sodium chloride (NaCl) |
| Periodic Table | Organized chart of all elements by atomic number | 118 elements arranged in 7 periods |
| Chemical Bond | Force that holds atoms together | Covalent, ionic, metallic |
| Acid / Base | Substances that donate (acid) or accept (base) protons | HCl (acid), NaOH (base) |
| Stoichiometry | Calculations that relate amounts of reactants/products | Moles, molarity, mass‑percent |
2. Key Points to Remember
| # | Topic | Important Facts |
|---|---|---|
| 1 | Atomic Structure | 1. Proton charge +1, 2. Neutron charge 0, 3. Electron charge –1, 4. Mass ≈ proton + neutron |
| 2 | Periodic Trends | 1. Atomic radius ↑ down a group, ↓ across a period, 2. Ionization energy ↓ down a group, ↑ across a period, 3. Electronegativity ↑ down a group, ↓ across a period |
| 3 | Types of Bonds | 1. Ionic: electron transfer, usually metal + non‑metal 2. Covalent: electron sharing, usually non‑metal + non‑metal 3. Metallic: “sea” of delocalised electrons |
| 4 | States of Matter | Solid (ordered), Liquid (less ordered), Gas (highly disordered). Phase changes: melting, boiling, sublimation, condensation, deposition |
| 5 | Acids & Bases | 1. Arrhenius: H⁺ in water (acid), OH⁻ in water (base) 2. Brønsted–Lowry: proton donor/acceptor 3. Lewis: electron pair donor/acceptor |
| 6 | pH Scale | pH = –log[H⁺]; 0–14; <7 acidic, >7 alkaline |
| 7 | Stoichiometry | 1. Moles = mass ÷ molar mass 2. Molarity (M) = moles solute ÷ volume solution (L) 3. Percent composition = (mass of element ÷ mass of compound) × 100 |
| 8 | Balanced Chemical Equations | 1. Conservation of mass – atoms same on both sides 2. Use coefficients to balance 3. Example: 2 H₂ + O₂ → 2 H₂O |
| 9 | Reactions Common to PSC | 1. Combustion (CₙH₂ₙ₊₂ + O₂ → CO₂ + H₂O) 2. Acid–base neutralisation (HCl + NaOH → NaCl + H₂O) 3. Redox (Fe²⁺ + Cu²⁺ → Fe³⁺ + Cu) |
3. Worked Example
Problem
A 0.25 M solution of hydrochloric acid (HCl) is prepared by diluting 10 mL of 1 M HCl to 100 mL. Verify the final molarity.
Solution
-
Find initial moles of HCl
[ n_{\text{initial}} = C_{\text{initial}} \times V_{\text{initial}} = 1,\text{M} \times 0.010,\text{L} = 0.010,\text{mol} ] -
Total final volume
[ V_{\text{final}} = 0.100,\text{L} ] -
Final concentration (molarity)
[ C_{\text{final}} = \frac{n_{\text{initial}}}{V_{\text{final}}} = \frac{0.010,\text{mol}}{0.100,\text{L}} = 0.10,\text{M} ]
Result: The final concentration is 0.10 M, not 0.25 M.
Common mistake: confusing dilution factor with final concentration.
4. Quick Revision Summary
- Atoms → Elements → Compounds → Molecules
- Periodic Table: key trends – atomic size, ionization energy, electronegativity.
- Three main bonds: ionic, covalent, metallic.
- States of matter and phase changes.
- Acids & bases: Arrhenius, Brønsted–Lowry, Lewis definitions.
- pH: measure of H⁺ concentration (0–14).
- Stoichiometry: moles, molar mass, molarity, percent composition.
- Balanced equations: obey conservation of mass.
- Common PSC reactions: combustion, neutralisation, redox.
Tip for PSC – Always check units (mol, L, g) and remember that “molarity” is moles per liter.
These notes cover the essentials you need to grasp for the General Awareness & Current Affairs section. Good luck!