Chemistry Basics – Study Notes for VDO (Village Development Officer) Exam
Purpose: Quick, clear refresher on essential chemistry concepts that frequently appear in VDO General Awareness & Current Affairs.
1. Simple Explanation
Chemistry is the science of matter and the changes it undergoes.
- Matter = any substance that occupies space and has mass.
- Elements = pure substances composed of one kind of atom (e.g., H, O, Fe).
- Compounds = substances made of two or more different elements chemically bonded (e.g., H₂O, NaCl).
- Mixtures = physical combinations of two or more substances that can be separated by physical means (e.g., air, soil).
Chemical reactions involve breaking old bonds and forming new ones, often releasing or absorbing energy.
2. Key Points
| Topic | What You Need to Know |
|---|---|
| Atoms & Molecules | • Atomic number = number of protons. • Mass number = protons + neutrons. • Electrons orbit the nucleus. |
| Periodic Table | • Groups (vertical columns) show similar valence electrons. • Periods (horizontal rows) show increasing atomic number. • Alkali metals (1A), Alkaline earth metals (2A), Halogens (7A), Noble gases (8A). |
| Chemical Bonding | • Ionic: electron transfer (e.g., Na⁺ + Cl⁻ → NaCl). • Covalent: shared electrons (e.g., H₂, O₂). • Metallic: “sea of electrons” in metals. |
| Chemical Formula | • Element symbols + subscript for atoms. • Empirical vs. molecular formula. |
| Chemical Equations | • Reactants → Products. • Balance atoms on both sides. |
| Stoichiometry | • Mole ratio from balanced equation. • Use molar mass to convert grams ↔ moles. |
| Acids & Bases | • Acid: donates H⁺ (pH < 7). • Base: accepts H⁺ or donates OH⁻ (pH > 7). • Neutralization: acid + base → salt + water. |
| Salts & Solubility | • Salt = product of neutralization. • Solubility rules (e.g., nitrates are soluble). |
| Redox Reactions | • Oxidation = loss of electrons. • Reduction = gain of electrons. • Oxidation number helps identify changes. |
| Common Industrial Chemicals | • Fertilizers (ammonia, urea). • Pesticides (organophosphates). • Plastics (polyethylene, polypropylene). |
| Environmental & Safety Aspects | • Chemical waste management. • Green chemistry principles (reduce, reuse, recycle). |
| Current Affairs (2023‑24) | • India’s “National Chemical Safety Policy” (2023). • Government push for “Green Chemistry” labs in rural areas. • Increased use of bio‑based fertilizers in the Green Revolution. |
3. Worked Example – Balancing a Simple Reaction
Problem: Balance the combustion of methane:
[ \text{CH}_4 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} ]
Step‑by‑Step:
-
Write unbalanced equation.
[ \text{CH}_4 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} ] -
Count atoms on each side.
| Element | Reactants | Products |
|---|---|---|
| C | 1 | 1 |
| H | 4 | 2 |
| O | 2 | 3 (2 in CO₂ + 1 in H₂O) |
-
Balance hydrogen first.
Need 4 H in products → 2 H₂O.
[ \text{CH}_4 + \text{O}_2 \rightarrow \text{CO}_2 + 2,\text{H}_2\text{O} ] -
Re‑count oxygen.
| Element | Reactants | Products |
|---|---|---|
| O | 2 | 4 (2 in CO₂ + 2 in 2H₂O) |
-
Balance oxygen.
Need 4 O on reactants → 2 O₂.
[ \text{CH}_4 + 2,\text{O}_2 \rightarrow \text{CO}_2 + 2,\text{H}_2\text{O} ] -
Check all atoms.
C: 1 = 1, H: 4 = 4, O: 4 = 4 → Balanced!
Answer:
[ \boxed{\text{CH}_4 + 2,\text{O}_2 \rightarrow \text{CO}_2 + 2,\text{H}_2\text{O}} ]
4. Quick Revision Summary
- Atoms: Building blocks; defined by protons, neutrons, electrons.
- Periodic Table: Organizes elements by properties; useful for predicting reactivity.
- Chemical Bonding: Ionic (metal + non‑metal), covalent (non‑metal + non‑metal), metallic (metal