Chemistry Basics – Study Notes
(For Haryana PSC – General Awareness & Current Affairs)
📌 Simple Explanation
Chemistry is the science that studies the matter around us – its composition, structure, properties and the changes it undergoes.
Key concepts you need to remember:
| Concept | What it means | Why it matters for PSC |
|---|---|---|
| Matter | Anything that has mass & occupies space | All physical and chemical phenomena |
| Element | Pure substance defined by a single atom type | Foundation of the periodic table |
| Compound | Substance made of two or more elements chemically bonded | Everyday substances (water, salt, etc.) |
| Mixture | Physical combination of substances that can be separated | Foods, solutions, alloys |
| Atom | Smallest unit of an element, contains protons, neutrons & electrons | Basis of chemical reactions |
| Periodic Table | Arrangement of elements by atomic number & properties | Quick identification of elements |
| Chemical Bond | Force that holds atoms together (ionic, covalent, metallic) | Determines compound behaviour |
| Acids & Bases | pH < 7 & pH > 7, respectively | Common household substances, pH scale |
| Stoichiometry | Quantitative relationship between reactants & products | Calculations for lab & industry |
🔑 Key Points
| Section | Core Take‑aways |
|---|---|
| 1. Atomic Structure | • Protons (positive) define the element. • Neutrons (neutral) affect mass. • Electrons (negative) determine reactivity. • Isotopes: same element, different neutrons. |
| 2. Periodic Table | • Rows: Periods – same electron shell. • Columns: Groups – similar valence electrons. • Trends: Atomic radius ↑ down a group, ↓ across a period. • Metals on left, non‑metals on right, metalloids in between. |
| 3. Chemical Bonding | • Ionic: Transfer of electrons (metal + non‑metal). • Covalent: Sharing of electrons (non‑metal + non‑metal). • Metallic: Delocalised electrons in a lattice. |
| 4. Acids & Bases | • Acid: Donates H⁺ (pH < 7). • Base: Accepts H⁺ or donates OH⁻ (pH > 7). • Neutralisation: Acid + Base → Salt + Water. |
| 5. States of Matter | • Solid: Fixed shape & volume. • Liquid: Fixed volume, takes container shape. • Gas: Neither fixed shape nor volume. • Plasma: Ionised gas (high temperature). |
| 6. Stoichiometry | • Mole concept: 1 mol = 6.022×10²³ entities. • Molar mass: grams per mole. • Balanced equations: atoms conserved. • Percent composition: (mass of element/total mass)×100. |
| 7. Important Equations | • Ideal Gas Law: PV = nRT • pH: pH = –log[H⁺] • Density: ρ = mass/volume • Heat: q = mcΔT |
| 8. Everyday Chemistry | • Food additives, preservatives. • Pharmaceuticals, fertilizers. • Environmental chemistry (air & water pollution). |
🧮 Worked Example
Problem:
Calculate the number of moles of sodium chloride (NaCl) in 58.44 g of NaCl.
Solution Steps
-
Find molar mass of NaCl
- Na = 22.99 g/mol
- Cl = 35.45 g/mol
- Molar mass = 22.99 + 35.45 = 58.44 g/mol
-
Use the mole formula
[ n = \frac{m}{M} ] where (m) = mass (g), (M) = molar mass (g/mol). -
Insert values
[ n = \frac{58.44\ \text{g}}{58.44\ \text{g/mol}} = 1.00 mol ]
Answer: 1.00 mole of NaCl.
Why this matters?
- Understanding molar mass and the mole concept is essential for stoichiometric calculations, which often appear in exam questions on yield, purity, or reaction safety.
📋 Quick Revision Summary
- Matter → Elements → Compounds → Mixtures
- Atomic structure: protons, neutrons, electrons
- Periodic table: Periods = electron shells; Groups = valence electrons
- Bonding: Ionic (metal + non‑metal), Covalent (non‑metal + non‑metal), Metallic (metal lattice)
- Acids & Bases: pH < 7 / > 7, neutralisation → salt + water
- States of matter: solid, liquid, gas, plasma
- Stoichiometry: mole concept, molar mass, balanced equations, percent composition
- Key equations: PV = nRT, pH = –log[H⁺], ρ = m/V, q = mcΔT
- Practical relevance: Food chemistry, pharmaceuticals, environmental issues
Final Tips for PSC Exam
- Read the question carefully – identify if it asks for an analysis (e.g., acid strength) or a calculation (e.g., moles).
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