Chemistry Basics – Study Notes
(Manipur PSC – General Awareness & Current Affairs)
1. Simple Explanation
Chemistry is the science that studies the composition, structure, properties, and changes of matter.
- Matter: Anything that has mass and occupies space.
- Atoms: The smallest units that retain the identity of an element.
- Molecules: Two or more atoms bonded together.
- Reactions: Processes where atoms rearrange to form new substances.
Understanding chemistry provides a foundation for many everyday phenomena – from cooking and medicine to technology and environmental science.
2. Key Points
| Concept | What It Means | Why It Matters |
|---|---|---|
| Atomic Structure | Nucleus (protons + neutrons) + electrons | Determines chemical behaviour |
| Protons | Positive charge, defines element (atomic number) | Gives identity of element |
| Neutrons | Neutral, different numbers → isotopes | Affects mass & stability |
| Electrons | Negatively charged, occupy shells | Drive bonding & reactivity |
| Periodic Table | 118 elements arranged by atomic number | Predicts properties & trends |
| Periodic Trends | Atomic radius, ionization energy, electronegativity, electron affinity | Explain reactivity patterns |
| Chemical Bonds | Ionic, covalent, metallic, hydrogen, van der Waals | Determine compound structure |
| States of Matter | Solid, liquid, gas, plasma | Different physical behaviours |
| Solutions | Homogeneous mixtures (solute + solvent) | Basis of many industrial processes |
| Acids & Bases | Acids donate H⁺; bases accept H⁺ (or donate OH⁻) | pH measurement, neutralisation |
| Redox Reactions | Oxidation = loss of e⁻; Reduction = gain of e⁻ | Energy transfer, batteries |
| Stoichiometry | Quantitative relation between reactants & products | Calculating yields, limiting reactants |
| Law of Conservation of Mass | Mass is neither created nor destroyed in a reaction | Basis of quantitative chemistry |
| Moles & Avogadro’s Number | 1 mole = 6.022×10²³ entities | Unit of measurement for atoms/molecules |
| Molar Mass | Mass of one mole (g/mol) | Convert between mass & moles |
| Density | Mass per unit volume (g/cm³) | Identify substances, calculate masses |
| Temperature & Pressure | Influence reaction rates & gas behaviour | Gas laws (Boyle, Charles, Avogadro, Ideal gas) |
3. Worked Example – Stoichiometry
Problem
A chemist burns 10.0 g of methane (CH₄) in excess oxygen. The reaction is:
[ \mathrm{CH_4 + 2,O_2 \rightarrow CO_2 + 2,H_2O} ]
Calculate the mass of carbon dioxide (CO₂) produced.
Step‑by‑Step Solution
-
Write the balanced equation – already given.
-
Find molar masses
- CH₄: (12.01 + 4(1.008) = 16.04) g/mol
- CO₂: (12.01 + 2(16.00) = 44.01) g/mol
-
Convert grams of CH₄ to moles
[ n_{\text{CH}_4} = \frac{10.0\ \text{g}}{16.04\ \text{g/mol}} = 0.624\ \text{mol} ]
-
Use the mole ratio from the balanced equation
1 mol CH₄ → 1 mol CO₂[ n_{\text{CO}_2} = 0.624\ \text{mol} ]
-
Convert moles of CO₂ to grams
[ m_{\text{CO}2} = n{\text{CO}2} \times 44.01\ \text{g/mol} ] [ m{\text{CO}_2} = 0.624 \times 44.01 \approx 27.5\ \text{g} ]
Answer
Approximately 27.5 g of CO₂ will be produced.
4. Quick Revision Summary
- Atoms – nucleus (protons + neutrons) + electrons.
- Element – fixed number of protons (atomic number).
- Molecule – bonded atoms.
- Periodic Table – organized by atomic number; trends:
- Radius ↑ left → right ↓;
- Ionization energy ↑ left → right ↓;
- Electronegativity ↑ left → right ↑.
- Bonds – ionic (opposite charges), covalent (shared electrons), metallic (electron sea).
- States – solid, liquid, gas, plasma; changes governed by temperature & pressure.
- Acids – donate H⁺; Bases – accept H⁺ or donate OH⁻.
- Redox – track electron transfer; oxidation number changes.
- Stoichiometry – use mole ratios from balanced equations.
- Law of Conservation – mass before = mass after.
- Mole concept – 1 mol = (6.022\times10^{23}) entities.
- Molar mass – grams per mole; use to convert between mass and moles.
- Density – (