Chemistry
Core concepts
Focus on atomic structure (protons, neutrons, electrons, isotopes), the periodic table and periodic trends, chemical bonding (ionic, covalent, metallic), formula writing and balancing equations, the mole concept, and basic acid–base and redox ideas. Know the difference between physical and chemical change and the signs of a reaction.
Common traps
- Balancing by changing subscripts - never; only coefficients change.
- Confusing atomic number with mass number.
- Mole calculations without converting grams to moles first (n = m/M).
- Ionic vs. covalent classification of compounds formed between metals and non-metals.
- pH scale direction: lower pH means more acidic, not more basic.
Worked example
Balance: Fe + O₂ → Fe₂O₃. Put 4Fe + 3O₂ → 2Fe₂O₃. Check: Fe 4 = 4; O 6 = 6. Balanced.
Rule of thumb
For stoichiometry, always convert to moles, use the balanced equation's coefficients as a ratio, then convert back to grams or litres.