Chemistry
Chemistry items cover atomic structure, the periodic table, bonding, the mole concept, acids and bases, metals, and basic organic and environmental chemistry. Expect balanced-equation questions, simple calculations and qualitative tests.
Core rules and concepts
- Atom: protons define the element (atomic number Z); mass number A = protons + neutrons; isotopes differ in neutrons only.
- Electronic configuration fills 2, 8, 8; outer-shell electrons determine group and chemical behaviour.
- Bonding: ionic between metals and non-metals (electron transfer), covalent between non-metals (sharing), metallic within metals.
- Mole: n = m/M; one mole contains 6.022 × 10²³ particles and occupies 22.4 dm³ of any gas at STP.
- Acids and bases: acids donate H⁺, bases accept H⁺; acid + base → salt + water; pH below 7 is acidic.
- Oxidation is loss of electrons and reduction is gain (OIL RIG); redox always occurs together.
- The reactivity series predicts displacement: a more reactive metal displaces a less reactive one from its salt.
- Air is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide.
Worked example
What is the mass of 0.25 mol of calcium carbonate, CaCO₃? Molar mass = 40 + 12 + (3 × 16) = 100 g/mol. Then m = n × M = 0.25 × 100 = 25 g. If the question had asked for the number of formula units instead, multiply 0.25 by 6.022 × 10²³ to get about 1.5 × 10²³.
Traps that cost marks
- Writing unbalanced formulae such as CaCl instead of CaCl₂.
- Applying the 22.4 dm³ molar volume to solids or liquids.
- Confusing mass number with relative atomic mass.
- Mixing up indicator colours for acids and alkalis.
Always balance the equation before attempting a mole ratio.