Science: Core Class-Level Concepts and Their Applications
Science in the NSO tests class-level concepts as a connected system. Questions often merge metals and non-metals with bonding, carbon compounds with homologous series, reproduction with plant and human physiology, electricity with household circuits, magnetic effects with direction rules, and control and coordination with reflex and hormone feedback.
Core rules
- Reactivity series predicts displacement: a more reactive metal displaces a less reactive metal from its salt solution, e.g. Zn + CuSO4 -> ZnSO4 + Cu.
- Metal oxides are usually basic; non-metal oxides are acidic. Amphoteric oxides such as Al2O3 and ZnO react with both acids and bases.
- Carbon is tetravalent and shows catenation. Saturated hydrocarbons burn with a clean flame; unsaturated ones tend to burn with a sooty flame. Homologous series members differ by -CH2-.
- Ohm's law is V = IR. Series resistance adds; parallel resistance obeys 1/R = 1/R1 + 1/R2. Electrical power is P = VI = I^2R.
- Magnetic field lines run from north to south outside a magnet; the right-hand thumb rule gives field direction around a current-carrying conductor.
Worked example
A 6 ohm and a 3 ohm resistor are connected in parallel across 12 V. Find the total current.
Parallel resistance: R = (6 x 3)/(6 + 3) = 2 ohm. Total current I = V/R = 12/2 = 6 A. Check by branches: 12/6 = 2 A and 12/3 = 4 A; 2 + 4 = 6 A.
Mark-losing traps
Using the series formula for parallel resistors; calling an alloy a compound; confusing pollination with fertilisation; saying field lines start at the south pole; assuming a reflex action is first processed by the brain.