Botswana JCE Science (Syllabus 14) cheat sheet

Independent variable: the factor deliberately changed; dependent variable: the factor measured in response; all other factors are controlled (kept constant).

Hypothesis is a testable prediction of the form 'If X is changed, then Y will happen, because Z'.

Fair test requires one independent variable only; repeat readings and average them to reduce random error.

Accuracy = closeness to the true value; precision = closeness of repeated readings to each other; a reading can be precise but inaccurate.

Mean = sum of readings ÷ number of readings; the mean reduces the effect of random variation.

Anomalous result = a reading far from the pattern; it should be checked and, if explained, excluded and reported.

Line graph used when both variables are continuous; bar chart used when the independent variable is categoric (named groups).

SI base units: metre (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount).

A conclusion must refer back to the hypothesis and be supported by the data collected, not by expectation.

Particle model: all matter is made of particles in constant motion; solids vibrate in fixed positions, liquids slide past each other, gases move freely and far apart.

Melting and boiling are physical changes: mass is conserved, particles separate but no new substance forms.

Chemical change signs: new substance formed, colour change, gas evolved, heat or light released, change not easily reversed.

Mixture separated by filtration (insoluble solid from liquid), evaporation or crystallisation (dissolved solid from solution), distillation (solvent from solution), magnetism (magnetic solid from non-magnetic).

Elements are single types of atom; compounds contain two or more elements chemically combined in fixed ratios; mixtures are not chemically combined.

Atomic structure: protons (positive, in nucleus), neutrons (neutral, in nucleus), electrons (negative, in shells); atoms are neutral because protons equal electrons.

Acids turn blue litmus red and have pH below 7; alkalis turn red litmus blue and have pH above 7; pH 7 is neutral.

Acid + base → salt + water (neutralisation); acid + metal → salt + hydrogen gas; acid + carbonate → salt + water + carbon dioxide.

Density = mass ÷ volume (g/cm³ or kg/m³); an object floats in a liquid if its density is less than the liquid's density.

Cell organelles: nucleus controls activities and holds genetic material; cytoplasm is where reactions occur; cell membrane controls entry and exit; mitochondria release energy by respiration; chloroplasts carry out photosynthesis.

Plant cells have a cellulose cell wall and often chloroplasts and a large vacuole; animal cells lack a cell wall and chloroplasts.

Photosynthesis: carbon dioxide + water → glucose + oxygen, using light energy trapped by chlorophyll; occurs in chloroplasts.

Respiration: glucose + oxygen → carbon dioxide + water + energy; happens in all living cells, day and night.

Food chain arrows show the direction of energy flow; producers are at the base; decomposers break down dead matter and return nutrients to the soil.

Human digestive order: mouth → oesophagus → stomach → small intestine → large intestine; absorption of digested food occurs mainly in the small intestine.

Blood transport: red blood cells carry oxygen using haemoglobin; white blood cells defend against pathogens; plasma carries dissolved food and waste; platelets help clotting.

Breathing mechanics: inhale - diaphragm contracts and flattens, ribs move up and out, volume increases and pressure falls; exhale is the reverse.

Classification hierarchy: kingdom, phylum, class, order, family, genus, species; the binomial name is genus + species.

Force = mass × acceleration (F = ma); force is measured in newtons (N); weight = mass × gravitational field strength (W = mg, g ≈ 10 N/kg on Earth).

Friction opposes motion between surfaces in contact; it produces heat and can be reduced with lubricants or rollers.

Pressure = force ÷ area (P = F/A); smaller area gives greater pressure for the same force (sharp edges, nails).

Work done = force × distance moved in the direction of the force (W = Fd), measured in joules.

Speed = distance ÷ time; average speed = total distance ÷ total time; velocity includes direction.

Energy stores: kinetic (½mv²), gravitational potential (mgh), elastic, chemical, thermal, nuclear; energy is conserved but transfers are not always useful.

Series circuit: one path, same current everywhere, supply voltage shared between components. Parallel circuit: branches, voltage same across each branch, currents add.

Ohm's law: V = IR; resistance in ohms (Ω); resistors in series add; adding branches in parallel lowers total resistance.

Magnetism: like poles repel, unlike poles attract; magnetic field lines run from north to south outside the magnet.

Earth's structure: crust (thin, solid rock), mantle (hot, semi-solid rock where convection occurs), outer core (liquid iron and nickel), inner core (solid iron and nickel).

Earth's rotation on its axis (about 24 hours) causes day and night; revolution around the Sun (about 365¼ days) with a tilted axis causes the seasons.

Moon phases arise from the changing angle between the Sun, Earth and Moon; a lunar eclipse occurs when Earth blocks sunlight from the Moon.

Solar system order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

The Sun's energy reaches Earth by radiation and drives weather, winds, the water cycle and photosynthesis.

Water cycle stages: evaporation, transpiration, condensation, precipitation, run-off and infiltration.

Igneous rock forms from cooled magma or lava; sedimentary rock forms from compacted layers of sediment and may contain fossils; metamorphic rock forms when existing rock is changed by heat and pressure.

Soil components: mineral particles, humus, air, water and living organisms; humus improves water retention and fertility.

Air composition by volume: about 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide, plus variable water vapour.