PNG Grade 10 Science common mistakes
PNG Grade 10 Science · Understand the traps and how to avoid them.
When explaining why a wilted plant recovers after watering, the candidate writes 'the plant drinks water because it is thirsty' or 'water goes into the cell to fill it up', naming no direction of movement and no concentration difference.
Everyday PNG phrasing ('the plant drinks') is used in class talk, and students retell the phenomenon instead of the mechanism; they have memorised the word 'osmosis' but not the sentence pattern examiners reward.
What to do instead
Practise a fixed template until it is automatic: 'Water moves by osmosis from a region of higher water concentration (the soil) to a region of lower water concentration (the vacuole) through the partially permeable cell membrane.' Say it for three cases: watering a wilted plant, a freshwater fish in sea water, and a root hair cell absorbing water.
In a monohybrid cross the candidate puts both parents' genotypes along the top of the Punnett square, or writes the phenotype ratio (3 tall : 1 short) as if it were the genotype ratio, and labels a heterozygous plant 'pure'.
The mechanics of the grid are copied from the board but the meaning of 'one allele from each parent' is never practised, so students cannot tell which row belongs to which parent, and the words genotype/phenotype/heterozygous are learned as vocabulary without examples.
What to do instead
Always draw the cross in four labelled steps: parent genotypes (Tt x Tt), gametes (T, t and T, t), grid, then a sentence stating both ratios. Do five crosses including a test cross (Tt x tt) where the ratio is 1:1, and write the phenotype next to every genotype so the two words stay separate.
In food web questions the arrows are drawn pointing from the predator to the prey, and the candidate says energy is 'recycled' through the ecosystem or that a hawk is a 'primary consumer' because it is big.
Arrows in everyday diagrams (flow charts, family trees) point to what comes next, so the convention 'arrow shows direction of energy flow' is counter-intuitive; energy and nutrients are also confused because both are described as 'moving through the food chain'.
What to do instead
Annotate one food web in class with the rule written on it: 'arrow points to the eater - it shows where the energy goes'. Then state the two separate facts: nutrients are recycled by decomposers, energy is not recycled - about 90% is lost at each level as heat and in waste, which is why food chains are short and pyramids narrow upward.
Chemical equations are left unbalanced or written with element symbols instead of formulae for the diatomic gases, e.g. 'H + O -> HO' or 'Mg + O2 -> MgO', and state symbols are omitted when the question asks for them.
Students move from word equations to symbol equations in one step and treat the formula as a label for the substance rather than a count of atoms; they also run out of time and stop at the first version that looks finished.
What to do instead
Use a counting ritual on every equation: list each element, count atoms on both sides, adjust coefficients only (never subscripts), recount. Memorise the seven diatomic elements (H2, N2, O2, F2, Cl2, Br2, I2) with the mnemonic 'HOFBrINCl', and practise balancing magnesium burning, hydrogen burning, and hydrochloric acid plus sodium hydroxide.
When asked to explain the bonding in sodium chloride and in carbon dioxide, the candidate says sodium chloride is held together by 'shared electrons' and describes it as small separate NaCl molecules, while covalent bonding is described as electrons being 'given away'.
The two mechanisms are learned in the same lesson and sound similar in notes; the word 'bond' suggests a physical object joining particles, and diagrams showing dots and crosses look alike whether the electrons are transferred or shared.
What to do instead
Apply a decision rule first: metal + non-metal means ionic (electrons transferred, ions formed, giant lattice, high melting point, conducts when molten or dissolved); non-metal + non-metal means covalent (electrons shared, molecules, low melting point, does not conduct). Draw the dot-and-cross diagram for NaCl showing the full outer shells of Na+ and Cl- before drawing CO2.
For an acid-base question the candidate identifies substances only by taste or by 'it feels soapy', says an acid plus a base gives only water, and predicts that a strong acid will always have a pH above 7 once it is diluted.
The pH scale is memorised as a number line without being tied to real substances, and neutralisation is remembered as one product because the word 'neutral' suggests nothing is formed; household experience substitutes for the indicator tests the question is testing.
What to do instead
Build a reference table of pH values for substances students actually meet (lime juice, vinegar, pure water, soap, bleach, drain cleaner) and use indicator results, not senses, as evidence. Write neutralisation always as acid + base -> salt + water with the salt named from the metal and the acid (HCl + NaOH -> NaCl + water), and note that dilution moves pH towards 7 but does not cross it.
Numbers are substituted straight into v = d/t, F = ma or V = IR without converting to SI units, so a distance given in centimetres or a mass in grams is divided by a time in seconds and the answer is left with no unit or the wrong one.
The formula triangles are drilled but the units attached to each quantity are not, and students treat 'cm' and 'm' as interchangeable labels rather than different amounts; there is also no habit of writing the final unit.
What to do instead
Force a three-line layout for every calculation: (1) list the given values and convert them (÷100 for cm to m, ÷1000 for g to kg, ×60 for minutes to seconds), (2) write the rearranged formula, (3) write the answer with its unit and a sense check. Practise ten conversions alone before doing any calculation.
Circuit questions are answered with 'the current is used up as it goes around' or 'two bulbs in series get less current because the second bulb takes some', and two cells in parallel are assumed to double the voltage.
The water-pipe analogy taught in lower grades implies current is consumed; series and parallel rules are memorised as two lists that get swapped under exam pressure because they are not linked to a mental picture of the paths the current can take.
What to do instead
Draw the two circuits side by side and label them with the rules that follow from the paths: in series there is one path, so current is the same everywhere and voltages add; in parallel there are branches, so voltage is the same across each branch and branch currents add back to the total. Include one worked example with three ammeter readings in parallel to prove the currents add.
Plate boundaries are described as 'the plates crash' or 'the ground shakes because it is hot underground', and the same answer is reused for a mid-ocean ridge and a deep ocean trench; PNG earthquakes and volcanoes are explained without naming the Pacific Ring of Fire or a specific boundary type.
The three boundary types are memorised as three names rather than three processes, and because PNG sits at a convergent boundary, students assume every boundary simply collides; the local hazard is experienced as a fact of life rather than as evidence to be interpreted.
What to do instead
Make a three-row table with columns for plate motion, landform produced and PNG example: divergent (plates move apart, magma rises, ridge and new crust), convergent (oceanic under continental, subduction, trench plus volcanic arc plus earthquakes), transform (plates slide past, fault, earthquakes, no volcanoes). Then label a simple sketch map of the Pacific Ring of Fire and mark where PNG's own volcanoes sit.
Seasons are explained as 'the Earth is closer to the Sun in summer', solar and lunar eclipses are drawn in the wrong order (or labelled 'the Moon blocks the Earth from the Sun' for a lunar eclipse), and rotation and revolution periods are confused when giving the length of a day and a year.
The tilted-axis explanation is genuinely counter-intuitive and competes with the everyday experience of heat and sunshine; the eclipse diagrams are copied as shapes without writing which body is in which shadow, and the two time periods are learned as a matching pair rather than as two separate motions.
What to do instead
Anchor seasons on the tilt: draw the Earth in four orbital positions with the axis always pointing the same way, and mark which hemisphere gets more direct sunlight. For eclipses, write the order out in words before drawing - solar: Sun, Moon, Earth; lunar: Sun, Earth, Moon - and note that a lunar eclipse happens at full moon and a solar eclipse at new moon. State that one rotation on the axis gives one day and one revolution around the Sun gives one year.