13. Assessed scientific skills

Exam analysis

Most tested topics

13.1 Handling information and problem-solving

Start from the task, not from the amount of information available. Select only facts, values and relationships that can help answer the stated chemistry question, then organise them so the reasoning can be checked.

Step	Action
locate	scan headings, labels, units, keys and named substances to find candidate evidence
select	keep information that bears on the exact command and reject distractors
organise	group related facts, put a process in order or align values in a table
present	use clear chemical names/symbols, headings, units and an answer form suited to the task

For a question comparing two reaction rates, select the changing quantity, time values and controlled conditions; align corresponding values before comparing. A decorative detail that cannot affect rate is not evidence.

More copied information is not automatically a better answer. Omitting a unit, table heading, key condition or substance identity can make otherwise correct evidence ambiguous.

Translation changes the representation while preserving the substances, quantities, directions and relationships in the source.

Source form	Translation check
verbal description	chemical symbols/formulae name the same substances and states
symbol or equation	words preserve reactants, products and proportions
data table	graph axes use the correct variables, scale and units; every plotted point matches a row
graph	numerical or verbal statement reports the correct coordinates, gradient direction or interval

The equation $2\mathrm{H_2}+\mathrm{O_2}\rightarrow2\mathrm{H_2O}$ translates to a 2:1:2 reacting ratio of particles or moles; it does not mean the masses are in that ratio.

A translation is not an interpretation. First preserve what the source explicitly shows; only then add a conclusion or explanation if the question asks for one.

Data manipulation is a traceable chain: align units, choose the relationship, substitute values with units, calculate without premature rounding and report a sensible precision.

Check	Question to ask
identity	which quantity does each value represent?
units	must cm³ become dm³, minutes become seconds or percentages become fractions?
operation	does the required relationship call for a ratio, difference, mean, gradient or rearranged formula?
arithmetic	were brackets, powers and proportional factors applied to the correct values?
output	is the unit present and is the precision justified by the data?

Estimate the order of magnitude before calculating and substitute the result back into the relationship afterwards. These two checks expose many misplaced decimal points and inverted ratios.

Do not round every intermediate step. Keep extra calculator digits during working and round only the final result unless the question specifies otherwise.

A pattern is a repeated or structured feature; a trend is the overall direction of change; a conclusion is a claim that answers the question using the evidence.

Evidence feature	Defensible report
values mostly rise as the independent variable rises	state an increasing trend and name both variables
rate of change itself changes	describe the curve or compare gradients over stated intervals
one point departs from the rest	retain it and identify it as a possible anomaly
evidence answers the investigation question	form a bounded conclusion and cite the supporting values or trend

Use comparative language tied to data: ‘as temperature increases from 20 °C to 40 °C, the measured rate increases’. Add ‘approximately’, ‘levels off’ or ‘except at…’ when the evidence requires it.

Do not explain why a trend occurs when the command is only ‘describe’. Do not claim causation from a pattern alone, and do not hide an anomalous value.

A reasoned explanation links an observed phenomenon or relationship to a relevant syllabus principle through an explicit causal chain.

Part	Function
phenomenon	state exactly what changes or is observed
evidence	cite the relevant value, pattern, equation or comparison
chemistry principle	select the particle, bonding, energy, equilibrium, rate or other taught idea that applies
causal link	show how the principle produces the observation
boundary	qualify the claim if another variable, anomaly or limited range matters

‘The reaction is faster at the higher concentration because there are more reacting particles per unit volume, so successful collisions occur more frequently’ links condition → particle model → rate.

Restating the observation is not an explanation. Every ‘because’ must introduce a mechanism or principle that actually accounts for the stated pattern.

A prediction extends an established relationship to a new case. State the relationship, check that the new case is comparable, then give the predicted outcome and any justified limit.

Step	Prediction move
identify	find the relevant pattern, family behaviour or proportional relationship
match	check that the new substance or condition shares the feature controlling that relationship
extend	infer the expected product, direction or approximate value
qualify	distinguish interpolation from less-certain extrapolation and retain the evidence range

If barium nitrate is stated to decompose in the same way as magnesium nitrate, transfer the given nitrate-decomposition pattern: predict nitrogen dioxide and oxygen as the gaseous products.

A prediction is not a guess based only on a familiar name. State the relationship that licenses the transfer, and do not invent a precise numerical value when the evidence supports only a direction or range.

An unfamiliar context changes the surface details, not the syllabus principles. Convert the prompt into known quantities, relationships and constraints before choosing a route.

Stage	Action
decode	identify the command, target quantity or required qualitative claim
inventory	list given data, units, conditions and relevant chemistry principles
connect	draw a short chain from givens to target; split a multi-step problem into intermediate results
execute	show substitutions, equations, comparisons or deductions in a checkable order
verify	test units, magnitude, chemical feasibility and whether the final statement answers the command

For a qualitative problem, eliminate claims that contradict the stated evidence, then connect the remaining evidence to a taught principle. For a quantitative problem, preserve units and proportional factors at every step.

Do not search for a memorised question with identical wording. Use only principles in the syllabus, and reject a numerical answer that is mathematically produced but chemically impossible or in the wrong unit.

13.2 Experimental skills and investigations

Choose a technique or apparatus by the job it must do, the required measurement quality and the substances involved. Safety then follows from the specific hazard and exposure route, not from a generic precaution list.

Need	Selection reason	Use or safety control
measure an accurate fixed liquid volume	volumetric pipette is designed to deliver one calibrated volume	use a pipette filler, never mouth pipette
deliver a variable liquid volume accurately	burette gives initial and final readings for volume delivered	clamp vertically; remove the filling funnel before readings
collect and measure a gas	gas syringe measures volume directly	make connections gas-tight before the reaction starts
heat a flammable mixture	avoid a naked Bunsen flame	use an appropriate non-flame heat source and keep ignition sources away
use a reactive or corrosive substance	minimise contact and splashing	wear eye protection and use the stated screen, distance or handling tool where justified

Follow the method in order because timing, mixing, heating and collection steps can affect both safety and the validity of the result. Identify apparatus from its function and explain why it suits the task.

A precaution must reduce the stated risk: eye protection reduces injury from splashes but does not prevent gas escape or ignition. Do not claim that every hazard is removed by gloves alone.

A complete plan changes one independent variable, measures one dependent variable and controls other factors that could alter the result, while specifying a safe repeatable method and how the data will address the investigation aim.

Planning decision	What to specify
question and prediction	expected relationship with a chemistry reason
independent variable	suitable number and range of values, including how each value is set
dependent variable	exactly what is measured, with apparatus and units
controlled variables	how each relevant factor is kept constant and why it could otherwise affect the result
method	quantities, apparatus, ordered steps, endpoint and repeat strategy
risk control	hazard, possible harm and a matching precaution
record and process	results-table headings, calculation or graph, and the comparison used for the conclusion

To investigate how acid concentration affects reaction rate, vary concentration across a suitable range, measure gas volume against time, and keep temperature, acid volume, carbonate mass and carbonate particle size constant.

‘Keep everything the same’ is not a plan. Name the relevant control variables and say how they are controlled. A variable is not controlled merely because it is measured.

Record what is seen or measured at the time it occurs. Observations describe evidence; measurements pair a numerical reading with a unit; estimates report a justified approximate value.

Evidence type	Strong record
qualitative observation	specific change such as ‘effervescence’, ‘solid disappears’ or ‘solution turns blue’
analogue reading	read at the correct eye position and to the nearest half-scale division where required
digital reading	record all displayed digits with the unit
repeated measurements	keep every reading, including a possible anomaly, before calculating or selecting concordant results
results table	independent variable first; units in headings; consistent precision within each measured column

Take enough measurements to reveal the relationship and repeat where appropriate. Constant mass is established by heating, cooling and weighing again until successive masses agree, not by assuming one heating was enough.

Do not change a reading to match the expected result and do not write an inference as an observation. ‘Hydrogen formed’ is an interpretation; ‘a gas was produced that gave a pop with a lighted splint’ records the evidence.

Interpretation turns processed observations or data into a conclusion; evaluation asks how strongly the data support it. Keep the observed pattern, the chemical explanation and the quality judgment distinct.

Stage	Evidence move
process	calculate required quantities or prepare values for plotting without premature rounding
represent	use an appropriate graph and best-fit line when the variables are continuous
interpret	describe the pattern with variables and cite relevant values or observations
conclude	answer the investigation question and justify it from the evidence
evaluate	assess repeats, spread, resolution, range and anomalies; state how each limits confidence
use graph	interpolate within the measured range; treat extrapolation beyond it as less certain; determine requested gradient or intercept

Concordant repeats are close enough to support a representative result under the stated criterion. If a result lies away from the pattern, keep it visible, repeat that condition if possible and exclude it from a mean only with a recorded reason.

A best-fit line need not pass through every point or the origin. An anomaly is not automatically a mistake, and a trend alone does not prove a proposed mechanism.

A useful evaluation links a concrete weakness to its likely effect on the result, then proposes a feasible change that reduces that effect while preserving the investigation's purpose.

Limitation or error source	Likely effect	Targeted improvement
gas escapes before the bung is fitted	measured gas volume is too low	assemble and test a gas-tight system and start collection as reactants mix
heat transfers to the surroundings	measured temperature change is smaller in magnitude	use an insulated container with a lid
liquid level is read from above or below	parallax shifts the volume reading	read the scale at eye level at the correct meniscus
one reading is used	random variation cannot be judged	repeat, identify concordant values and calculate a representative result
independent-variable range is narrow	the pattern or turning point may be missed	use more suitably spaced values across a wider safe range
an uncontrolled factor changes	comparison is not fair	state how that factor will be held constant

Write evaluation as cause and remedy: ‘Some gas escaped, so the measured volume was too low; make the apparatus gas-tight before mixing.’ This is testable and tied to the result.

Avoid vague labels such as ‘human error’, ‘use better equipment’ or ‘repeat for accuracy’. Name the source, its direction or consequence where known, and how the proposed change addresses it; repeats mainly improve reliability and reveal random variation.

13.3 Mathematical and data-presentation skills

Treat every chemistry calculation as one controlled chain: express the data in compatible forms and units, choose the relationship, rearrange if necessary, substitute, calculate, then round only the final answer.

Requirement	Reliable move
fraction, decimal, percentage	convert to the form needed; percentage = fraction × 100%
ratio	divide all terms by the same factor; preserve the order of quantities
reciprocal	$1/x$ reverses a non-zero factor or expresses an inverse relationship
standard form	write $a\times10^n$ with $1\leq a<10$ and integer $n$
equation	keep units consistent, substitute known values and use inverse operations to isolate the unknown
direct proportion	$y=kx$ ; doubling $x$ doubles $y$
inverse proportion	$y=k/x$ ; doubling $x$ halves $y$

$c=\frac{n}{V},\quad n=cV=0.250\times0.0400=0.0100\ \mathrm{mol}$

Keep extra calculator digits through intermediate steps. Report the final result with decimal places or significant figures appropriate to the given data and the measurement context.

Do not round each intermediate value or substitute quantities in inconsistent units. A numerically correct calculator display is not a complete result when a unit is required.

A unit must match the quantity and scale. Convert before substitution when an equation requires consistent units, and attach the unit to the final result.

Quantity	Exact relationship	Smaller unit → larger unit
volume	$1000\ \mathrm{cm^3}=1\ \mathrm{dm^3}$	divide by 1000
mass	$1000\ \mathrm{mg}=1\ \mathrm{g}$ ; $1000\ \mathrm{g}=1\ \mathrm{kg}$	divide by 1000 for each step
energy	$1000\ \mathrm{J}=1\ \mathrm{kJ}$	divide by 1000
pressure	$1000\ \mathrm{Pa}=1\ \mathrm{kPa}$	divide by 1000

Term	Meaning used in measurement
radius / diameter	centre-to-edge distance / full distance through the centre; diameter = 2 × radius
circumference	distance around a circle
square / rectangle	four-sided shapes used for area; a square has equal sides
diagonal	line joining opposite corners
angle / curve	amount of turn / a line whose direction changes

$40.0\ \mathrm{cm^3}=0.0400\ \mathrm{dm^3}$ ; the conversion changes the numerical value but not the physical volume.

Cubic units are already volume units: the syllabus conversion is $1000\ \mathrm{cm^3}=1\ \mathrm{dm^3}$ , not a factor of $10$ and not $10^6$ for this pair.

Choose a representation that matches the variables, then read only what its scale and range support. A graph displays a relationship; a mean combines comparable repeated values.

Task	Mathematical tool	Interpretation
show how one continuous variable changes with another	line graph or scatter graph with best-fit line/curve	direction, shape, intercept and values within the plotted range
compare separate categories	suitable chart	compare bar lengths or sector proportions using the labelled scale/key
estimate between measured points	interpolation	estimate within the evidence range
extend beyond measured points	extrapolation	prediction with greater uncertainty
rate of change on a straight line	gradient $=\Delta y/\Delta x$	include units from the two axes
starting value	intercept	value where the line crosses an axis
summarise repeats	mean $=\Sigma x/n$	use only values selected by the stated repeat/anomaly rule

Direct proportionality appears as a straight line through the origin: $y=kx$ . A straight line with a non-zero intercept is linear but not directly proportional.

Interpolation and extrapolation are not equally secure. Do not claim direct proportion merely because the graph is straight, and do not average quantities that are not comparable repeats.

The recorded value must reflect what the instrument can resolve. Read an analogue scale to the nearest half of the smallest division where required and preserve appropriate precision in the table.

Feature	Required convention
analogue reading	interpolate to the nearest half-smallest division where appropriate
measured value	decimal places and significant figures reflect the instrument's precision
calculated value	use the same number of significant figures as the least precise raw datum used in that calculation
table heading	quantity or symbol followed by solidus and unit, e.g. `time / s`
table body	numbers only; do not repeat units in each cell
repeated readings	record each reading before calculating or selecting a representative value
ratio	write in the order requested as $x:y$

Values measured with the same instrument in one table column should normally show consistent decimal places, including trailing zeros that communicate the instrument's resolution.

Do not add unjustified digits to a coarse reading or remove meaningful trailing zeros. Significant figures describe precision; they do not make an inaccurate method accurate.

A graph must make the data relationship readable without distorting it: variables, units, scale, plotted points and the best-fit relationship all carry meaning.

Step	Construction rule
axes	independent variable on $x$ and dependent variable on $y$ , unless instructed otherwise
labels	transfer quantity and unit from the table heading, e.g. `time / s`
scale	use sensible 1, 2 or 5-based intervals and more than half the grid in both directions
points	mark small crosses or encircled dots to half a smallest square
best fit	draw one thin, smooth line or curve with roughly even scatter on both sides; ignore a clearly anomalous point when fitting
readings	interpolate/extrapolate and read values or intercepts to half a smallest square
straight-line gradient	use $\Delta y/\Delta x$ from a marked triangle whose hypotenuse spans at least half the best-fit line

Gradient units are $y$ -axis units divided by $x$ -axis units. Use two points on the best-fit line or its large gradient triangle, not necessarily two experimental points.

Do not join plotted points dot-to-dot unless instructed. A best-fit line need not pass through the origin or every point, and an anomaly remains plotted even when ignored for the fit.

For Extended candidates, the gradient of a curve at one point is the gradient of the tangent there. The tangent is a straight line with the same local direction as the curve.

Step	Action
1	locate the required point on the smooth curve
2	draw a straight tangent that touches the curve at that point and follows its local direction, with roughly balanced separation on either side
3	choose two well-separated points on the tangent, not on the curve
4	draw a large gradient triangle and read the coordinate differences from the axes
5	calculate $\Delta y/\Delta x$ and attach $y$ -unit per $x$ -unit

$\text{instantaneous gradient}=\frac{y_2-y_1}{x_2-x_1}$

A steeper tangent has a larger gradient magnitude. A horizontal tangent has gradient zero; a downward tangent gives a negative gradient with the usual axis directions.

Do not calculate between two points on the curve: that gives an average gradient over an interval. The tangent must be drawn at the specified point before the triangle is chosen.

13. Assessed scientific skills

Exam analysis

Most tested topics

13.1 Handling information and problem-solving

Select and present the information a chemistry task needs

Translate chemical information without changing its meaning

Manipulate chemistry data accurately

Separate patterns, trends and conclusions

Build a reasoned chemical explanation

Make predictions from chemical relationships

Solve unfamiliar qualitative and quantitative problems

13.2 Experimental skills and investigations

Choose apparatus and control laboratory risk

Plan a fair and testable chemistry investigation

Record observations and measurements faithfully

Interpret results and judge data quality

Turn method limitations into specific improvements

13.3 Mathematical and data-presentation skills

Control the numerical chain in chemistry calculations

Choose and convert measurement units

Choose and interpret graphs, charts and statistics

Record readings and tables with matched precision

Construct and analyse a chemical-data graph

Find the instantaneous gradient of a curve