Practical Skills in Chemistry A2 II

Keep observations separate from explanations. 'Brown fumes form', 'the solution turns green', 'the temperature falls' and 'effervescence occurs' are observations; naming the species or process responsible is the inference. If asked what is seen, do not replace visible evidence with 'a gas is produced'.

Use only information relevant to the actual setup. Check limiting reagents, state, concentration, temperature, time and apparatus before transferring a familiar rule to an unfamiliar practical.

When investigating how concentration affects rate, concentration is independent and time or initial rate is dependent. Keep temperature constant with a thermostated bath, total volume constant by adjusting water, and the same quantities and surface area of other reactants. Use the same endpoint rule and mixing procedure each run.

Control choices come from the chemical model: temperature changes the rate constant, total volume changes concentrations, and particle size changes available surface area. State why each uncontrolled factor would change the dependent variable.

Record raw readings before derived values. For a burette, retain initial and final readings as well as titre; for repeated data, show all trials, identify any justified exclusion and calculate a mean from the retained values. Match decimal places to the instrument resolution.

Give units once in the heading, not after every entry. Use chemical symbols, state labels and significant figures consistently so another person can reconstruct every calculation from the table.

State the relationship and its boundary, then quote data or graph features that support it. Compare an experimental value with a reference only after applying uncertainty. A value inside the uncertainty interval is consistent with the reference; that does not prove the method is error-free.

A yield below 100% can arise from incomplete reaction, side reactions, transfer loss, product remaining dissolved or loss during purification. Identify the stage and direction rather than writing only 'human error'.

For a difference between two readings, include uncertainty from both readings; a burette titre therefore contains two reading uncertainties. For multiplication or division, add percentage uncertainties as the conservative course method. Larger measured quantities give smaller percentage uncertainty when absolute uncertainty is fixed.

If a $25.00\,\mathrm{cm^3}$ pipette has absolute uncertainty $\pm0.06\,\mathrm{cm^3}$, its percentage uncertainty is $0.06/25.00 imes100=0.24\%$. Report an uncertainty with its unit or as a percentage, not as an unlabelled number.

Write each comment as: specific feature → effect on result → improvement. For immiscible reactants, continuous stirring increases contact; for a reaction sampled over time, rapid quenching fixes the composition at the sampling time; when one reagent is deliberately in excess, its exact volume may not control calculated moles.

Where possible, predict the direction of error. Continued reaction before titration can increase product measured; loss of product during transfer lowers yield; dilution may change concentration without changing moles.

Use a large triangle on a straight best-fit line, not two raw points unless they lie suitably on the line. For an instantaneous rate, draw a tangent at the specified time and calculate its gradient. Read an intercept or intersection with units and sensible precision.

Interpolation uses the supported data range; extrapolation is less secure. A curved graph has a changing gradient, so quote the position at which a tangent was taken.

Label intermediate quantities so mole ratios and scale factors can be checked. When a graph is required, show the construction on the graph; when a result comes from a calibration line, do not read beyond the calibrated range without flagging extrapolation.

Carry extra digits through intermediate calculations and round only the final result. For multiplication and division, the least precise measured input usually limits the final significant figures; follow any explicit instruction in the question. Exact stoichiometric coefficients and defined conversion factors do not limit precision.

Round an absolute uncertainty sensibly, then round the measured value to the same decimal place: for example, $12.347\pm0.062\,\mathrm{g}$ becomes about $12.35\pm0.06\,\mathrm{g}$. Keep raw table readings at the instrument's consistent decimal places.

Precise but inaccurate results cluster away from the accepted value, often from systematic bias. Accurate but imprecise results scatter around the accepted value, often from random variation. Results may be both, either or neither.

Improve precision by controlling conditions and using finer-resolution apparatus; improve accuracy by calibration, blank correction, complete transfer and a method that measures the intended quantity. Compare differences with combined uncertainty before claiming disagreement.

Check that the expected value lies within the instrument range, the resolution makes percentage uncertainty acceptable, and the material tolerates heat, pressure and reagent. Choose a thermometer/probe whose range covers the whole change, not merely the starting value.

In a permanganate titration, add $\ce{KMnO4}$ from the burette and approach the endpoint dropwise while swirling; the first permanent pale-pink colour is the endpoint. Wash flask walls with deionised water because this changes volume but not analyte moles.

Range is the interval of values an instrument can measure. Resolution is the smallest change its scale or display can distinguish. Select the smallest practical range that still contains every expected reading, because this often provides finer resolution and smaller percentage uncertainty.

For an analogue scale, reading uncertainty is commonly related to half the smallest division; for a digital instrument, use the stated manufacturer uncertainty or display convention. Apply the actual uncertainty supplied in the question when one is given.

Hazard is the inherent capacity to cause harm; risk combines severity with likelihood and exposure. Read the supplied hazard data, identify the route—skin, eyes, inhalation, ingestion, fire or pressure—then state how the control interrupts it.

For concentrated nitric acid and nitrogen dioxide, acid-resistant handling controls corrosive contact while a fume cupboard controls toxic gas inhalation. These are different hazards requiring different controls.

Use fresh portions, correct acidification and sufficient reagent. Record observation first, then inference and, when requested, a net ionic equation with charge and atoms balanced.

Choose tests that separate the remaining candidates and use fresh portions to avoid reagent carry-over. A 2,4-DNPH result finds a carbonyl; Tollens' then distinguishes an aldehyde from a ketone. A negative result is useful only when conditions and reagent quality were valid.

Preserve the distinction between the directly measured quantity and the desired chemical quantity. Every conversion needs an equation, unit and assumption; every evaluation should identify which assumption the method may violate.

Use repeats and uncertainty to judge differences, not appearance alone. Improve the dominant limitation: better insulation for calorimetry, a larger titre for lower percentage uncertainty, rapid quenching for timed samples, or a thermostated bath for rates/equilibria.

Track material at every transfer. Use washings for quantitative transfer, cold wash solvent to limit product loss, an ice-water bath for full surface contact, and a desiccator rather than an oven when heat could melt or decompose the solid.

Explain a low yield by the actual stage—incomplete reaction, side reaction, mechanical transfer, product solubility or purification loss. Explain purity with an independent measurement, not yield alone.