What the investigation does
The student prepares a pooled silver nanoparticle solution and compares lithium, sodium, potassium and ammonium chloride at five added-solution concentrations. Four trials per combination generate 80 spectra; time-dependent absorbance near 407 nm is used as a proxy for aggregation rate, alongside spectra collected after one minute.
A calibration derived from published peak-wavelength and diameter data estimates particle size. The results support greater aggregation with increasing salt concentration but do not support a simple cation-radius ranking. The evaluation identifies a large diameter uncertainty, limited wavelength resolution, sedimentation and handling effects.
Why this example is worth reading
The design separates salt identity from concentration and controls the starting nanoparticle mixture by pooling successful preparation batches.
The student adapts data capture to the equipment by recording the changing spectrum display, allowing time comparisons despite software limitations.
The discussion tests two hypotheses separately and accepts the failure of the proposed ionic-radius ordering rather than forcing all observations into it.
What to examine critically
Absorbance change is an indirect indicator of aggregation, and the wavelength-to-diameter calibration has an uncertainty of about 10 nm. Some relative size uncertainties exceed 50%, limiting fine comparisons.
Sedimentation can bias the sampled optical path, and one-minute spectra need not represent a true endpoint in every condition. Avoid presenting an empirical salt ranking as proof of a single causal mechanism.
Apply it to your own work
Explain why the chosen observable is an appropriate proxy for the process being investigated.
Carry calibration and instrument limitations into the strength of the final claim.
Distinguish support for one hypothesis from rejection of another within the same experiment.
Essay & assessment material
Read the original material alongside the breakdown, or download it to keep.
The source includes an examiner mark sheet on PDF page 3 recording 34/36 and handwritten annotations in the essay. No separate narrative commentary PDF is included; the teaching analysis does not attribute untranscribed annotations to the examiner.
Source & assessment context
IB Chemistry English exemplars, May 2015. The source PDFs retain their original attribution. The analysis on this page is AsterDraft’s independent teaching commentary.
Examiner mark: 34/36 (legacy criteria). The source includes an examiner mark sheet on PDF page 3 recording 34/36 and handwritten annotations in the essay. No separate narrative commentary PDF is included; the teaching analysis does not attribute untranscribed annotations to the examiner.