A water company routinely monitors the quality of drinking water supplied to homes. Chemists at the treatment plant use instrumental methods of analysis to check for trace amounts of metal ions and organic contaminants in water samples. Explain why instrumental methods are used for this purpose rather than traditional chemical tests.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Water companies are legally required to ensure drinking water meets strict safety standards, including limits on certain metal ions measured in parts per million (ppm).
Model answer (5 marks)
Instrumental methods are more sensitive than traditional chemical tests, allowing detection of very low concentrations of metal ions and organic contaminants.
They can identify and quantify trace amounts that colour‑impression or precipitation tests cannot, ensuring compliance with ppm limits.
Results are obtained much faster; for example, spectrophotometry or ICP‑MS gives readings in minutes versus hours for manual tests.
The techniques are more accurate and reliable, giving reproducible data with lower analytical error and less operator bias.
Finally, many instruments can analyse large numbers of samples automatically or in parallel, increasing throughput and reducing labour costs.
They can identify and quantify trace amounts that colour‑impression or precipitation tests cannot, ensuring compliance with ppm limits.
Results are obtained much faster; for example, spectrophotometry or ICP‑MS gives readings in minutes versus hours for manual tests.
The techniques are more accurate and reliable, giving reproducible data with lower analytical error and less operator bias.
Finally, many instruments can analyse large numbers of samples automatically or in parallel, increasing throughput and reducing labour costs.
Examiner tips
- Use the exact points from the mark scheme; each point counts one mark.
- Show understanding of sensitivity, speed, accuracy and throughput.
- Keep sentences concise and use technical terms like ‘sensitivity’, ‘trace amounts’, ‘analytical error’.
Common mistakes
- Confusing sensitivity with selectivity; both are important but the scheme rewards sensitivity. Failing to mention the speed or automation aspects. Using vague language such as ‘better’ instead of ‘more accurate’ or ‘more reliable’.
Mark scheme (5 marks)
- Instrumental methods are more sensitive than traditional chemical tests
- Instrumental methods can identify/detect very small quantities (traces) of substances that traditional tests cannot
- Instrumental methods give faster/quicker results than traditional chemical tests
- Instrumental methods are more accurate/reliable than traditional chemical tests
- Instrumental methods can analyse many samples or handle large numbers of samples automatically/at the same time
Key terms in this question
instrumental methods · trace amounts · traditional chemical tests
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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