Chemistry • Year 12 • Module 8 • Lesson 8
Heavy Metal Contamination & Analysis
Lock in the key heavy metals, their sources and health effects, and the core vocabulary for AAS-based monitoring — the foundation for every data question on this topic.
1. Term–definition match
Write the matching term from the list into the right-hand column. Each term is used once.
Terms: heavy metal · bioaccumulation · biomagnification · hollow cathode lamp · atomisation · maximum contaminant level (MCL) · EDTA complexometry · detection limit · Beer–Lambert law · ADWG
10 marks
| # | Definition | Matching term |
|---|---|---|
| 1.1 | A dense metal with atomic mass > 20 u that is toxic at low concentrations; examples include Pb, Hg, Cd and As. | |
| 1.2 | The gradual build-up of a contaminant in the tissues of a single living organism over time. | |
| 1.3 | The increase in concentration of a contaminant at each successive trophic level of a food chain. | |
| 1.4 | The element-specific light source used in AAS; contains a cathode made of the target metal and emits only that element’s characteristic wavelengths. | |
| 1.5 | The process in AAS of converting the sample into free ground-state atoms, usually in a flame or graphite furnace, so that absorption can occur. | |
| 1.6 | The highest permissible concentration of a contaminant in drinking water, set to protect health. | |
| 1.7 | A titration method in which EDTA forms stable 1:1 complexes with metal ions; the moles of EDTA used equal the moles of metal ion, allowing metal ion concentration to be determined. | |
| 1.8 | The lowest concentration that an instrument can reliably distinguish from blank signal, often in the ppb range for AAS. | |
| 1.9 | The relationship stating that absorbance is proportional to both the concentration of the analyte and the path length through which light travels (A = εlc). | |
| 1.10 | Australian Drinking Water Guidelines — the national document that sets MCLs for contaminants including Pb (< 0.01 mg L−1), As (< 0.01 mg L−1) and Hg (< 0.001 mg L−1). |
2. True or false — with correction
Circle T or F. If false, write the corrected version on the line below. 10 marks (1 T/F + 1 correction each)
2.1 AAS directly measures the concentration of metal ions as they exist in solution. T / F
2.2 The ADWG maximum contaminant level for lead in drinking water is < 0.01 mg L−1. T / F
2.3 Heavy metal contamination is only a problem in industrial areas; natural geological sources cannot produce dangerous concentrations. T / F
2.4 Minamata disease is a neurological illness historically associated with mercury poisoning through biomagnification in a food chain. T / F
2.5 Phytoremediation uses a selective membrane to force water through and remove dissolved metal ions. T / F
3. Function recall
Answer each in 1–2 sentences using precise lesson terms. 10 marks (2 each)
3.1 What is the function of the hollow cathode lamp in AAS?
3.2 Why is a calibration curve needed in AAS analysis of a heavy metal?
3.3 What is EDTA complexometry and how does it allow a metal ion concentration to be determined?
3.4 Why does bioaccumulation mean that a low metal concentration in water can still create a high risk in organisms?
3.5 What is the function of chemical precipitation as a heavy metal remediation strategy?
4. Fill in the blanks
Complete the paragraph using the word bank below. Each word is used once. 8 marks
Word bank: atomised · absorbance · ground-state · calibration · element-specific · hollow cathode · Beer–Lambert · ppb
In atomic absorption spectroscopy (AAS), the water sample is first _______________ so that the target metal exists as free _______________ atoms. Light from a _______________ lamp passes through the atomised sample; because the lamp is _______________, only the target element absorbs the light strongly. The degree to which light is absorbed — the _______________ — follows the _______________ law: it is proportional to concentration and path length. By comparing the absorbance of the unknown sample against a _______________ curve constructed from known standards, the concentration of the metal can be determined. AAS is valuable for heavy metal monitoring because its detection limits reach the _______________ range, matching the low concentrations at which these metals become environmentally significant.
5. Connect the concepts
Draw labelled arrows between the five terms below to show how they are related. Each arrow must carry a linking phrase. Aim for at least 5 labelled arrows. 5 marks
Terms: heavy metal in water · bioaccumulation · biomagnification · top predator tissue · AAS monitoring
Q1 — Term–definition matches
1.1 heavy metal • 1.2 bioaccumulation • 1.3 biomagnification • 1.4 hollow cathode lamp • 1.5 atomisation • 1.6 maximum contaminant level (MCL) • 1.7 EDTA complexometry • 1.8 detection limit • 1.9 Beer–Lambert law • 1.10 ADWG.
Q2 — True / false
2.1 False. Correction: AAS does not measure ions in solution directly. The sample is first atomised; AAS then measures absorption by free ground-state atoms.
2.2 True.
2.3 False. Correction: Heavy metals can arise from natural geological sources (e.g. arsenic-bearing groundwater) as well as industrial and agricultural sources.
2.4 True.
2.5 False. Correction: Phytoremediation uses plants to absorb or stabilise contaminants. It is reverse osmosis that uses a selective membrane to remove dissolved metal ions by forcing water through under pressure.
Q3 — Function recall
3.1 The hollow cathode lamp emits light at only the characteristic wavelengths of the target element (the metal whose cathode the lamp is made from). This makes AAS element-specific: only the target metal absorbs the light significantly, so other elements in the sample cause little interference.
3.2 A calibration curve is built from standards of known concentration. It relates absorbance to concentration so that the absorbance measured for an unknown sample can be used to determine that sample’s concentration. Without the calibration, the absorbance value alone has no quantitative meaning.
3.3 EDTA complexometry is a titration technique where EDTA (ethylenediaminetetraacetic acid) forms a stable 1:1 complex with most metal ions. The sample containing the metal ion is titrated with a standard EDTA solution; the moles of EDTA used at the endpoint equal the moles of metal ion in the sample. This allows metal ion concentration to be determined analytically without requiring AAS instrumentation.
3.4 Bioaccumulation means the metal builds up inside an organism over time to a concentration much higher than that of the surrounding water. Even if the water concentration is below the MCL, the internal tissue concentration in aquatic organisms may still reach a level that is toxic, especially with prolonged exposure.
3.5 Chemical precipitation converts dissolved metal ions into insoluble solid compounds (e.g. metal hydroxides or sulfides) that can then be removed by sedimentation or filtration from the water. This physically removes the metal from the aqueous phase.
Q4 — Cloze answers (in order)
atomised · ground-state · hollow cathode · element-specific · absorbance · Beer–Lambert · calibration · ppb
Q5 — Sample concept map
Acceptable arrows include:
- heavy metal in water —is taken up by organisms through→ bioaccumulation
- bioaccumulation —leads to increasing concentration at higher trophic levels via→ biomagnification
- biomagnification —results in highest concentrations in→ top predator tissue
- AAS monitoring —quantifies concentration of→ heavy metal in water
- AAS monitoring —can also analyse→ top predator tissue (tissue sample prepared and analysed by AAS)
Award 1 mark per correctly directed, labelled arrow (max 5).