How do chemists measure and judge the quality of water?
Describe the key indicators of water quality, including dissolved oxygen, BOD, pH, turbidity and ion concentrations.
Dissolved oxygen, biochemical oxygen demand, pH, turbidity, salinity and nutrient ions as indicators of water quality, and what each reveals about a water body.
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What this dot point is asking
You need to know the main water-quality indicators, what each measures, and how to interpret high or low values. This connects to the analytical techniques (titration, spectroscopy) used to measure them.
The main indicators
Dissolved oxygen and BOD
Aquatic life needs dissolved . DO decreases as temperature rises (gas solubility falls with temperature) and as organic pollution increases.
BOD measures how much oxygen microorganisms consume while breaking down organic matter in a sample, usually over 5 days at . A high BOD means a lot of biodegradable organic pollution (such as sewage or fertiliser run-off), which depletes DO and can suffocate aquatic life.
pH and turbidity
Most aquatic organisms tolerate a narrow pH range (about 6.5-8.5). Acid run-off (e.g. from mine drainage or acid sulfate soils) lowers pH and can mobilise toxic metal ions.
Turbidity is cloudiness caused by suspended solids such as silt, clay or microorganisms. High turbidity blocks light for aquatic plants and can indicate erosion or pollution.
Nutrient ions and eutrophication
Excess nitrate and phosphate, often from fertiliser run-off, cause eutrophication: rapid algal growth (algal blooms). When the algae die, microbial decomposition raises BOD and depletes DO, killing fish.
Measuring the indicators
Many indicators are measured with techniques from this topic: ion concentrations by titration or atomic absorption spectroscopy, organic species by chromatography, and DO/pH by electronic probes.
When asked to interpret a data set, link each indicator value back to a cause and a consequence for water quality, rather than just quoting the number.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2022 SACE Stage 24 marksHazelnut crops require less fertiliser than other biofuel crops. Explain how excess fertilisers in waterways cause environmental problems.Show worked answer →
For 4 marks, build the eutrophication sequence.
Fertilisers contain soluble nitrogen and phosphorus nutrients (nitrate and phosphate ions) that run off into waterways.
These extra nutrients cause rapid growth of algae and aquatic plants (an algal bloom).
When the algae die, decomposing bacteria break them down, consuming dissolved oxygen in the water (high biochemical oxygen demand).
The dissolved oxygen level falls so low that fish and other aquatic organisms cannot survive and die. This process is eutrophication. One mark per stage.
2024 SACE Stage 23 marksAmmonium ions formed from a fertiliser bind to silicate minerals in the soil. Explain how ammonium ions that are bound to soil silicates become available to plants.Show worked answer →
The ammonium ions are held on the silicate by ion exchange, and the same process releases them.
Silicate (and aluminosilicate) minerals carry a negative surface charge that adsorbs positive ammonium ions (NH4+) by electrostatic attraction.
Other cations in the soil water (for example H+, Ca2+, or K+) can exchange with the adsorbed NH4+.
These cations displace the NH4+ from the mineral surface, releasing it into the soil water where it dissolves and can be taken up by plant roots. One mark each for adsorption, exchange, and release into solution.