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A student investigating the water quality of stormwater in a drain near the school collected samples for testing in the school laboratory - HSC - SSCE Chemistry - Question 28 - 2011 - Paper 1
Question 28
A student investigating the water quality of stormwater in a drain near the school collected samples for testing in the school laboratory. The student conducted the ... show full transcript
Worked Solution & Example Answer:A student investigating the water quality of stormwater in a drain near the school collected samples for testing in the school laboratory - HSC - SSCE Chemistry - Question 28 - 2011 - Paper 1
Step 1
Hardness
Answer
The hardness of water is primarily due to the presence of calcium and magnesium ions. The chemical principle involved is the formation of insoluble precipitates when these ions react with specific reagents.
Procedure:
- Collect a sample of the water to be tested.
- Add a few drops of a suitable reagent, such as soap solution. If the water lathers, then it has low hardness, while a lack of lather indicates high hardness.
- Alternatively, perform a titration using EDTA (ethylenediaminetetraacetic acid) to quantify the hardness by determining the amount of calcium and magnesium present.
Step 2
Biochemical oxygen demand
Answer
Biochemical Oxygen Demand (BOD) measures the amount of dissolved oxygen consumed by microorganisms in the water sample over a specified period. This test indicates the organic pollution level in the water.
Procedure:
- Collect a water sample in a BOD bottle, making sure to fill it completely to avoid any air bubbles.
- Incubate the sample at 20°C for five days in the dark to prevent photosynthesis.
- After five days, measure the dissolved oxygen level in the sample. The difference between the initial and final dissolved oxygen levels gives the BOD value.