To determine the biochemical oxygen demand (BOD) of the water in a lake, two samples were taken and one was tested immediately - Leaving Cert Chemistry - Question 9 - 2016
Question 9
To determine the biochemical oxygen demand (BOD) of the water in a lake, two samples were taken and one was tested immediately.
(i) Under what conditions was the se... show full transcript
Worked Solution & Example Answer:To determine the biochemical oxygen demand (BOD) of the water in a lake, two samples were taken and one was tested immediately - Leaving Cert Chemistry - Question 9 - 2016
Step 1
Under what conditions was the second sample stored?
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The second sample should be stored under conditions of 20°C in the dark to prevent any biological activity that could alter the BOD results.
Step 2
After what time interval was it tested?
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The second sample was tested after a time interval of five days, allowing for a realistic measurement of the biochemical oxygen demand.
Step 3
The result of the BOD test suggested that eutrophication had occurred in the lake.
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Eutrophication is defined as the over-enrichment of water with nutrients, primarily nitrates and phosphates, leading to excessive growth of algae and plants which can reduce water transparency and deplete oxygen levels.
Step 4
Suggest a possible cause of eutrophication.
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A possible cause of eutrophication could be runoff from agricultural activities, such as the use of fertilizers that contain nitrates and phosphates, as well as pollutants from industrial waste and sewage.
Step 5
Why is there concern about lead in drinking water?
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Lead is a toxic substance that poses health risks, particularly to young children and pregnant women, as it can affect cognitive development and cause serious health issues.
Step 6
Name an instrumental method used to analyse water for heavy metals.
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An instrumental method used to analyze water for heavy metals is Atomic Absorption Spectroscopy (AAS).
Step 7
How can lead be removed from a contaminated water supply?
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Lead can be removed from contaminated water supplies through various methods such as precipitation, displacement reactions, and methods involving reverse osmosis or ion exchange.
Step 8
How does temporary hardness arise in rainwater flowing over limestone?
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Temporary hardness arises when rainwater dissolves carbon dioxide (CO₂) in the atmosphere, forming carbonic acid (H₂CO₃). This interacts with limestone (mainly calcium carbonate, CaCO₃), leading to calcium bicarbonate (Ca(HCO₃)₂).
Step 9
Write a balanced equation to describe the formation of limescale when this water is boiled.
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The balanced equation for the formation of limescale when water with temporary hardness is boiled is:
ightarrow ext{CaCO}_3 (s) + ext{H}_2 ext{O} + ext{CO}_2 (g)$$
Step 10
Explain in words or by balanced equations how this water with temporary hardness is deionised using a cation-exchange resin (RH) and an anion-exchange resin (R-OH).
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When water with temporary hardness passes through a cation-exchange resin (RH), calcium ions (Ca²⁺) are exchanged for hydrogen ions (H⁺) from the resin. This process can be represented as:
ightarrow ext{CaR}_2 + 2 ext{H}^{+}$$
Next, for the anion-exchange resin (R-OH), bicarbonate ions (HCO₃⁻) in the water are exchanged with hydroxide ions (OH⁻) from the resin. The process can be shown as:
$$ ext{HCO}_3^{-} + ext{R-OH}
ightarrow ext{R-CO}_3^{2-} + ext{H}_2 ext{O}$$
This combination effectively removes temporary hardness from the water.
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