Water chemistry fundamentals: Which statements about pH and its interpretation for acidity/alkalinity are correct for potable water assessment?
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ANeutral water has a pH value of 7
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BMaximum acidity corresponds to pH approaching 0
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CMaximum alkalinity corresponds to pH approaching 14
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DAll of the above
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EOnly (a) and (b)
Answer
Correct Answer: All of the above
Explanation
Introduction / Context:pH is a master variable in water treatment, influencing corrosion control, coagulation efficiency, disinfection kinetics, and consumer acceptability. Understanding its scale and endpoints is foundational for process control.
Given Data / Assumptions:
- Standard pH scale from 0 to 14 at 25°C.
- Neutrality defined at pH 7 for pure water at 25°C.
- Extremes represent strong acidity or alkalinity.
Concept / Approach:
By definition, pH = −log10[H+]. Pure water at 25°C has [H+] = 10^-7 mol/L (pH 7). Lower pH values indicate increasing acidity; higher values indicate increasing alkalinity. Thus statements (a), (b), and (c) are simultaneously true.
Step-by-Step Solution:
Confirm neutrality at pH 7.Recognise pH 0 as strongly acidic (maximum acidity conceptual limit).Recognise pH 14 as strongly alkaline (maximum alkalinity conceptual limit).Therefore choose “All of the above”.Verification / Alternative check:
Any basic water-chemistry text lists pH endpoints and neutrality at 25°C; treatment setpoints often target pH 6.5–8.5 for distribution system stability.
Why Other Options Are Wrong:
- Option (e) excludes the valid statement (c).
Common Pitfalls:
- Forgetting temperature dependence of neutrality (slight shifts with temperature).
Final Answer:
All of the above.