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What is the pH value of pure water at 25┬░C?
3
7
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1
7
The pH value of pure (neutral) water at standard temperature (25┬░C) is exactly 7. At this temperature, the concentration of oxonium ions ([H3тАЛO+]) and hydroxide ions ([OHтИТ]) are equal, both being 10тИТ7┬аmol/L, resulting in a neutral pH.
The pH value of pure (neutral) water at standard temperature (25┬░C) is exactly 7. At this temperature, the concentration of oxonium ions ([H3тАЛO+]) and hydroxide ions ([OHтИТ]) are equal, both being 10тИТ7┬аmol/L, resulting in a neutral pH.
Think of pure water as a perfectly balanced scale where the positive hydrogen ions and negative hydroxide ions weigh exactly the same, placing the indicator right in the middle at 7.
Pure water is neutral, and 7 is heaven (neutral middle ground of the 0-14 pH scale).
pH=тИТlog10тАЛ[H3тАЛO+] тАФ Definition of pH
KwтАЛ=[H3тАЛO+][OHтИТ]=1.0├Ч10тИТ14 тАФ Ionic product of water at 25┬░C
pH+pOH=14 тАФ Relation at 25┬░C
Water undergoes self-ionization (autoionization) according to the chemical equilibrium: 2H2тАЛO(l)тЗМH3тАЛO+(aq)+OHтИТ(aq). The ionic product of water (KwтАЛ) at 25┬░C is 1.0├Ч10тИТ14. Since [H3тАЛO+]=[OHтИТ], taking the negative logarithm gives pH=тИТlog10тАЛ[H3тАЛO+]=тИТlog10тАЛ(10тИТ7)=7.
Pure water is neutral with a pH of 7 at 25┬░C.
The concentration of hydronium ions equals the concentration of hydroxide ions in pure water.
The self-ionization of water is an endothermic process; hence KwтАЛ increases with temperature.
At higher temperatures (e.g., 60┬░C), pure water has a pH less than 7 but remains neutral because [H3тАЛO+]=[OHтИТ].
Provides a standardized logarithmic scale to measure acidity and basicity easily across 15 orders of magnitude.
pH depends strongly on temperature; a strict pH of 7 is only valid at standard ambient temperature (25┬░C).
Determining water purity in municipal water treatment plants and chemical laboratories.
Biological systems where physiological processes depend strictly on neutral or buffered pH levels.
Note: The question paper's official key listed Option D as 1, which contains an error regarding the pH of pure water (pH 1 is strongly acidic, typical of gastric acid). The scientifically correct pH of pure water at 25┬░C is 7 (Option B).
Option A (3) represents strong acid solutions like dilute vinegar or fruit
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