The pH scale is logarithmic, so dropping one unit means ten times more H⁺: lemon juice at pH 2 is a hundred times more acidic than coffee at pH 4. Across the whole 0 to 14 range, [H⁺] and [OH⁻] always move in opposite directions, because their product is pinned at 10⁻¹⁴, the ion product of water at 25°C. Indicators change colour over their own particular pH bands, with phenolphthalein turning pink only above 8.2, while a buffer is what keeps human blood clinging to 7.4 no matter what we eat, as a weak acid and its conjugate base swallow whatever H⁺ or OH⁻ arrives.
The pH scale is logarithmic, so dropping one unit means ten times more H⁺: lemon juice at pH 2 is a hundred times more acidic than coffee at pH 4. Across the whole 0 to 14 range, [H⁺] and [OH⁻] always move in opposite directions, because their product is pinned at 10⁻¹⁴, the ion product of water at 25°C. Indicators change colour over their own particular pH bands, with phenolphthalein turning pink only above 8.2, while a buffer is what keeps human blood clinging to 7.4 no matter what we eat, as a weak acid and its conjugate base swallow whatever H⁺ or OH⁻ arrives.
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