Pure water has pH 7, so still, freshly distilled water should have a pH of 7.0. However, interaction with the atmosphere allows carbon dioxide to dissolve into it, forming carbonic acid. As a result, agitating the water or allowing it to sit for a while will leave you with an aqueous solution that drifts down in pH. Because there are no natural buffers in distilled water, the pH can go down as low as 5.0.
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