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A strong acid has a very low pH value, typically ranging from 1 to 3. This is due to the high concentration of hydrogen ions (H⁺) present in a strong acid solution, which results in a significant decrease in pH. The universal indicator used to measure pH will reflect this acidity by displaying a color change. In the case of a strong acid, the indicator would turn red, signifying a highly acidic environment.

The choice of pH values and the corresponding color of the universal indicator for strong acids is consistent with what is understood about their chemistry. Strong acids dissociate completely in water, leading to a high concentration of H⁺ ions, which is responsible for the low pH and red color of the indicator.