Which change in the h+ ion concentration of an aqueous solution represents a decrease of one unit on the ph scale?

The answer would be a tenfold increase
The pH scale is calculated based on the concentration of H+ ion in the solution. The formula is using log10, so to decrease 1 unit from the scale it will be 10^1= 10 fold of increase. For 2 unit, you will need 10^2= 100 fold of increase.

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