In a series of solutions with pH of 2.5, 3.5, 7.0 and 8.0, which is likely to turn red moist litmus paper blue?

Litmus is an acid-base indicator. Red litmus paper turns blue only in the presence of a base (alkaline solution), which has a pH greater than 7.

Examining the given pH values:

• pH 2.5 - strongly acidic (no colour change; red litmus stays red)
• pH 3.5 - acidic (no colour change)
• pH 7.0 - neutral (no colour change; litmus remains the same colour)
• pH 8.0 - basic/alkaline (red litmus turns blue)

Only the solution with pH 8.0 is alkaline, so it is the only one that will turn red moist litmus paper blue. Acidic and neutral solutions cannot cause this change.