A student puts a drop of reaction mixture of a saponification reaction first an a blue litmus paper and then on a red litmus paper. He may observe that :

A saponification reaction involves the hydrolysis of fats or oils with a strong base like sodium hydroxide ($NaOH$) to produce soap (sodium salt of fatty acids) and glycerol. Since soap is a salt of a weak acid (fatty acid) and a strong base ($NaOH$), its aqueous solution is alkaline (basic) in nature. Additionally, there may be unreacted sodium hydroxide left in the reaction mixture, which is also strongly basic.

• Basic solutions turn red litmus paper blue.
• Basic solutions have no effect on blue litmus paper.
• Therefore, the red litmus paper turns blue, and the blue litmus paper remains unchanged.