Final answer:
The blue litmus paper turning red upon contact with the unknown substance indicates that the substance is acidic, with a pH less than 7.
Step-by-step explanation:
When Holly placed blue litmus paper into the unknown substance and it turned red, this indicates that the substance is acidic. This is because blue litmus paper is known to change color from blue to red when it comes into contact with an acidic solution. Acids have a pH less than 7, so given this reaction, the pH of Holly's substance would also be expected to be less than 7.
Litmus paper is a type of pH indicator used to test whether a substance is acidic or basic. Basic or alkaline substances, on the other hand, would turn red litmus paper blue, and since this did not occur, we can conclude that the substance is not basic. To gain a more accurate measurement of pH, Holly could use pH paper, which contains a set of indicators that change color at different pH values.