Suppose you have already compared the ph values of two substances using 0-14 paper. when would a follow-up test using 4.5-7.5 paper be most ?

When both substances had a pH value of about 6 on the 0-14 paper  

Explanation:

The other options contemplate the case in which both substances had a pH of 3 or a pH of 9. These options are incorrect because there are outside the range of the 4.5-7.5 paper.

Another option stated that one substance has a pH of 5 and the other one a pH of 7. These values are far enough to be distinguished with the 0-14 paper.

Finally, when both substances had a pH value of about 6 on the 0-14 paper a more precise comparison is needed and the 4.5-7.5 paper can be used.

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