Question

A student measures the pH of a solution to be 6.8. Which should the student add if she wants to decrease the pH of the solution?

NaCl
H2O
HF
KOH

Answer 1

HF is the answer because it is an acid so it decreases overall ph. Did you understand?

Answer 2

The neutral pH is 7. Less than 7 indicates an acid and more than 7 indicates a base (up to 14).
NaCl - it's a salt (we can't measure the pH)
H2O - it can be an acid but also a base (the pH it is almost neutral,meaning close to 7 )
HF - it is a strong acid
KOH - it is a strong base (pH=14)

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He needs to use HF (Hydrogen fluoride) to decrease the pH.