The one word that completes the sentence "The pH scale is a measure of how _________ a solution is" is "acidic." The pH scale measures the acidity of a solution by quantifying the concentration of hydrogen ions (H⁺) it contains. It is a logarithmic scale ranging typically from 0 to 14, where values below 7 indicate acidity, 7 is neutral, and above 7 indicates basicity (alkalinity). Therefore, the pH scale specifically measures how acidic a solution is, but it also reflects how basic or alkaline a solution is depending on the pH value.

Hence, the best single word to fill the blank in the sentence is "acidic."