A range of values used to express the acidity or alkalinity (basicity) of a system; on this scale, a pH of 7 is neutral, less than 7 is acidic, and greater than 7 is basic.

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The main idea here is the pH scale, a system for expressing how acidic or basic a solution is. On this scale, 7 is neutral, values below 7 are acidic, and values above 7 are basic. The term pH scale is the best fit because it names the entire measurement system. A single pH value is just a number taken from that scale, not the system itself. The other terms aren’t the standard name for this measurement. Remember, the pH scale is logarithmic, so every step up or down represents about a tenfold change in hydrogen ion concentration, which is why small changes in pH can have big effects in chemistry and environmental contexts.