What is the pH scale, and what does it measure?

Question

Accepted Answer

A logarithmic scale from 0 to 14 that measures the acidity or basicity of a solution based on the concentration of hydrogen ions. The pH scale is a logarithmic scale ranging from 0 to 14 that measures the acidity or basicity (alkalinity) of an aqueous solution. It is based on the concentration of hydrogen ions (H⁺) in the solution. A pH of 7 is neutral (pure water), pH values below 7 indicate acidity (higher H⁺ concentration), and values above 7 indicate basicity (lower H⁺ concentration). Because the scale is logarithmic, each unit change represents a tenfold change in H⁺ concentration - a solution with pH 3 is ten times more acidic than one with pH 4, and one hundred times more acidic than pH 5.

Atmospheric pressure is measured by a barometer in units like millibars or inches of mercury, not on the pH scale. Temperature is measured on the Celsius, Fahrenheit, or Kelvin scales, not the pH scale. Density is measured as mass per unit volume (kg/m³ or g/mL), which is an entirely different physical property.

The pH scale and acid-base chemistry are important topics on the DSST Principles of Physical Science I exam. Students should know the pH scale range, understand the logarithmic nature (each unit = 10× change), know common examples (stomach acid ≈ pH 1-2, lemon juice ≈ pH 2, water = pH 7, baking soda ≈ pH 9, bleach ≈ pH 12-13), understand acids (proton donors) and bases (proton acceptors), and know how indicators and pH meters are used to measure pH.

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