What the pH scale measures
The pH scale tells you how acidic or basic (alkaline) a water-based solution is. It is defined from the concentration of hydrogen ions in solution:
pH = −log₁₀[H⁺], which rearranges to [H⁺] = 10⁻ᵖᴴ mol/L.
Because the definition uses a base-10 logarithm, the scale is logarithmic: every whole step is a ten-fold change in hydrogen-ion concentration. A solution at pH 4 has ten times more H⁺ than one at pH 5, and one hundred times more than pH 6. That is why a small-looking pH difference can mean a huge chemical difference.
Slide the marker above and watch three things update together: the pH value, its acidity classification, the calculated [H⁺], and the closest everyday substance at that point on the scale.
Reading the scale
Most familiar solutions fall between 0 and 14, with pure water at pH 7 (neutral at 25 °C, where [H⁺] = 1×10⁻⁷ mol/L):
- Acids sit below 7 — lemon juice (~2), vinegar (~3), coffee (~5). Lower pH means more H⁺ and stronger acidity.
- Neutral is right around 7, where hydrogen-ion and hydroxide-ion concentrations are balanced.
- Bases sit above 7 — baking soda solution (~9), hand soap (~10), bleach (~13). Higher pH means fewer H⁺ (and more OH⁻).
Values can technically fall below 0 or above 14 for very concentrated strong acids and bases, but the everyday examples here keep things on the familiar 0–14 range.
Why pH matters
pH controls chemistry that students meet across biology, earth science, and the lab. Human blood is held in a tight range near pH 7.4; soil and ocean pH shape which organisms thrive; and titrations in the chemistry lab track pH as acid and base react. Connecting an abstract number to a real substance — and to the actual [H⁺] behind it — makes the scale concrete.
Using this with a class
Project the scale and ask students to predict the pH of a substance before dragging to check it, or give a [H⁺] value and have them work back to the pH. It is free to embed on a class site or LMS page.