Understanding pH

The chemistry of the "Potential of Hydrogen"

The Definition

In chemistry, pH is a measure of how acidic or basic a water-based solution is. The scale is defined by the concentration of hydrogen ions (H+) in the solution.

pH = -log[H+]

When acids are dissolved in water, they increase the concentration of H+ ions. When bases are dissolved, they either add hydroxide ions (OH-) or consume hydrogen ions, thereby decreasing the H+ concentration.

The Logarithmic Scale

The most important thing to grasp for students is that the pH scale is logarithmic (base 10). This means that each number on the scale represents a ten-fold change in the acidity.

pH 3 is 10x more acidic than pH 4
pH 3 is 100x more acidic than pH 5
pH 3 is 1,000x more acidic than pH 6

Key Thresholds

0 – 6.9

Acidic

7.0

Neutral

7.1 – 14.0

Alkaline / Basic

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Common Misconceptions

Mistake

"Higher numbers mean more acidic."

Reality: Because it is an inverse log scale, lower numbers signify a higher concentration of hydrogen ions (higher acidity).

Mistake

"pH only goes from 0 to 14."

Reality: While rare in school labs, extremely concentrated acids or bases can actually have pH values below 0 or above 14.