The pH scale trap
A free TEAS® chemistry explainer with an original practice question
The pH scale is logarithmic. pH is the negative log of the hydrogen ion concentration, so each step of 1 on the scale is a tenfold change in [H⁺].
Two things trip people up. The first is treating the scale as a straight line, so a difference of 3 looks like 3 times. It is 10 × 10 × 10 = 1,000 times. The second is direction: a lower pH means more H⁺ and a more acidic liquid, not less.
To compare two liquids, subtract the pH values, turn the difference into a power of 10, and give the larger [H⁺] to the liquid with the lower pH.
Try a sample question
Tomato juice has a pH of about 4. Pure water has a pH of 7. How does the hydrogen ion concentration [H⁺] in tomato juice compare with that in pure water?
- A.3 times greater
- B.1,000 times smaller
- C.1,000 times greater
- D.30 times greater
Show the answer
C. 1,000 times greater
The [H⁺] in tomato juice is 1,000 times greater. Each drop of 1 pH unit means 10 times more H⁺. Tomato juice is 3 units lower than pure water, so it has 10 × 10 × 10 = 1,000 times more H⁺. Lower pH means more acidic.
Why the other answers tempt:
- A. The pH values differ by 3, but each pH unit is a tenfold change, not a step of 1.
- B. A lower pH means more H⁺, not less. Tomato juice is the more acidic one.
- D. This multiplies the difference by 10 once. Each of the 3 steps is its own factor of 10: 10 × 10 × 10.
Every pH step is a ×10 step. Three steps is ×1,000.
Checked against OpenStax Chemistry 2e, 14.2 pH and pOH. Original question, not from a real test.
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