The pH Scale
pH = −log[H⁺] measures acidity.
About this topic
The pH scale measures acidity: pH = −log10[H+]. A solution is acidic when pH is below 7, neutral at 7 and basic above 7. Each unit change represents a tenfold change in hydrogen ion concentration, so pH 3 is ten times more acidic than pH 4 and a hundred times more than pH 5.
At 25 degrees Celsius, water self-ionises to give [H+] = [OH-] = 1.0 x 10^-7 M, hence pH 7, and the ion product is Kw = [H+][OH-] = 1.0 x 10^-14. This means pH + pOH = 14, with pOH = −log[OH−]. Indicators like litmus, phenolphthalein and universal indicator change colour with pH, and pH meters give accurate values.
Worked example: 0.010 M HCl is a strong acid, so [H+] = 1.0 x 10^-2 and pH = 2. For 0.010 M NaOH, [OH-] = 10^-2, pOH = 2 and pH = 12. Typical values: stomach acid around 1.5 to 2, lemon juice about 2, black coffee 5, blood 7.4, seawater 8, soap 10, bleach 13. Blood must stay between 7.35 and 7.45 for health.
Misconceptions: that pH cannot be negative or above 14 (it can for very strong solutions) and that a small change in pH is small in effect. Because of the logarithm, one pH unit is a tenfold change.
Key ideas
- pH 7 is neutral
- Each unit is a factor of 10
- Lower pH means more acidic
Quick check
Use cases: where The pH Scale helps
- Homework: review the key idea behind The pH Scale before you start a problem set.
- Exam revision: use the quick check to test whether you can recall The pH Scale without notes.
- Visual learning: play with the interactive lab to see how The pH Scale changes when you adjust its inputs.
- Teaching a friend: explain The pH Scale in your own words, then compare your version with the key ideas.
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Frequently asked questions
What is The pH Scale?
pH = −log[H⁺] measures acidity.
What will I learn in The pH Scale?
pH 7 is neutral. Each unit is a factor of 10. Lower pH means more acidic.
Is The pH Scale part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the The pH Scale lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study The pH Scale?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.