Beginner Topic 517 of 1102 Acids, Bases & Electrochemistry
pH Scale
A logarithmic measure of acidity, pH equals minus log of hydrogen ion concentration.
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In-depth explanation of pH Scale
pH is defined as the negative base-10 logarithm of the hydrogen ion activity, approximated by concentration in dilute solutions: pH = -log10[H+]. Because the scale is logarithmic, each unit corresponds to a tenfold change in hydrogen ion concentration, so a solution at pH 3 is ten times more acidic than one at pH 4. At 25 C, pH 7 is neutral, values below 7 are acidic and values above 7 are basic. Typical values range from stomach acid at about 1.5 to 2, through coffee near 5 and blood near 7.4, up to household ammonia near 11. The logarithmic form makes it convenient to express concentrations spanning many orders of magnitude.
Going deeper: pH Scale
Key ideas: pH = -log10[H+], and each unit is a tenfold change in [H+]. A pH change from 3 to 5 means 100 times less acid. Significant figures: the digits after the decimal in pH equal the sig figs in the concentration. Convert back with [H+] = 10^(-pH).
Common mistakes and connections: Errors: forgetting the minus sign, treating pH as linear when averaging solutions (average concentrations, not pH values), and assuming pH must lie between 0 and 14: 2 M HCl has pH near -0.3. Very dilute acid (1e-8 M) must include water autoionization, giving pH near 6.98, never 8.
Visualization of pH Scale
Each pH unit is a tenfold change in H+; common substances span the scale from stomach acid near 1.5 to drain cleaner near 13. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A hydrogen ion concentration of 3.2 x 10^-4 M gives a pH of 3.5.
Frequently asked questions about pH Scale
What is pH Scale?
A logarithmic measure of acidity, pH equals minus log of hydrogen ion concentration.
Is pH Scale a beginner, intermediate or advanced topic?
pH Scale is a beginner topic in Acids, Bases & Electrochemistry. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before pH Scale?
Study these earlier topics in Acids, Bases & Electrochemistry first: Conjugate Acid-Base Pairs, Amphoteric Substances, Autoionization of Water.
What comes after pH Scale?
The next topics in Acids, Bases & Electrochemistry are pOH and pKw and Strong Acids.
Can I practise pH Scale with quizzes?
Yes. The interactive lesson includes a self-check question on pH Scale, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Where pH Scale fits in Acids, Bases & Electrochemistry
pH Scale is topic 7 of 105 in Acids, Bases & Electrochemistry.