pH scale withe pictures
The pH scale is a numerical scale for acidity in water-based solutions. Lower numbers indicate higher acidity (higher hydronium ion concentration), the middle region is near neutral, and higher numbers indicate basicity (lower hydronium ion concentration and relatively higher hydroxide ion concentration).
Meaning of pH numbers
pH is defined using the hydronium ion concentration (often approximated by molar concentration in introductory general chemistry):
\[ \mathrm{pH} = -\log_{10}\!\bigl[\mathrm{H_3O^+}\bigr] \]
The scale is logarithmic. A difference of 1 pH unit corresponds to a factor of \(10\) change in \(\bigl[\mathrm{H_3O^+}\bigr]\).
\[ \frac{\bigl[\mathrm{H_3O^+}\bigr]_{(\mathrm{pH}=a)}}{\bigl[\mathrm{H_3O^+}\bigr]_{(\mathrm{pH}=b)}} = 10^{\,b-a} \]
A solution at pH \(3\) has \(10^{5-3} = 10^2 = 100\) times more \(\mathrm{H_3O^+}\) than a solution at pH \(5\).
Visualization: pH scale with common substances
pH ranges and typical interpretations
| pH range | Interpretation | Representative examples |
|---|---|---|
| \(0\) to \(3\) | Strongly acidic (high \(\bigl[\mathrm{H_3O^+}\bigr]\)) | Battery acid (conceptual example), stomach acid |
| \(3\) to \(6\) | Moderately acidic | Lemon juice, vinegar, many soft drinks |
| Near \(7\) | Near-neutral aqueous solutions | Pure water (at a specified temperature), many salt solutions close to neutral |
| \(8\) to \(11\) | Moderately basic | Seawater (slightly basic), baking soda solutions, ammonia solutions |
| \(12\) to \(14\) | Strongly basic (low \(\bigl[\mathrm{H_3O^+}\bigr]\), relatively high \(\bigl[\mathrm{OH^-}\bigr]\)) | Bleach, concentrated hydroxide solutions |
Connection to pOH and water autoionization
A related measure is pOH:
\[ \mathrm{pOH} = -\log_{10}\!\bigl[\mathrm{OH^-}\bigr] \]
At \(25^\circ\mathrm{C}\), the water ion-product constant is commonly taken as
\[ K_w = \bigl[\mathrm{H_3O^+}\bigr]\bigl[\mathrm{OH^-}\bigr] = 1.0 \times 10^{-14} \]
which leads to the familiar relationship
\[ \mathrm{pH} + \mathrm{pOH} = 14.00 \]
Temperature dependence matters because \(K_w\) changes with temperature, so “neutral pH” is not exactly \(7.00\) at all temperatures.
Common pitfalls
- Logarithmic spacing: equal distances on the pH number line represent powers of ten in \(\bigl[\mathrm{H_3O^+}\bigr]\), not equal concentration differences.
- Mixtures and buffers: buffers resist pH changes, so pH does not track simple dilution in the same way as strong acids or strong bases.
- Activity vs concentration: rigorous pH is based on activity; dilute-solution concentration approximations are most accurate at low ionic strength.