Acid rain: pH scale and comparison to normal rainwater (pH = −log10[H+]. Unpolluted rain in equilibrium with atmospheric CO2 is weakly acidic)
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pH = −log10[H+]. Unpolluted rain in equilibrium with atmospheric CO2 is weakly acidic (~5.6). Acid rain refers to rain with pH lower than this due to H2SO4/HNO3 from SOx/NOx.
What each symbol means
| Symbol | What it stands for |
|---|---|
| pH | Negative log10 of [H+] |
| 5.6 | Typical textbook value for CO2-saturated distilled water reference |
| [H+] | Hydrogen ion activity (mol/L scale in introductory problems) |
When to use this
pH 5.6 is approximate didactic anchor; ionic strength and temperature shift it slightly.