pH is a scale used to specify how acidic or basic (alkaline) a water-based solution is.
The letters stand for "potential of Hydrogen" (or power of Hydrogen).
It measures the concentration of free hydrogen ions (H+) floating around in a liquid.
The pH scale typically runs from 0 to 14. It is a logarithmic scale, which means each whole number change represents a tenfold increase or decrease in acidity.
For example, a pH of 5 is ten times more acidic than a pH of 6.
pH < 7 (Acidic): Solutions with a high concentration of hydrogen ions. They taste sour and can be corrosive.
pH = 7 (Neutral): Pure water sits right in the middle. The hydrogen ions ($H^+$) perfectly balance out the hydroxide ions (OH-)
pH > 7 (Basic/Alkaline): Solutions with a low concentration of hydrogen ions (and a high concentration of $OH^-$).
| Type | pH Level | Common Examples |
| Highly Acidic | 0 – 3 | Battery acid, stomach acid, lemon juice, vinegar |
| Mildly Acidic | 4 – 6 | Tomato juice, black coffee, milk, healthy skin |
| Neutral | 7 | Pure water, human blood (tightly regulated around 7.4) |
| Mildly Basic | 8 – 10 | Baking soda, seawater, antacids, hand soap |
| Highly Basic | 11 – 14 | Ammonia, soapy water, bleach, liquid drain cleaner |
Math: pH is defined as the negative logarithm (base 10) of the molar concentration of hydrogen ions:
pH equals negative logarithm base 10 of the hydrogen ion concentration.
pH = −log₁₀[H⁺]
Because it is a negative log, a higher number of hydrogen ions results in a lower pH value.
In human physiology, keeping the pH within a narrow, specific range is vital because even a tiny shift can completely alter chemical reactions and biological functions.
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