- Neutral
- Acidic
- Basic
- Alkaline earth
A solution with pH less than 7 is acidic, meaning it has a higher concentration of hydrogen ions (H⁺) than hydroxide ions (OH⁻). The pH scale, ranging from 0 to 14, is logarithmic—each unit decrease represents a tenfold increase in H⁺ concentration. Strong acids like hydrochloric acid (HCl) fully dissociate, producing pH values near 0-1, while weak acids like acetic acid (vinegar) partially dissociate, typically pH 2-6. Acidity arises from substances that donate protons (Brønsted-Lowry theory) or accept electron pairs (Lewis theory). Common acids include citrus fruits (citric acid), battery acid (sulfuric acid), and stomach acid (hydrochloric acid). Acidic solutions taste sour, turn blue litmus red, and react with metals to produce hydrogen gas. In the environment, acid rain (pH ~4) damages ecosystems and structures. Human blood maintains strict pH around 7.35-7.45; acidosis occurs below this range. Understanding acidity is crucial in chemistry (reaction rates, equilibrium), biology (enzyme function, cellular processes), medicine (gastric pH, drug absorption), and countless industrial applications from food preservation to chemical manufacturing.
