The pH scale is chemistry's universal ruler for measuring acidity and basicity. Introduced in 1909 by Danish chemist Soren Sorensen while working at the Carlsberg brewery laboratory (yes, the beer company), pH stands for "power of hydrogen." The scale runs from 0 to 14: pure water sits at 7 (neutral), anything below 7 is acidic, and anything above 7 is basic (alkaline). But here is the crucial detail that trips up many students: the scale is logarithmic, not linear. Each whole number step represents a tenfold change in hydrogen ion concentration. Lemon juice at pH 2 is not slightly more acidic than tomato juice at pH 4 — it is 100 times more acidic.
Look around and you will find pH governing everything. Your blood is maintained at a precise pH of 7.35-7.45 — your body has elaborate buffering systems to keep it there, because even a shift of 0.3 units can be fatal. Your stomach operates at a brutal pH of 1.5-3.5 to digest food. Swimming pools are kept at pH 7.2-7.8 for comfort and effective chlorine disinfection. Soil pH determines which plants can grow: blueberries thrive in acidic soil (pH 4.5-5.5), while lavender prefers alkaline conditions (pH 6.5-8.0). Aquarium enthusiasts monitor pH religiously because most freshwater fish need pH 6.5-7.5 to survive.
Measuring pH can be as simple as dipping a strip of litmus paper into a solution — red for acid, blue for base — or as precise as using a digital pH meter that measures voltage differences caused by H⁺ ions. Industries rely on pH monitoring constantly: winemakers track pH to control fermentation, wastewater treatment plants adjust pH before releasing water into rivers, and cosmetics are formulated to match skin's natural pH of about 5.5. The pH scale is one of chemistry's most elegant inventions: a single number that instantly tells you the chemical character of any aqueous solution.