acid and base

Acids and bases are chemistry's great opposites — two sides of the same coin. The simplest way to tell them apart is a single tiny particle: the proton (a hydrogen ion, H⁺). An acid is a substance that gives protons away; a base is one that grabs them. Lemon juice and vinegar taste sharp because they're loading your tongue with loose protons; soap and baking soda feel slippery — a classic hallmark of bases, which react with the natural oils on your skin.

Put an acid and a base together and something wonderful happens: they cancel each other out, a reaction called neutralization. The donated protons rush into the waiting base, and what's left behind is usually water and a salt. For the common acids and bases around the house that salt is often harmless, but the exact product — and whether it turns out mild, acidic, or basic — depends on which acid and which base met. It's why a spoonful of baking soda settles an acid stomach; and when that same baking soda fizzes and froths, the bubbles are carbon dioxide gas escaping from the bicarbonate, not the bare acid-and-base reaction itself.

We measure how acidic or basic something is on the pH scale, a ruler that usually runs from about 0 to 14. Below 7 is acidic, above 7 is basic, and 7 — pure water — sits right in the middle, though very strong, concentrated acids or bases can spill past either end. One common mix-up: a "strong" acid isn't simply one that's concentrated. Strength is about how eagerly a substance lets its protons go. Stomach acid is fierce yet dilute; a strong acid can be made gentle by watering it down.