hydr

The root hydr comes from Greek hydōr, meaning simply "water." To the Greeks this was an everyday word for the rivers, rain, and seas around them, but it became one of the most productive roots in modern science. Almost every time a chemist, geologist, or engineer needs to say "water," they reach for hydr- (or its variants hydro- and hydra-).

The most famous example is hydrogen. When eighteenth-century chemists burned this light gas in air, the only thing left behind was water. So they named it the "water-maker": hydro- (water) + -gen (producer), the same -gen found in oxygen and nitrogen. Hydrogen literally means "the element that generates water."

From there the family spreads in two directions. One branch is about water doing physical work. Hydraulic machines (think of the lift under a car at a garage) are powered by liquid pressure; the word goes back to Greek hydraulis, a "water-pipe" organ. A hydrant is the upright pipe firefighters open to draw water from the main.

The other branch is about water combining or being removed. To hydrate something is to add water to it; a hydrate is a compound that holds water molecules inside its crystals. Strip that water out and you get dehydrated (de- away + hydr water): a dehydrated hiker, dehydrated fruit. Push it to the absolute extreme — no water at all — and you have anhydrous (an- without + hydr water), a word chemists use for perfectly dry salts and solvents.

Water also shows up as a building block in compound names. A hydrocarbon is a molecule of only hydrogen and carbon (oil and natural gas are made of these), and a carbohydrate is literally "carbon + water" — early chemists noticed sugars contained hydrogen and oxygen in the same 2:1 ratio as water, so they called them "watered carbon." On the largest scale, the hydrosphere is the whole watery layer of the planet — every ocean, lake, and river — set against the land and the atmosphere.

So whenever you meet hydr-, picture water: water being made (hydrogen), water doing work (hydraulic, hydrant), water added or taken away (hydrate, dehydrated, anhydrous), and water as a chemical and planetary ingredient (hydrocarbon, carbohydrate, hydrosphere).