General:
* Hydrology: The study of water and its movement on, in, and above the Earth.
* Hydrophilic: Having a strong affinity for water.
* Hydrophobic: Having a strong aversion to water.
* Hydrolysis: The chemical breakdown of a compound due to the reaction with water.
* Hydrate: To combine with water.
Specific compounds:
* Hydrocarbon: A compound composed solely of hydrogen and carbon atoms.
* Hydroxide: A compound containing the hydroxide ion (OH-).
* Hydrogen: The chemical element with the symbol H.
* Hydrazine: A colorless, flammable liquid used as a rocket fuel.
* Hydrochloric acid: A strong acid composed of hydrogen and chlorine.
Other:
* Hydroponics: The process of growing plants in a water-based nutrient solution.
* Hydroelectric: Relating to the generation of electricity from the movement of water.
* Hydrotherapy: The use of water for therapeutic purposes.
* Hydrocephalus: A condition involving an abnormal buildup of cerebrospinal fluid in the brain.
This is not an exhaustive list, but it provides a good overview of common words ending in "-hydro".