Other forms: liquefied; liquefying; liquefies
When you turn a solid into a liquid, like when you set an ice cube in the sunshine and watch it melt into a puddle of water, you liquefy it.
You can liquefy a solid by heating it until it melts, and you can also say the substance itself liquefies. A jar of coconut oil that's hard and solid in the refrigerator will liquefy if you leave it on the counter on a warm summer day. The Latin root of liquefy is liquefacere, "make liquid," from liquere, "be fluid," and facere, "to make."