The antonyms for the phrase "became liquid" are "solidify," "harden," "freeze," and "thicken." These words refer to the process wherein a substance loses its fluidity and hardens into a solid state. While "became liquid" represents the state of matter transitioning from a solid or gas to a liquid, these antonyms describe a reverse process. Solidification occurs when a liquid cools and loses its heat energy, while hardening is when a material solidifies or becomes stiffer through a process such as drying or curing. Freezing pertains to the change from liquid to solid, while thickening is the process of making a liquid denser, often by adding a thickening agent or reducing the liquid content.