The antonyms for the phrase "becomes soft" would be "hardens," "stiffens," and "solidifies." These words indicate the opposite of softness, and rather suggest a rigidity or firmness of texture. For example, dough that is left out for too long without being covered will harden and become difficult to work with. Similarly, a cable that has been exposed to extreme temperatures may stiffen and become less flexible. In contrast to becoming soft, solidification often occurs when a liquid or gas changes state and becomes a solid. Antonyms for "becomes soft" provide an important vocabulary tool for expressing the opposite of this physical state.