The molten steel is poured into the ingot, changed from liquid to solid and solidified; Dewdrops are small water droplets formed by cold liquefaction of water vapor in the air, so they are liquefied; Popsicles melt in people's mouths, and only by absorbing heat can people feel cool. White gas is small water droplets formed by cold liquefaction of water vapor exhaled by people.
So the answer is: solidification, liquefaction, melting, liquefaction.