If you mean solid as a state of matter, there are three (the other two are liquid and gas) so there is not one that is exactly an opposite of one ofthe others.
If you are using solid to mean sturdy or well-constructed, one opposite would be flimsy.
solid.
Liquid
The opposite phase change (solid to liquid) is liquefaction.Where it means to strengthen or coalesce, the opposite could be fragmentation.
Liquid
sturdy, solid
solid
Stuipid
Yes, an antonym is the opposite, and since solid means that it is full on the inside, while hollow means to be empty, so they are the opposite
An non-example of a crystalline solid would be the complete opposite of it, an amorhous solid.
If the type of solid (die) is a Platonic solid, then the opposite sides add to the number of faces plus one. For example, a cube's opposite sides add to seven.
The opposite of congeal is liquefy. When a substance liquefies, it changes from a solid to a liquid state.
When something solidifies, it turns from a gas or a liquid to a solid. The opposite of something turning from a liquid or gas to a solid would be something turning from a solid to a liquid or gas. Now, there's a word for something turning from a solid to a gas, but it's not in common use; and very few things go straight from solid to gas (dry ice is one example). So the most appropriate answer would be the word for something turning from solid to liquid, which is "liquify".