"Melt" is closer than "thaw". I would probably say "dissolve" or "liquefy".
What will happen to a bar of soap if I put it into the bathtub full of water? I know eventually it will either become really small or disappear depending on how much water there is in the tub. But will it "melt" or "thaw"?
Things can only thaw if they are/were/have been frozen.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
'Melt' is generally used for breaking down by the application of heat. It's used colloquially for 'dissolve'. It depends on how physically/chemically correct you want to be. I'd use 'dissolve' since that's both correct and colloquial.
One more vote for dissolve.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
I am not a teacher.
What really happens to a bar of soap when put into a bathtub full of water is that it becomes slippery.
Then it dissolves.