General:
* Melts
* Dissolves
* Turns to liquid
* Becomes liquid
* Flows
* Runs
* Thins
More specific:
* For solids:
* Fuses (for metals)
* Dissolves (for salts)
* Liquefies (most precise)
* For gases:
* Condenses
* Liquefies (most precise)
The best synonym will depend on the context of your sentence.