Considering its chemical function:
* Freezing agent: This would cause a substance to freeze more readily. However, this term isn't commonly used.
* Ice promoter: Similar to the previous term, this would encourage the formation of ice.
Considering its use in cars:
* Coolant: This term refers to any liquid used to keep a car's engine cool, including antifreeze. However, "coolant" isn't strictly the opposite of "antifreeze" because it can also be used without the anti-freezing properties.
Considering its effect on freezing:
* Freezing enhancer: This is more of a scientific term referring to something that promotes freezing.
* Freezing agent: Similar to the previous term, this would cause a substance to freeze more readily.
Ultimately, there's no perfect one-word antonym for "antifreeze." The best word to use depends on the specific context and what you are trying to communicate.