Hi Zhuoyuan, your second sentence is correct. The ice refers to a specific piece of ice. "If you heat ice, it will melt" refers to all ice or ice in general. Your first sentence is incorrect, actually. Hope this helps! :)

Of course you can say that. But if you add the definite article "the", it means you are referring to some specific ice rather than to ice in general.

The sentence is a statement of fact, and your sentence is also grammatically correct.