In the first pair of your examples, I'd rather say:
If she heats ice it'll melt., unless you want to put some emphasis on her extraordinary abilities to melt ice, which is not that exceptional at all.
Otherwise, in a general sense:
If one heats/you heat ice it melts/it'll melt.
In reported speech, I'd say:
He said that if she heated ice it would melt.
He said that if one heats/you heat ice it melts/it'll melt. or with back-shifting :
He said that if one/you heated ice it melted/it would melt.
Similarly, with your other pair of examples.
When speaking in a general sense, you can also use When, Whenever, Every time, etc. in place of If.