Can someone help me with this, what even is this? I didn't even learn this

100mg magnesium reacts with 20ml of 1.0m hcl

What does the symbol (–hfus) indicate in a phase change?

Some liquids can be distilled, but only at temperatures that are so high that it is impractical, or so high the compound decomposes. explain why distillation such compounds at significantly less than atmospheric pressure (some degree of vacuum) would solve this problem.