magnesium is a kind of alkaline earth metal & fluorine is a kind of gas & magnesium and fluorine react chemically producing magnesium fluoride ->  magnesium fluoride is a new substance produced by magnesium and fluorine reacting chemically  magnesium fluoride is a new substance produced by magnesium and fluorine reacting chemically & chemical reactions cause new substances to form ->  a chemical reaction happened between magnesium and fluorine  a chemical reaction happened between magnesium and fluorine & combining two substances chemically causes chemical reactions -> a chemical reaction is caused by magnesium and fluorine combined chemically