when substances react chemically to form new substances is due to changes in their chemicals structures as a result of a rearrangement of their atoms. there is also the breaking of chemical bonds as well as the formation of other bonds
when glucose is mixed with yeast it ferments to make alcohol. Glucose (C6H12O6) is a chemical compound which enzymes in yeast can use to make ethyl alcohol (C2H5OH).
Another good example is iron rusting. When iron (Fe) rusts, it combines in a
complex fashion with oxygen to form another reddish-colored compound known as
ferric oxide (Fe2O3).