iron is a kind of metal & nails are often made of iron ->  nails are made of metal  nails are made of metal & metals in contact with water and oxygen will rust ->  nails in contact with water and oxygen will rust  nails in contact with water and oxygen will rust & rust forming is a kind of chemical change ->  nails rusting is a kind of chemical change  nails rusting is a kind of chemical change & chemical change is a kind of chemical reaction -> nails rusting is a kind of chemical reaction