sentences: a hammer often is made of metal & when a metal rusts , that metal becomes flaky on the surface ->  when a hammer rusts, the hammer will becom flaky on the surface  when a hammer rusts, the hammer will becom flaky on the surface & kim's hammer is flaky on the surface ->  the hammer has rusted  the hammer has rusted & rust forming is a kind of chemical change -> the hammer has undergone a chemical change