Answer
a. Melting of iron
b. Lighting a match
c. Grinding corn
d. Souring of milk
Work Step by Step
a. This is a physical change because the substance we are referring to (iron) does not change composition, but only state (liquid iron is still iron).
b. This is a chemical change; the substances in the head of the match react with oxygen to produce fire. This change is a combustion reaction.
c. This is a physical change. None of the chemicals in corn change in the grinding process. The only change is that corn is cracked.
d. This is a chemical change. When milk becomes sour, it is because bacteria in the milk have consumed lactose (a carbohydrate) and produced lactic acid. The lactic acid is what makes milk turn sour. In this process, we see that one substance is turned into another via bacteria. Therefore, the chemical identity of the substance changes.
