Concept:A new substance is formed only when a chemical change occurs.Explanation:Cooling water to form ice is a physical change because water changes state but remains H2O.Dissolving sugar in water and mixing iron dust and sand only produce mixtures, with no new substances formed.Adding saliva to cooked yam is a chemical change because the enzyme amylase in saliva breaks down starch into a new substance, sugar/maltose.Answer:B. Adding saliva to cooked yam