a) Mothballs gradually vaporize in a closet: This is a physical change. The vaporization of mothballs involves a change in state from solid to gas without altering the chemical composition of the substance.
b) Calcium chloride lowers the temperature at which water freezes and can be used to melt ice on city sidewalks and roadways: This is a physical change. The process involves the dissolution of calcium chloride in water, which lowers the freezing point of water. The chemical composition of calcium chloride and water remains unchanged.
c) An antacid tablet dissolves in water: This is a chemical change. When an antacid tablet dissolves in water, it reacts with the water to produce new substances, such as carbon dioxide gas and a solution of the antacid’s active ingredients. This indicates a change in the chemical composition of the substances involved.