Final answer:
A chemical change occurs when the composition of a substance is altered and new substances are formed, such as when a leaf changes color in the fall because of the breakdown of chlorophyll. Options a, b, and c are physical changes, thus the correct option that represents a chemical change is d.
Step-by-step explanation:
A chemical change occurs when the composition of a substance changes and new chemical bonds are formed or existing ones are broken. This leads to the formation of a substance with different properties. Examples of chemical changes include souring of milk, burning of gasoline, and corrosion of metals like iron. In contrast, physical changes do not alter the chemical composition of a substance and are often reversible, such as melting, freezing, and dissolving.
For the options provided:
(a) Solidifying of minerals to form a crystal is often considered a physical change as it's a change of state.
(b) Purifying ethanol by distillation is a physical change as the chemical identity of the ethanol remains the same.
(c) Formation of salt from evaporated seawater is a physical change where the water changes its state from liquid to gas leaving behind salt.
(d) A leaf changing color in the fall is a chemical change because it involves the breakdown of chlorophyll and the change in the chemical composition of the leaf.
Therefore, the correct answer is (d) a leaf changes color in the fall, as this process involves a chemical change where new substances are produced by the breakdown of chlorophyll.