Which of the following is a definite indicator of a chemical change? a) A new substance called a precipitate is formed. b) The shape of the substance is changed. c) The color of the substance is changed. d) A substance is converted from a liquid to a gas. Choices b & d describe physical changes. While choices a & c are both indicators of chemical changes, but I need one definite indicator of a chemical change. Please help.

A physical change is a change in the shape, size, or state of a substance. Also, a physical change does not mean that the change has occurred in the substance itself (i.e., no new substance is formed). Thus, a change from a liquid to a gaseous state (choice d) is only a physical change, as no new substance is formed—only the state of the matter has changed. Similarly, a change in shape (choice b) is also a physical change.
In comparison, a chemical change refers to a change in the substance itself. This means that a new substance is formed. Hence, the formation of a new chemical—a precipitate (choice a)—is a chemical change. Similarly, the color change (choice c) refers to a chemical reaction and the formation of a different substance; hence, a chemical change has occurred.
In short, the formation of a new substance is a chemical change, while physical change does not entail the formation of a new substance.
Hope this helps.