Which of the following represents a chemical change?

The oxidation of iron is a clear example of a chemical change due to the fact that it involves a reaction between iron and oxygen from the environment, resulting in the formation of iron oxide, commonly known as rust. This process alters the chemical composition of the iron; it transforms from a metallic state to a compound (iron oxide) with different properties.

In contrast, mixing sand and salt involves a physical change as both substances retain their individual properties and can be separated by physical means. Boiling water, similarly, does not change the chemical identity of H2O; it only changes its state from liquid to gas. Breaking a wooden stick is also a physical change because the wood remains wood, and while its shape may change, its chemical structure does not. Therefore, the oxidation of iron exemplifies a fundamental chemical change where new substances are created through a reaction.