Which of the following involves a chemical change?

The process of burning natural gas is an example of a chemical change because it involves a reaction between the gas and oxygen in the air, resulting in the formation of new substances: primarily carbon dioxide and water vapor. During combustion, the molecular structure of the natural gas, which is primarily methane (CH₄), is altered as it reacts with oxygen (O₂). The heat generated during this reaction indicates a transformation that is not reversible under standard conditions.

In contrast, melting steel, sublimation of carbon dioxide, and evaporation of alcohol are physical changes. These processes involve transformations in the physical state of materials—such as solid to liquid, solid to gas, or liquid to gas—without altering the chemical composition of the substances involved. In these cases, the original materials can be recovered without any significant chemical alteration, which differentiates physical changes from chemical changes like combustion.