Which of the following represents a physical change?

A physical change involves a change in the form or appearance of a substance, but does not alter its chemical composition. Mixing salt in water exemplifies this concept perfectly. When salt dissolves, it breaks down into individual sodium and chloride ions; however, these ions can be recovered if the water evaporates, thus demonstrating that the original substance (salt) remains chemically intact throughout the process.

In contrast, burning wood is a chemical change because it transforms the wood into ashes and gases, fundamentally altering its chemical composition. Rust formation on iron is also a chemical change, as it involves a reaction between iron and oxygen resulting in iron oxide, changing its properties and appearance permanently. Baking a cake represents a chemical change due to the reactions that occur when ingredients combine and undergo heating to form new substances, which cannot simply be reverted back to the original elements.

Thus, mixing salt in water is identified as a physical change since it maintains the integrity of the original chemical compound.