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Physical changes are transformations in a substance that do not alter its molecular composition. Such changes include altering the state, shape, or size of a material. For example, a physical change occurs when water freezes into ice or when it evaporates into steam. In both instances, the H₂O molecules remain unchanged, regardless of being in a solid, liquid, or gaseous state.

In the context of separating mixtures, physical changes are exclusively at play because the individual substances in a mixture retain their original properties after separation. For instance, when saltwater undergoes evaporation, the water and salt are physically separated, yet neither is chemically changed.

Chemical changes, in contrast to physical changes, involve a substance undergoing a chemical reaction, resulting in a new substance with a different molecular composition. The signs of a chemical change can include a change in color, temperature, formation of a precipitate, or the release of gas.

When baking a cake, for instance, the ingredients undergo chemical changes due to the heat and interactions between the components. The result is a new substance that looks, tastes, and smells different from the original ingredients. Similarly, rust forming on iron is a chemical change, where oxygen combines with iron to create iron oxide, a compound with entirely different properties from the initial iron.

Separation techniques are the methods by which components of mixtures are divided based on their physical properties. Some common techniques include:

• Filtration, which separates solids from fluids (liquids or gases) using a medium through which only the fluid can pass.
• Distillation, which separates substances based on their different boiling points.
• Chromatography, which separates components based on their movement through a stationary phase.
• Evaporation, which removes a liquid from a solution to leave a solid residue.

Each of these techniques exploits a particular physical property, like boiling point or particle size. The choice of method depends on the mixture's composition and the characteristics of its individual components. The implementation of these techniques ensures that separations remain physical processes, with the molecular composition of each component left intact.

The molecular composition of a substance is the unique arrangement of atoms that defines its chemical identity. A molecule's composition determines its physical and chemical properties, as well as how it interacts with other substances.

In the scope of separations, paying close attention to molecular composition is crucial. The objective is to partition the components of a mixture without altering their molecular compositions. For example, when making tea, the water extracts compounds from the tea leaves, but individual molecules like caffeine or tannins remain unchanged even when they move from the leaves to the water. This emphasizes why separation of mixtures is deemed a physical change. Understanding molecular composition aids in predicting how a substance will behave during separation and is key to selecting the appropriate separation technique.