The correct option is A. Dissolving one substance in another.
A physical change involves alterations in the physical state or form of a substance without changing its chemical composition. Among the given options:
A. Dissolving one substance in another is a physical change. Dissolving is a process where a substance (solute) disperses uniformly in another substance (solvent) to form a solution. It does not result in the creation of a new substance; rather, it involves the intermolecular forces between particles.
B. Making a new substance and D. Changing one substance into another** both describe chemical changes. Chemical changes involve the formation of new substances with different chemical compositions. This includes the breaking and forming of chemical bonds, leading to a transformation at the molecular level.
C. Breaking apart one substance into two substances can be ambiguous. If it refers to a physical separation without altering the chemical composition, it could be considered a physical change. However, if it involves a chemical process leading to the creation of two distinct substances, it would be a chemical change.
Therefore, among the options, A. Dissolving one substance in another is a physical change as it involves a change in the physical state without altering the chemical composition.