Final answer:
The scientist observed that a solid substance dissolved in a liquid substance, forming a solution. This is a physical change where the properties of the substances did not change.
Step-by-step explanation:
The scientist observed that a solid substance dissolved in a liquid substance. This observation can be categorized as a physical change because the properties of the substances did not change, only their form. When a solid dissolves in a liquid, it forms a homogeneous mixture called a solution. In this case, the solid substance dissolved into the liquid, resulting in a solution of the two substances.