Which term describes a substance that expands indefinitely and takes the shape of its container?

The term that describes a substance that expands indefinitely and takes the shape of its container is gas. Gases do not have a fixed volume or shape; they will fill any container they are placed in and can expand to occupy a larger space if allowed. This behavior is due to the high kinetic energy of gas molecules, which move freely and are far apart from one another compared to solids and liquids.

In contrast, liquids have a definite volume but take the shape of their container, while solids maintain both a definite shape and volume. Aerosols comprise tiny particles or droplets suspended in a gas, which means they are more complex and do not simply describe a substance's ability to expand and take the shape of a container. Therefore, gas correctly identifies the property of a substance that expands indefinitely and conforms to the shape of its environment.