When a physical change occurs, the mass of the substance is conserved. this means that the total mass of the substance remains the same from beginning to end. the physical properties of the substance, such as size and shape, may change, but the amount of matter in the substance does not change.

Scientists follow specific processes in order to determine valid explanations and conclusions from observations. david observed strange lights in the sky above his home outside of roswell, new mexico. the only explanation that makes sense to him is that there are ufos in the area. what should david do next to verify his explanation?