The answer is volume, in gasses, volume depends on the matter, but in solids, volume can be measured.
Compounds are made of two or more atoms of different elements, such as water (H2O) and methane (CH4). Atoms are not drawn to scale. Molecules of compounds have atoms of two or more different elements.
Since Alai is hitting each with a hammer, the physical property which he must be comparing on the two materials must be hardness. Solubility can be tested when you put them in a solvent. Odor can be tested with smell. Conductivity can be tested with electricity. Hardness can be tested with its resistance to the force that hits it.
A compound is a pure substance composed of two or more different atoms chemically bonded to one another. A compound can be destroyed by chemical means. It might be broken down into simpler compounds, into its elements or a combination of the two.
