Final answer:
The density of 0.5 g/cm³ does not provide sufficient information to conclusively identify the state of matter or the type of substance; hence, the substance is unknown.
Step-by-step explanation:
The density of a substance indicates its mass per unit volume. The value given, 0.5 g/cm³, falls within the typical range for solids and liquids, as most have densities between about 0.7 g/cm³ and 19 g/cm³. However, gas densities are typically much lower and measured in g/L. Without additional context, we cannot determine the specific state of matter (solid, liquid, or gas), nor can we ascertain if the substance is a mixture or a pure substance. Based on this information, the correct answer is E) The substance is unknown.