Final answer:
The density of air decreases with altitude. Density can be used to identify substances in objects. Additional information is needed to identify substances in an object composed of more than one material.
Step-by-step explanation:
The density of air decreases with altitude. For a column of air of a constant temperature, the density decreases exponentially with altitude. This means that as you go higher in the Earth's atmosphere, the air becomes less dense.
One example where density is used to identify the substance composing an object is in flotation. When an object is placed in a fluid, it will either float or sink depending on its density relative to the density of the fluid. For example, if you have a piece of wood and a piece of metal with different densities, you can determine which is which by observing their behavior in water.
To identify the substances in an object composed of more than one material, you would need additional information beyond just the average density. This could include measurements of different properties such as hardness, melting point, or conductivity.