When a substance is heated, its density
A. Increases
B. Decreases
C. Remains same
D. First decreases then increases

Hint: When a body’s temperature is changed then the interatomic vibrations of the molecules of the body also change. On increasing the temperature, these vibrations increase in magnitude pushing the molecules away from each other.

Detailed step by step solution:
According to the kinetic theory of gases, we know that the temperature of a body gives us the measure of kinetic energy of molecules in that body. A higher temperature means that the molecules of the body are vibrating with large kinetic energy while a lower temperature means that the molecules of the body are vibrating at lower energies.
The vibrational motion of the motion opposes the attractive forces between the molecules which binds them together. With increase in temperature, the molecules start pushing away from each other which leads to increase in the volume of the material.
As we know that density of a substance is given as the mass contained in the body per unit volume. Higher the volume of the material, lower is its density if its mass is kept constant.
$\rho = \dfrac{m}{V}$
Now when temperature of a substance is raised, the volume increases while mass will stay constant in ideal situations. Therefore, with the increase in temperature the density of the substance will decrease.
Hence, the correct answer is option B.

Note: The above conclusion that the density decreases with increase in temperature holds true for the majority of materials but there are some exceptions also to this. For example, in the case of ice, due to the spacious lattice structure, when it is melted at 0$^\circ C$ its density increases up to 4$^\circ C$ and decreases beyond it.