What happens to the density of a fluid as it is heated?

When a fluid is heated, its density generally decreases. This phenomenon occurs because heating a fluid typically causes its molecules to move faster and spread apart. As the temperature rises, the increased kinetic energy leads to expansion of the fluid, which results in a decrease in density because density is defined as mass per unit volume.

For a given mass of fluid, if the volume of the fluid increases due to heating, the overall density will decrease as the same mass occupies a larger volume. This behavior is particularly observable in liquids and gases.

In contrast, other choices do not accurately reflect this relationship. For example, an increase in density would typically occur upon cooling or compressing a substance rather than heating it. Similarly, density remaining constant would imply that temperature changes do not affect the distance between molecules, which isn't consistent with the observed expansion of fluids upon heating. Fluctuating density would imply erratic changes, which is not characteristic of the stable and predictable behavior of fluids under temperature changes.