What happens to molecules as temperature increases?

As temperature increases, the kinetic energy of molecules also increases, causing them to move more rapidly. This increased movement leads to a greater distance between the molecules, which results in them spreading out. When molecules spread out, the overall density of the substance decreases because density is defined as mass per unit volume. When the same amount of mass occupies a larger volume due to the expanded spacing of the molecules, the density must decrease. This principle is observed in many states of matter, particularly in gases and in the thermal expansion of liquids and solids.

It's important to note that in the context of temperature changes, the behavior of molecules is influenced significantly by their state; for example, gases tend to expand more significantly with increases in temperature compared to liquids and solids. This dynamic contributes to a variety of natural phenomena, such as why warmer air rises in the atmosphere or how hot water expands when heated.