What is the relationship between volume and density when temperature increases?

When the temperature of a substance increases, its volume generally increases due to thermal expansion. As particles gain heat energy, they tend to move more vigorously, causing them to occupy more space. This increase in volume, when not accompanied by a corresponding increase in mass, leads to a decrease in density since density is defined as mass per unit volume.

For example, in gases, the impact of temperature is very pronounced. When gas is heated, it expands significantly, resulting in lower density at higher temperatures. In liquids and solids, although thermal expansion still occurs, the changes in volume are comparatively smaller.

Thus, the correct relationship when temperature increases is that the volume increases while the density decreases.