What is the effect of increasing pressure on the density of a substance?

Increasing pressure on a substance typically results in the molecules of that substance being forced closer together. This is due to the fact that pressure compresses the volume occupied by the molecules, which leads to a greater number of molecules per unit of volume. Consequently, since density is defined as mass per unit volume, as the volume decreases while the mass remains constant, the density of the substance increases. This compression is particularly noticeable in gases, where the effect of pressure on density is more pronounced than in solids or liquids. Thus, the correct understanding is that with increasing pressure, molecules indeed move closer together, leading to an increase in density.