The density of a substance is an intrinsic property that measures the amount of mass per unit volume. While there can be variations in density due to factors such as temperature or pressure, the density of a pure substance remains constant for a given set of conditions.

However, if the substance is not pure or is a mixture of different components, the density can vary depending on the composition and proportions of the components. For instance, if you mix water with salt, the density of the resulting solution will increase compared to pure water due to the presence of dissolved salt.

Therefore, the density of a substance is constant for a pure substance under specified conditions, but it can vary for mixtures or impure substances depending on their composition.