Relative density is the ratio of the density of a substance to the density of a reference material. For liquids and solids the reference is usually water. Because it is a ratio of two densities, relative density has no units, and a value above one means the substance is heavier than water. Food analysts use it to check the strength of syrups and brines, the concentration of beverages such as beer wort and spirits, and the purity of edible oils, where deviations from the expected value reveal dilution, adulteration, or process faults.