In chemistry, the mass concentration (or ) is defined as the mass of a constituent divided by the volume of the mixture . For a pure chemical the mass concentration equals its density (mass divided by volume); thus the mass concentration of a component in a mixture can be called the density of a component in a mixture. This explains the usage of (the lower case Greek letter rho), the symbol most often used for density. Definition and properties The volume in the definition refers to the volume of the solution, not the volume of the solvent. One litre of a solution usually contains either slightly more or slightly less than 1 litre of solvent because the process of dissolution causes volume of liquid to increase or decrease. Sometimes the mass concentration is called titre. Notation The notation common with mass density underlines the connection between the two quantities (the mass concentration being the mass density of a component in the solution), but it can be a source of confusion especially when they appear in the same formula undifferentiated by an additional symbol (like a star superscript, a bolded symbol or varrho). Dependence on volume Mass concentration depends on the variation of the volume of the solution due mainly to thermal expansion. On small intervals of temperature the dependence is : where is the mass concentration at a reference temperature, is the thermal expansion coefficient of the mixture. Sum of mass concentrations - normalizing relation The sum of the mass concentrations of all components (including the solvent) gives the density of the solution: Thus, for pure component the mass concentration equals the density of the pure component. Units The SI-unit for mass concentration is kg/m3 (kilogram/cubic metre). This is the same as mg/mL and g/L. Another commonly used unit is g/(100 mL), which is identical to g/dL (gram/decilitre). Usage in biology In biology, the "%" symbol is sometimes incorrectly used to denote mass concentration, also called "mass/volume percentage". A solution with 1 g of solute dissolved in a final volume of 100 mL of solution would be labeled as "1%" or "1% m/v" (mass/volume). The notation is mathematically flawed because the unit "%" can only be used for dimensionless quantities. "Percent solution" or "percentage solution" are thus terms best reserved for "mass percent solutions" (m/m = m% = mass solute/mass total solution after mixing), or "volume percent solutions" (v/v =