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Knowledge of Chemical Unit Operations 1: The Concept of Density. Density is the mass per unit volume. The international unit is kilograms per cubic meter (kg/m3); grams per cubic centimeter (g/cm3) is also commonly used. For liquids or gases, kilograms per liter (kg/L) and grams per milliliter (g/mL) are also used. But g/L is generally not used. Physical meaning: The physical meaning of density is that it represents a property of a substance which does not change with variations in mass and volume, but rather changes only with variations in the substance’s state, temperature, and pressure. The ratio of the mass of a substance to its volume, that is, the mass of that substance per unit volume, is called the density of that substance. Taking water as an example, its density at 4°C is 103 kilograms per cubic meter, or 1 gram per cubic centimeter; in other words, 1.0×10³ kg/m³. This means that the mass of water in one cubic meter is 1000 kilograms. Density is usually denoted by the symbol “ρ”, and is pronounced as “ròu”. The average density of the Earth is 5518 kilograms per cubic meter. The average density of dry air under standard conditions is 0.001293×103 kilograms/m3. Density is a physical quantity used to describe the mass of a substance per unit volume. Density can also be interpreted as the ratio of a quantity to a range, and this is used as an abbreviation in such cases; for example, population density, magnetic flux density, also known as magnetic induction strength. The definition of density refers to the mass per unit volume of a substance at a specified temperature, that is, the ratio of the mass to the volume of that same substance. It is expressed as kilograms per cubic meter for kg/m^3, or grams per cubic centimeter for g/cm^3. For objects of the same mass, the smaller the volume, the greater the density; the larger the volume, the lower the density. It is mainly used for converting quantities and measuring delivery acceptance, as well as for quality control of certain oils and for simply assessing the performance of oils. Density conversions: 1 pound/ft³ = 16.02 kilograms/m³; Specific gravity at 141.5/15.5°C = 131.5; 1 pound/gal = 99.776 kilograms/m³; Specific gravity at 140/15.5°C (Baume scale) = 130; 1 pound/in³ = 27,679.9 kilograms/m³; 1 pound/gal = 119.826 kilograms/m³; 1 pound/bbl = 2.853 kilograms/m³; 1 kilograms/m³ = 0.001 grams/cm³; 0.001 grams/cm³ = 0.0624 pounds/ft³. These are the formulas for calculating density as well as the units used for expressing density. ρ=M/V. M=ρV. V = M/ρ. The commonly used unit is grams per cubic centimeter. g/cm3。 kilograms per cubic meter. kg/m3。 Their conversion relationship is 1 g/cm3 = 10-3 kg/10-6 m3 = 1000 kg/m3. 1000 kg/m3 = 1 g/cm3. 2700kg/m3=2.7 g/cm3. Understanding the patterns is convenient in calculations related to density.