Formulators often are faced with product specifications for which the tests are not chemical but physical. Oftentimes, the terms describing these physical traits are not clearly defined. Two such terms are density and specific gravity, which are defined here by industry expert Tony O'Lenick.

* Density *is the mass of a material per unit volume. The unit for density is the kg/m

^{3}or g/mL. The measurement can easily be made using a pycnometer. This apparatus is a vessel with a very carefully controlled volume. The liquid is added to full the unit, then the weight is determined. Density is important to the formulator because in cases of systems in which there are insoluble materials (i.e., oil, water and silicone), the less dense fluids float on more dense fluids.

Density is also used in fitness. Body density measures mass per unit volume. One method to determine body density is to look at the volume of water an individual's body displaces from a pool and their weight. From this calculation, one can determine how much fat versus lean mass make up his or her body. Lean mass consists of muscle, bones, internal organs, blood and water. Fit individuals have a higher body density while sedentary individuals typically have a lower body density.

* Specific gravity *is a unit of measure that compares the ratio of density of a material to another, typically water. Consequently, the specific gravity is a unitless number. Temperature and pressure must be specified for both the sample and the reference. The specific gravity is of interest to the formulator since it can help determine a formula's weight per volume.

For example, a gallon of water weighs weighs 8 pounds and has a specific gravity of 1.00. As a comparison:

- Water weighs 8.00 pounds per gallon and has a specific gravity of 1.00;
- Diesel weighs 7.29 pounds per gallon and has a specific gravity of 0.911; and
- Gasoline weighs 6.2 pounds per gallon and has a specific gravity of 0.775.

**Test Your Skills
**Calculate the following weights by their specific gravities, in comparison with water:

1. Olive oil has a specific gravity of 0.917; what does a gallon of it weigh?

2. Sea water has a specific gravity of 1.201; what does a gallon of it weigh?