Caustic concentration is a chemical designation that describes how corrosive a particular solution will be when it comes in contact with common metals such as zinc, tin, bronze, brass, copper and aluminum. Concentration is typically defined as what amount of the total solution the caustic agent makes up.
The specific gravity of a substance is a ration of its density to that of fresh water at 4 degrees Celsius (unless otherwise specified). Items with a specific gravity of less than one will float in fresh water while those with higher ratios will sink. Specific gravity is always given in terms of grams per centimeter cubed.
Generally, the higher the concentration of caustic substances within a solution, the higher the specific gravity of that solution will rise (due to those caustic substances making the substance denser than water). For instance, sodium hydroxide at a solution of 20 percent has a specific gravity of 1.22 g/cm3 but the more caustic 40 percent solution has a specific gravity of 1/43 g/cm3.