Glossary / Physical properties
Relative density
Relative density is the density of a substance divided by the density of a reference material, which is usually water at a stated temperature. It is the term used in GHS-aligned SDSs and is effectively the same property that older documents call specific gravity.
Explanation
GHS lists relative density as one of the required entries in Section 9 of the SDS. A value less than 1 means the material is less dense than water and floats; a value greater than 1 means it is denser and sinks. Like Specific gravity, it is dimensionless, because the units cancel in the ratio.
The reference conditions matter. The SDS should state the temperature of the sample and, where relevant, the temperature of the reference water. Some suppliers also quote the property for the vapor, expressed relative to air; that figure is the Vapor density, a different property with a different reference.
Why you need it: it tells you how a spill will behave in water, how much a container will weigh, and how a mixture may layer or separate. It is also useful when comparing similar products or when checking that the material in a tank is what the label says it is.
Relative density is only one clue. Solid products may report bulk density rather than a true density, and for powders the number can change with how tightly the material is packed. If it is critical to your process or a spill plan, ask the supplier which measure the SDS value represents.
What to do and watch for
- Read the reference temperature and reference material before comparing values between products.
- Use it with Solubility in water to predict how a spill will behave in water.
Where it appears on an SDS
- Section 9: Physical and chemical properties. Reports relative density under that name (or specific gravity on older SDSs), with the measuring conditions where given.
SDS content and wording vary by supplier. This page is general reference and does not replace the current SDS for your product.