Here's why:

* Scalar quantities have only magnitude (size or amount).

* Vector quantities have both magnitude and direction.

Specific gravity is the ratio of the density of a substance to the density of a reference substance (usually water at 4°C). It's a dimensionless number, meaning it doesn't have units like meters or kilograms.

For example, if a substance has a specific gravity of 2.5, it means it's 2.5 times denser than water. This value doesn't have any direction associated with it.