My textbook says that the specific gravity is "the ratio of the mass of a mineral compared with the mass of an equal volume of water."

Is this any different to density? If so, how? If not, why does it have its own name in geology?

  • $\begingroup$ As mentioned in the other answers, SG is a ratio of densities. Turns out that density in g/cm³ has the same numerical value as SG. g/cm³ is used very commonly in geology, therefore these two are used interchangeably. So they are not exactly the same thing, but the number is the same and they mean generally the same. $\endgroup$ – Gimelist May 22 '16 at 10:25
  • $\begingroup$ Using the metric system, SG & density have the same numerical value because the density of water is 1 g/cm³. In antiquated systems of units like the British or US system of units, numerically there will be differences between SG & density. $\endgroup$ – Fred May 23 '16 at 14:54

It is different to density but they are closely related. This is easily seen by considering the dimensions of the two quantities (side note: always consider the dimensions of quantities - it is invariably useful).

If you read your definition carefully you will realise that specific gravity is defined as the ratio of the density of the material to that of water. $$\mathrm{SG = \frac{\rho_{sample}}{\rho_{water}}}$$

Historically I suspect that specific gravity was used rather than density because it is easy to measure by measuring the mass of the displaced water when you place the sample into a jar of water, and the mass of the sample itself. You don't actually need to know the density of water in order to calculate the specific gravity.

  • $\begingroup$ The density of water is 1. So it's a ratio of 1 to X, which is X...? Am I misunderstanding? $\endgroup$ – Tim May 21 '16 at 21:21
  • $\begingroup$ No you are correct - but dimensionally they are different so they cannot be the same quantity and the the key point is that you can measure specific gravity without knowing anything about the density of water. $\endgroup$ – bon May 21 '16 at 21:35
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    $\begingroup$ Yes, if we assume 4 degree C as reference temperature and Standard atmospheric pressure as reference pressure. If reference Temp and pres differ from these values, then SG and rho differ. Although, I agree with you that - except for the different unit - SG and rho have equal values. $\endgroup$ – daniel.neumann May 21 '16 at 21:47

It's simple. Specific Gravity is dimensionless. It's just a ratio. Density is dimensional, such as grams per cubic centimeter, kilograms per litre, tonnes per cubic metre, etc.


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