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Q & A: What is specific gravity?

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Can you explain specific gravities of liquids? What is it?
- Jake
Nevada, USA
Excellent question!!!  The specific gravity ("Sp.G.") of a liquid tells you how much more or less dense the liquid is than water.  Water has a specific gravity of 1.000 (near 4įC).  If a liquid is more dense than water, then its specific gravity is greater than 1.  If it is less dense than water, then the specific gravity is less than 1.  To calculate the specific gravity of a liquid, you have to know its density.  Take the density of the liquid, divided by the density of water (1 gm/cm3), and you will get the specific gravity of the liquid:

            Sp.G. = (density of liquid) / (1 gm/cm3)

Specific gravity is also related to buoyancy.  The denser a liquid is, the greater the force of buoyancy will be on an object floating in/on it (the higher it will float).  You can calculate the magnitude of the buoyancy force like this:

            Fbuoyancy = (density of liquid) x (volume of object) x (acceleration of gravity)

In the subjects I study (biology & medicine), specific gravity is important for determining how well a person or animalís kidney is working.  The job of the kidney is to filter unwanted molecules out of the blood and into the urine.  But to keep you from getting totally dehydrated, it also has to be able to make the urine very concentrated (so you donít lose too much water).  The more concentrated the urine is, the higher its specific gravity is.  So an easy way for a doctor or veterinarian to check how well the kidneys are working is to check the urineís specific gravity.


(published on 05/14/07)

Follow-Up #1: specific gravity of liquids

how to determine the specific gravity of liquid?
- Anonymous
One uses a device called a hydrometer to measure the specific gravity of liquids.  There are a variety that are used for specific purposes such as:   the charge of a car battery,  alcoholic content of water-alcohol mixtures,  freezing point of radiator fluid in a car, etc.   They all work on the principle of a weighted glass vial with a long neck calibrated in the appropriate units.
The vial floats to a certain level and one reads off the specific gravity on the scale.
See the various wikipedia articles for more detailed explanations:


(published on 07/01/07)

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