The name of the instrument used to compare the density of sea water to the density of fresh water is hydrometer.
Density is a measure of the compactness of the molecules in a substance.
Density is given mathematically as the ratio of the mass and volume of a substance.
The instrument that would be used to compare the density of sea water to the density of fresh water is a hydrometer.
A hydrometer is an instrument used to compare the density of different fluids.
In conclusion, density is the ratio of mass and volume.
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