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Weird Science: Hydrometers and Specific Gravity

Aquarists, people who maintain aquaria, use hydrometers to quickly measure the salinity of their saltwater tanks. Some fish are very susceptible to changes in salinity, so the salt concentration of their tanks must be kept constant. However, hydrometers are also used to determine information about the concentration of many other types of solutions. For these other solutions, the measurement is not salinity, but specific gravity. Specific gravity is the ratio of the density of a substance to the density of a reference substance, generally water, at a certain temperature and pressure. Substances with a specific gravity of one are neutrally buoyant in water, substances with a specific gravity of less than one are less dense than water, and substances with a specific gravity greater than one are more dense than water.


<p><strong>SF Fig. 2.8.</strong> Different grades of maple syrup</p>Urine hydrometers are used to quickly determine a person’s hydration level. When a person is dehydrated, their urine will often have a dark color due to the presence of concentrated salts and minerals. A well-hydrated person’s urine will be lighter in color and less dense. Low specific gravity urine is also is also associated with medical conditions like diabetes and chronic kidney failure.


A maple syrup hydrometer is used to measure the density of maple syrup. Boiling maple tree sap produces maple syrup by evaporating off the water and making a dense sap, which we call syrup. Between 20 and 50 liters (L) of sap are boiled to produce 1 L of syrup. The specific gravity of the syrup, determined by the concentration of sugar in the syrup, and the clarity of the syrup, is used to assign the maple syrup a grade (SF Fig. 2.8). Different grades of syrup are associated with different maple flavors.

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.