Specific Gravity
Specific gravity is a measurement that compares the densities of two substances, commonly using pure water as the reference point. It is expressed as a dimensionless ratio, calculated by dividing the density of the substance by the density of water, which has a standard specific gravity of 1. This means that if a substance has a specific gravity less than 1, it will float in water, while a specific gravity greater than 1 indicates that it will sink. Specific gravity can provide valuable insights into the physical properties of materials and is used in various scientific applications, such as determining the makeup of urine samples in medical tests.
To measure specific gravity, scientists often utilize a hydrometer, a device that can indicate the specific gravity of liquids and can also be specialized for specific purposes, such as measuring alcohol content or sugar levels in solutions. While specific gravity is primarily associated with liquids and solids, gases can also be analyzed in this way, with their specific gravity calculated against the density of air. Understanding specific gravity can assist in differentiating between materials and assessing their behaviors in different environments.
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Specific Gravity
Specific gravity is a comparison of the densities of two substances, one of which is usually pure water. Specific gravity is measured as a ratio. It is a dimensionless quantity, which means that it has no units of measurement associated with it. When the specific gravity of a substance is measured, it is usually measured in air rather than in a vacuum. (A vacuum is an environment in which no matter, including air, exists.) The term specific gravity is often abbreviated as sp gr.
Background
The specific gravity of a substance is related to its density. Indeed, one must find the density of a substance before trying to determine its specific gravity. Density is a physical property of matter. The density of matter is its mass per unit of volume. Density may be expressed using various combinations of units for mass and volume, but a common unit of density is grams per cubic centimeter (g/cm3). Very few substances have the exact same density, so scientists use density as a physical property that can help to identify substances. The formula for finding density is density = mass / volume. A heavy rock will have a higher density than a lightweight foam ball of the same size because the rock has more mass. Once the density of a substance is known, one can use it to find the substance's specific gravity.
Specific gravity is found by creating a ratio. The ratio is the density of one substance in comparison to the density of a reference substance. The formula for finding specific gravity is as follows: specific gravity = density of substance / density of reference substance. Most often, the reference substance used to measure specific gravity is pure water because the specific gravity of pure water at room temperature (or 4°C) and standard air pressure is 1. Thus, the ratio may also be written this way: specific gravity = density of substance / 1 g/cm3. Because the density of the reference substance is almost always 1, the specific gravity of a material is usually the same as its density. However, specific gravity is a dimensionless quantity, which means that it is measured without units. This is because the units in the numerator of the ratio and the units in the denominator of the ratio cancel each other out. For example, a mineral sample may have a density of 3 g/cm3. The mineral's specific gravity can be calculated with the ratio 3 g/cm3 / 1 g/cm3. Thus, the specific gravity is 3. It is important for people calculating specific gravity to use the same units in the numerator and denominator of the ratio so that they cancel out and the resulting measurement has no units.
Overview
Observers cannot measure specific gravity without special equipment. However, observers usually can tell when two materials have very different specific gravities. For example, graphite has a specific gravity of 2.23, and silver has a specific gravity of 10.5. An observer holding a one-centimeter piece of graphite and a one-centimeter piece of silver could tell a difference between the two objects. The piece of silver would feel heavier, even though it is the same size as the piece of graphite.
Understanding a substance's specific gravity can help one determine whether that substance will sink or float. The specific gravity of pure water is 1. Materials with specific gravities less than 1 are less dense than water. So, materials with specific gravities less than 1 will float in water. Materials with specific gravities more than 1 are denser than water. That means they will sink in water.
Scientists measure specific gravity for a number of reasons. Specific gravity can tell scientists more about a particular substance and its physical characteristics. Scientists can differentiate between materials when they understand their specific gravities. Specific gravity measurements may be used in real-world applications, too. For example, a common medical test is measuring the specific gravity of urine samples. Testing the specific gravity of urine helps medical professionals understand the makeup of a patient's urine. The test can help them determine whether a patient has an infection, is dehydrated, or has kidney or liver problems.
Scientists can use a tool called a hydrometer to measure specific gravity. This tool looks similar to a thermometer and indicates the specific gravity of the substance being tested. Observers can place a hydrometer in liquid to measure the liquid's specific gravity. Some hydrometers are created for specific purposes. Some measure the alcohol content of an alcohol-and-water mixture. Other hydrometers can measure the salinity of salt water. Still other hydrometers can measure the sugar content in maple sap and maple syrup. For example, sap is boiled so that water evaporates. The sugar content increases as water boils away. Different grades of maple syrup have different sugar contents. Specialized hydrometers help syrup manufacturers know the amount of sugar in their maple syrup.
Scientists can also find the specific gravities of gases. They first find the density of the gas in kilograms per meter cubed (kg/m3). Then, they divide that density by the density of air at room temperature. This is approximately 1.29 kg/m3. Thus, unlike liquids and solids, gases' densities are different from their specific gravities. Gases that have specific gravities of less than 1.29 will often rise higher in a container or a room. Gases that have specific gravities of more than 1.29 will often sink lower in a container or a room.
Bibliography
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"Specific Gravity." The Department of Chemistry at the University of Illinois at Urbana-Champaign, Apr. 2010, www.chem.uiuc.edu/WebFunChem/specificgravity/sg.htm. Accessed 25 Nov. 2024.
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"Weird Science: Hydrometers and Specific Gravity." College of Education, University of Hawai'i, manoa.hawaii.edu/exploringourfluidearth/physical/density-effects/measuring-salinity/weird-science-hydrometers-and-specific-gravity. Accessed 25 Nov. 2024.