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Jai Ganesh

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Relative Density

Relative Density

Gist

Relative density on the other hand can be defined as the ratio of the density of a substance to the density of the standard substance. Relative Density is also called Specific Gravity.

Density is the ratio between the mass and the volume of a body. Relative density, on the other hand, is the ratio between the density of an object (substance) and the density of some other reference object (substance) at some given temperature. Density is measured in, while relative density is dimensionless.

Summary

Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nearly always measured with respect to water at its densest (at 4 °C or 39.2 °F); for gases, the reference is air at room temperature (20 °C or 68 °F). The term "relative density" (abbreviated r.d. or RD) is preferred in SI, whereas the term "specific gravity" is gradually being abandoned.

If a substance's relative density is less than 1 then it is less dense than the reference; if greater than 1 then it is denser than the reference. If the relative density is exactly 1 then the densities are equal; that is, equal volumes of the two substances have the same mass. If the reference material is water, then a substance with a relative density (or specific gravity) less than 1 will float in water. For example, an ice cube, with a relative density of about 0.91, will float. A substance with a relative density greater than 1 will sink.

Temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm (101.325 kPa). Where it is not, it is more usual to specify the density directly. Temperatures for both sample and reference vary from industry to industry. In British brewing practice, the specific gravity, as specified above, is multiplied by 1000.[4] Specific gravity is commonly used in industry as a simple means of obtaining information about the concentration of solutions of various materials such as brines, must weight (syrups, juices, honeys, brewers wort, must, etc.) and acids.

Details

The difference between the specific gravity and density is that at room temperature and pressure, 1gram per 1 cubic cm is the density of water this density is treated as a standard and the density of any other material (usual liquids) is calculated relative to this is called relative density or specific gravity.

Hence, specific gravity is the ratio of the mass of a substance to that of a reference substance, let’s consider the density of honey is approx. 1.42 grams/{cm}^3, so its specific gravity would be 1.42/1 = 1.42. Notice that specific gravity is a ratio, therefore specific gravity does not have a unit, and hence specific gravity is a dimensionless physical quantity.

The specific gravity of a substance will let us know if it will float or sink, it gives us the idea about relative mass or relative density. If the specific gravity of a substance is below 1 then it will float and if it is greater than 1 it will sink.

Several liquids with different specific gravity are given below for reference.

Material  :  Density(gram/{cm}^3)

Rubbing Alcohol  :  0.79
Lamp Oil  :  0.8
Baby Oil  :  0.83
Water  :  1.0
Milk  :  1.03
Liquid Soap  :  1.06
Corn Syrup  :  1.33
Maple Syrup  :  1.37
Honey  :  1.42

Archimedes Principle

Archimedes principle states that if a body is submerged partially or fully inside a fluid, the body will experience a force that will be equal to the weight of the fluid displaced. This principle is used to determine the purity of gold and other metals, designing of ships, and in lactometers which are used to determine the quality of milk.

Frequently Asked Questions – FAQs

Q1 : Why does relative density have no unit?
A1 : Relative density is the ratio of two similar quantities, so it does not have a unit.

Q2 : Is the relative density of a material constant?
A2: The relative density of any substance is directly relative to its reference. The relative density of a substance relative to oil will be different from the relative density with respect to water.

Q3 : What is the difference between density and relative density?
A3 : Density is defined as the ratio of the mass and the volume of a body. On the other hand, relative density is defined as the ratio between the density of a body and the density of some other reference body.

Q4 : What is the common unit of density?
A4: Grams per cubic centimetre and kilograms per meter cube.

Q5 : At what temperature, the density of water is maximum?
A5 : At 4°C, water has the maximum density.

Additional Information

Specific gravity is the ratio of the density of a substance to that of a standard substance.

The usual standard of comparison for solids and liquids is water at 4 °C (39.2 °F), which has a density of 1.0 kg per litre (62.4 pounds per cubic foot). Gases are commonly compared with dry air, which has a density of 1.29 grams per litre (1.29 ounces per cubic foot) under so-called standard conditions (0 °C and a pressure of 1 standard atmosphere). For example, liquid mercury has a density of 13.6 kg per litre; therefore, its specific gravity is 13.6. The gas carbon dioxide, which has a density of 1.976 grams per litre under standard conditions, has a specific gravity of 1.53 (= 1.976/1.29). Because it is the ratio of two quantities that have the same dimensions (mass per unit volume), specific gravity has no dimension.

Buoyancy (the ability of an object to float in water or air) is intimately related to specific gravity. If a substance has specific gravity less than that of a fluid, it will float on that fluid: helium-filled balloons will rise in air, oil will form a slick on water, and lead will float on mercury. The specific gravity of a substance is characteristic; it is the same for different samples of a substance (if pure, the same in composition, and free from cavities or inclusions) and is used to help identify unknown substances. Specific gravity has many other applications: gemologists use it to distinguish similar gems; chemists, to check on the progress of reactions and the concentration of solutions; and auto mechanics, to test battery fluid and antifreeze.

Specific gravity is the basis of methods used throughout history to concentrate ores. Panning, jigging, shaking, spiral separation, and heavy-medium separation are among the methods that depend on differences in specific gravity to obtain concentrated ore. Specific gravity is highest in rocks rich in iron, magnesium oxide, and the heavy metals and lowest in those rich in alkalies, silica, and water.

The ease with which specific gravity can be precisely determined leads to its widespread use in chemical science and technology; for example, determination of the specific gravity is part of the routine characterization of a new liquid compound. The specific gravity of most organic compounds containing only carbon, hydrogen, and oxygen is less than one. Among the devices used to measure specific gravity are the Westphal balance, the pycnometer, and the hydrometer.

What is Relative Density?

In the practice, the term relative density is also used (relative density is the ratio of the density of that substance at a certain temperature, to the density of water at the same or some other temperature). Determination of the relative density is performed by determining the ratio of the mass to the determined volume of the test solution and the mass of the same volume of water at a temperature of 20 °C. Relative density is dimensionless. Density and relative density at 20 °C are measured on experimental sample by using the reference method (picometry) or the usual method (hydrometry or densitometry using hydrostatic scale).

Difference Between Density and Relative Density

Density is the ratio between the mass and the volume of a body. Relative density, on the other hand, is the ratio between the density of an object (substance) and the density of some other reference object (substance) at some given temperature. Density is measured in, while relative density is dimensionless. Density is unique for each body, while the same body can have numerous relative densities (compared to different reference bodies).

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