[Physics Class Notes] on Specific Gravity Pdf for Exam

Specific gravity is an essential chapter in Physics. It has several real-life implications and applications. For example, when young children start learning to swim, they use floaters. Similarly, when we travel on a boat, we expect the boat not to have any leaks. Several factors contribute to the floating of any object on any liquid. One such factor is specific gravity. We will learn about the particular gravity in detail in this chapter. In this chapter about Specific Gravity, we will be going to deal with Explanation, Formula, Calculation, and its applications. 

Definition: What is Specific Gravity?

Physics is more of a practical subject. And, if you want to understand it properly and in-depth, you must be thorough with the concept of specific gravity.

Specific gravity is also known as relative gravity. It does not have any dimension which implies that it is a dimensionless quantity. It can be defined as the ratio of a substance’s density to that of water at a particular given temperature. It is important to note that temperature plays a vital role in determining the specific gravity of any substance. Therefore, specific gravity is a property of the substance at a particular temperature. Besides, pressure also affects the specific gravity of any substance.

According to the definition of specific gravity, it can be mathematically expressed as :

[SG = frac{rho_{substance} }{rho _{water}}]

In general, the specific gravity of any substance is defined relative to water at 4⁰C. This temperature is considered because the water density at that temperature is 1000kg/m³ or 1g/cm³. The density of water in other units can also be used to calculate any substance’s specific gravity value. The different densities of water are 62.43 pounds per cubic foot and 0.036 pounds per cubic inch.

[Specific Gravity = frac{Density , of , the , object}{Density , of , water rho object} = frac{rho object}{rho H_{2}O}]

What is the Specific Gravity of Water?

According to the meaning of specific gravity, the specific gravity of water value is equal to 1. However, the temperature of water whose specific gravity is considered needs to be considered here. At different temperatures, the density of water is different. Therefore, the specific gravity of water value can also change depending on temperature. Additionally, the specific gravity of water can also change due to the presence of impurities in it. Depending on the kind of impurity present in water, water density will vary, and so its specific gravity.

In ideal conditions, the specific gravity of water value for pure water at 4⁰C is 1.

Specific Gravity of Other Substances

If you now understand the meaning and concept of specific gravity, you will be able to calculate the specific gravity of any substance. There are several applications of specific gravity based on these values. In general, specific gravity for gases is the least, while that of the solids are the most. For example, the specific gravity of dry air is 0.0013. However, the presence of moisture in the air will increase the specific gravity value. Specific gravity for alcohol is 0.82. Following the specific gravity definition, the value for carbon dioxide is 0.00126. Exceptions will be rubber and wood oak, which are solids but have a specific gravity value less than water (0.96 and 0.77, respectively). All these values make up for the different applications of specific gravity, which is discussed in the subsequent segments.

How to Calculate Specific Gravity?

Till now, it has been clarified to you what the meaning of specific gravity is. With the application of the concept that you’ve understood, you will be able to calculate the specific gravity of any substance.

For calculation purposes, you will need to know the density of the substance at a particular temperature. As stated before, the temperature affects the density of the substances. You can calculate the substance’s density by using a densitometer or divide its mass by its volume. It would help if you also observed what the specific gravity of water is as the base value. For measuring the specific gravity of gases, you must calculate it concerning air. The density of air is 1.20 kg/m³ at room temperature.

Applications of Specific Gravity

There are several applications of specific gravity. Some of them are:

• The purity of gems can be measured by comparing their value with the ideal sets. 

• Gemologists use it to distinguish similar gems.

• Geologists and mineralogists also use these values to determine the mineral content of any rock.

• Urinalysis experts measure the specific gravity of urine to determine its contents.

• Checking on the progress of reactions and the concentration of solutions.

• Testing  battery fluid and antifreeze.

• Students and researchers also learn how to find specific weight from specific gravity. If you want to know how to find specific weight from specific gravity, multiply the specific gravity value with the 1000 (density of water) and 10 (acceleration due to gravity).

Points to ponder about Specific Gravity

• Specific gravity is a dimensionless quantity. That means it can be represented as fundamental units raised to power zero. M0 L0 T0. This also means that specific gravity does not have any units.

• One of the reasons why specific gravity is a unitless quantity is that it is the ratio of two quantities with same dimensions or units.

• Specific gravity is simply the ratio of two densities of two objects or substances.

• In most of the cases of specific gravity, it is defined as the ratio of density of the substance with respect to density of water. 

• Specific gravity of water is unit i.e Specific Gravity (water) = 1

• If the specific gravity of a substance is greater than 1, it will sink into the water. Objects such as rocks, metals, etc which are very dense in nature are most likely to float as they have specific gravity more than 1.

• If the specific gravity of the substance is less than 1, the substance will float on the surface of the water instead of sinking into it. As such objects will have density values less than that of water, the ratio will be less than one. Paper, hair, dust, oil, etc are such examples where the speci
  fic gravity value is less than 1.

• Specific gravity values are affected by the temperature and pressure of the environment or the surroundings.

• Generally, specific gravity is measured at 40 C. The specific gravity of water is equal to 1 at 40 C. It is very important to note that this value will change as there will be a change in temperature.

• As temperature increases, a substance can undergo melting (solid to liquid) or evaporation (liquid to gas), or sublimation (solid to gas directly). This will result in a change in the density of the substance as solids are said to be the most dense among three and gases hold less density while the density of liquid lies between these two values. Hence, specific gravity changes with a change in temperature.

• To compare two specific gravities, make sure both of them are at the same temperature and pressure configuration. 

• The pressure surrounding the system of the substance affects specific gravity as pressure and volume are inversely proportional. Also, volume is inversely proportional to the density of the substance. Hence, we can conclude that specific gravity is directly proportional to the pressure experienced by the substance in the given system.

• The specific gravity of water at 1 atm pressure and 40 C = 1

The specific gravity of water at 1 atm pressure and 250 C = 0.9970