Definition:
Specific Gravity is defined as the ratio of the weight density (or Density) of a fluid to the weight density (or density) of a standard fluid. For liquids, the standard fluid is taken water and for gases, the standard fluid is taken air.
Specific gravity is also called Relative Density.
· For Liquids: The standard fluid used for comparison is water.
· For Gases: The standard fluid used for comparison is air.
It is dimensionless quantity and is denoted by the symbol 'S'.
Material | Specific Gravity |
Dry Air | 0.0013 |
Alcohol | 0.82 |
Carbon dioxide | 0.00126 |
Cast Iron | 7.20 |
Petrol | 0.72 |
Rubber | 0.96 |
Wood Oak | 0.77 |
Nylon | 1.12 |
Factors Affecting Specific Gravity:
Specific gravity is not always constant. It is heavily influenced by:
Temperature: Changes in temperature alter the volume of a fluid, which changes its density. Water reaches its maximum density at 4 °C.
Pressure: Particularly for gases, an increase in pressure increases density, thereby increasing the specific gravity relative to the standard.
Standard temperature and pressure are used in the calculation of specific gravity. If failed to regulate the outside influences, then the specific gravity will change.
Applications of Specific Gravity:
Specific gravity has a wide range of applications, following are a few of the applications:
· Gemology: Professionals determine the gem purity by comparing its specific gravity with the already measured high purity level of another gem. This allows the gem’s value to be determined very quickly.
· Mineralogy: Specific gravity is used by mineralogists and geologists to determine the mineral content of the rock.
· Urinalysis: Urinary specific gravity is a measure of the concentration of solutes in the urine. It helps Doctors to evaluate kidney function and hydration levels. The test is a routine part of a urinalysis.
· Buoyancy and Stability: It tells us if an object will sink or float. Any substance with a specific gravity less than 1.0 will float on water.
· Pressure Calculations: It is used to calculate pressure head and manometric heights in hydraulic systems.
· Automotive: It is used to check the state of charge in lead-acid batteries by measuring the specific gravity of the electrolyte.
Frequently Asked Questions on Specific Gravity (FAQ)
Q1: What does a Specific Gravity value tell us?
The specific gravity of a substance indicates whether a substance will sink or float in the reference fluid. For example, if an object's specific gravity is less than 1.0, it will float on water.
Q2: What is the difference between specific gravity and specific weight?
Specific weight (ω) is the ratio of the weight of an object to the object’s volume (ω = ρg) measured in N/m3. In the case of Specific Gravity (S), it is the ratio of an object’s density to the density of water.
Q3: Which metal has the highest specific gravity?
Lead has the highest specific gravity.
Q4: Which metal has the lowest specific gravity?
Lithium has the lowest specific gravity.
Q5: Which instrument is used to measures the specific gravity of liquids?
A Hydrometer is used to measure the specific gravity of liquids.
Q6: Is specific gravity the same as mass density?
No, mass density (ρ) is the mass per unit volume and has units like kg/m3.
Specific gravity (S) is a dimensionless ratio and does not have any units.
Q7: What is a urine specific gravity test?
Urine specific gravity test compares the density of urine with the density of water. The test gives information about how well the kidneys concentrate the urine and a person’s hydration status.
Q8: Which substance is denser Mercury or Water?
Mercury is 13.6 times denser than Water. Since water's density is 1000 kg/m3, the density of mercury is 13,600 kg/m3.
Q9: What is specific gravity in a gemstone?
The specific gravity in a gemstone is the ratio of the density of the gemstone to the density of water.
Q10: At What temperature water has the highest density?
4 °C is the temperature at which water has the highest density.
Q11: What is the specific gravity of Petrol & rubber?
The specific gravity of petrol is 0.72 kg/m3 and rubber is 0.96 kg/m3.
Q12: What standard fluids are used for comparison?
For liquids, the standard fluid is always taken as water.
For gases, the standard fluid is taken as air.
Q13: What is the main application of specific gravity?
The concept of specific gravity is utilized by geologists and mineralogists to analyse the mineral constituents of rocks.