How to Distinguish Density from Specific Gravity in Science
Understanding the Difference Between Density And Specific Gravity is crucial for solving questions in Physics and Chemistry exams. This comparison helps students distinguish between absolute and relative properties of matter, which frequently appear in NEET, JEE, and board examinations.
Definition of Density
Density is defined as the mass per unit volume of a substance. It expresses how much matter is packed within a given volume of material.
The SI unit of density is kilogram per cubic metre (kg/m³), and it is considered an absolute property. For more details, refer to the Difference Between Density And Volume.
Definition of Specific Gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance, usually water at 4°C. It is a dimensionless and relative quantity.
Specific gravity indicates whether a substance is heavier or lighter compared to the reference material. It has no units, making it different from density. Learn more at Difference Between Density And Specific Gravity.
Difference Table
| Density | Specific Gravity |
|---|---|
| Mass per unit volume | Ratio of two densities |
| Absolute quantity | Relative quantity |
| SI unit is kg/m³ | No units (dimensionless) |
| Depends only on substance | Depends on both substance and reference |
| Expresses compactness of matter | Compares compactness to reference |
| Varies with temperature and pressure | Affected by temperature and pressure changes |
| Measured directly by mass and volume | Calculated from densities |
| Used for identification of substances | Used for characterization and comparison |
| Can be any positive value | Usually between 0 and greater than 1 |
| Example: 1000 kg/m³ for water | Example: 1 for water (relative to itself) |
| Specific for each material | Changes with reference choice |
| Used in equations like mass/volume | Used in ratio format |
| Directly involved in buoyancy calculation | Indicates if object floats or sinks |
| Intensive and intrinsic property | Intensive but relative property |
| Standard laboratory measurement | Useful in industries for identification |
| Helps distinguish materials by mass/volume | Compares to a standard material |
| Important in fluid mechanics | Critical for design of hydrometers |
| Constituent of Properties Of Solids And Liquids | Used in mineral and fluid identification |
| Expressed in g/cm³, kg/m³, etc. | Has no physical units |
| Remains fixed for pure material | Varies if reference substance changes |
Key Differences
- Density has units, specific gravity does not
- Density is absolute, specific gravity is relative
- Density measures compactness, specific gravity compares it
- Density is for single substance, specific gravity uses a reference
- Density is used directly in calculations, specific gravity is a ratio
Examples
The density of pure water is 1000 kg/m³, and its specific gravity is 1 with respect to itself.
Iron has a density of about 7870 kg/m³, so its specific gravity relative to water is 7.87, which shows iron is much heavier than water. For more real-world comparisons, refer to Difference Between Mass And Weight.
Applications
- Density helps in material identification and purity testing
- Specific gravity is used to check battery acid concentration
- Density is important in fluid dynamics and buoyancy studies
- Specific gravity aids mineral and petroleum industries
- Density determines floating or sinking in fluids
- Specific gravity is used in hydrometers for liquids
One-Line Summary
In simple words, density measures mass per unit volume, whereas specific gravity compares the density of a substance to a reference, typically water.
FAQs on Understanding the Difference Between Density and Specific Gravity
1. What is the difference between density and specific gravity?
Density refers to the mass per unit volume of a substance, while specific gravity is the ratio of a substance’s density to the density of water.
Key differences include:
- Density has units (g/cm³ or kg/m³), specific gravity is unitless.
- Density is an absolute measurement, specific gravity is a relative measurement.
- Specific gravity allows comparison with a standard (usually water for liquids and solids).
2. Define density and write its SI unit.
Density is the mass of a substance per unit volume.
SI Unit:
- Density = kilogram per cubic meter (kg/m³)
3. What is specific gravity? How is it calculated?
Specific gravity is the ratio of the density of a substance to the density of water at 4°C.
Formula:
- Specific gravity (SG) = Density of substance / Density of water
It is a dimensionless quantity (has no units).
4. Why is specific gravity unitless?
Specific gravity is unitless because it is a ratio of two densities expressed in the same units, so the units cancel out.
- Both numerator and denominator are in kg/m³ or g/cm³.
- Only their proportion is considered, not the actual measurement units.
5. How is density measured experimentally?
Density can be measured by dividing the mass of a substance by its volume.
Steps:
- Find mass using a balance (in grams or kilograms).
- Measure volume using a measuring cylinder or displacement method (in cm³ or m³).
- Density = Mass / Volume
6. What is the importance of specific gravity in real-life applications?
Specific gravity is important because it helps compare the heaviness of substances relative to water and is commonly used:
- In industries to identify substances and check purity
- In geology for mineral identification
- To test battery acid strength in automobiles
- For measuring alcohol content in beverages
7. Can two substances have the same specific gravity but different densities?
Substances at the same temperature and pressure with different actual densities may have the same specific gravity if compared with their respective reference substances.
However, for most solids and liquids using water as reference, similar specific gravity indicates similar density values.
8. What is the relation between mass, volume, and density?
Density is directly related to mass and inversely related to volume.
Mathematically:
- Density = Mass / Volume
- Mass = Density × Volume
- Volume = Mass / Density
9. Is specific gravity affected by temperature?
Yes, specific gravity changes with temperature because the density of substances and water can change as temperature changes.
- Always specify the temperature (eg: at 4°C for water) when quoting specific gravity values.
10. What is the density and specific gravity of pure water at 4°C?
At 4°C, pure water has:
- Density: 1 g/cm³ or 1000 kg/m³
- Specific gravity: 1 (using water as reference)
11. Why does specific gravity have no unit but density does?
Density is measured as mass per unit volume, so it has units (like kg/m³), but specific gravity is a dimensionless ratio, so it has no units.
- Density units indicate physical measurement
- Specific gravity is just a comparison or ratio
12. State one use each of density and specific gravity in daily life.
Density helps determine if an object will float or sink in water, while specific gravity is used to check the concentration of solutions (like battery acid).
- Density: Predicting whether a wood log will float
- Specific gravity: Testing quality of milk or syrup
