Relative Density vs. Specific Gravity: How Are They Different?
Relative density is a key concept in physics and everyday life, especially when comparing how dense a substance is compared to a standard material like water or air. Whether for problem-solving or real-world applications, knowing about relative density helps us understand why some objects float while others sink, how minerals are identified, or why pure and impure samples behave differently. In this article, we’ll explore the meaning, formula, characteristics, measurement, and uses of relative density, ensuring you have a clear grasp of this important topic.
What is Relative Density?
Relative density is the ratio of the density of a specific substance to the density of a reference substance, typically water for liquids and solids, and air for gases. It is a dimensionless quantity—meaning it has no units. For example, if gold has a relative density of 19.3, this means gold is 19.3 times denser than water at the same temperature and pressure. In physics, the concept of relative density is also referred to as specific gravity.
Relative Density Formula
The relative density formula provides an easy way to compare densities:
- Relative Density (RD) = Density of Substance / Density of Reference
- Or in symbols: RD = ρsubstance / ρreference
Here, “ρ” (rho) denotes density, and the reference is usually water (for solids/liquids) or air (for gases). For more details on related concepts like density units and calculations, see unit of density and density of water.
Relative Density Unit and Symbol
Relative density does not have a unit because it is a ratio between two quantities with the same units. The SI unit is not applicable, and its symbol is usually RD or sometimes “SG” for specific gravity. The dimensional formula is simply M0L0T0 (dimensionless).
Key Properties and Characteristics of Relative Density
Understanding the properties of relative density allows us to predict the behavior of objects in fluids:
- It is dimensionless and has no units.
- If relative density is less than 1, the object floats in water; if greater than 1, it sinks.
- Defines how much heavier or lighter a material is compared to the reference.
- It does not depend on the amount of material, making it an intensive property.
You can learn more about related fluid concepts in mechanical properties of fluids and buoyancy.
Factors Affecting Relative Density
Several factors can influence the relative density of a material:
- Temperature: As temperature increases, density usually decreases due to expansion, impacting relative density values.
- Pressure: Raising pressure compresses substances, increasing their density, thus changing their relative density.
- Purity: Impurities—like dissolved salts in water—increase the relative density.
- Porosity: Materials with more air pockets (porous) have lower relative density compared to non-porous materials.
For a deeper understanding of the effect of temperature and pressure on materials, read effect of change of temperature and effect of change of pressure.
How to Measure Relative Density
There are different experimental techniques for determining relative density depending on the material and desired precision:
- Hydrometer: A device that floats in liquids to indicate their relative density based on how deep it sinks.
- Pycnometer: A precise glass vessel used for weighing liquids or solids, allowing for accurate density and relative density calculations.
- Gravimetric Method: Involves careful measurement of both mass and volume followed by calculation using the relative density formula.
Measurement tools and methods often connect with practical physics topics such as measurement and fluid mechanics.
Relative Density of Common Materials
Let’s look at the typical relative density values for a few standard substances:
| Material | Relative Density | Reference |
|---|---|---|
| Water | 1.00 | Water at 4°C |
| Iron | 7.8 | Water at 4°C |
| Copper | 8.96 | Water at 4°C |
| Air | ~0.0012 | Water at 4°C |
| Gold | 19.3 | Water at 4°C |
These values demonstrate how much heavier or lighter substances are compared to water. Most metals have a high relative density, while air has a very low value, showing why it rises above water.
Relative Density vs Density
It’s important to distinguish relative density from simple density. Here is a quick comparison:
| Property | Density | Relative Density |
|---|---|---|
| Definition | Mass per unit volume of a substance | Ratio between the density of a substance and a reference |
| Unit | kg/m3, g/cm3, etc. | No units (dimensionless) |
| Symbol | ρ | RD or SG |
| Reference | None | Needs a reference like water or air |
For further reading, visit difference between density and volume.
Relative Density vs Specific Gravity
In most cases, relative density and specific gravity are used interchangeably in science. Both describe how dense a substance is compared to water. However, specific gravity is often mentioned in engineering and industry, while relative density is more common in pure sciences. Both are calculated using the same formula.
You can learn about the engineering uses in specific gravity.
Practical Applications of Relative Density
The concept of relative density finds diverse use in daily life and science:
- Identifying minerals and evaluating ore purity.
- Determining if an object will float or sink in a fluid.
- Quality control for liquids in industries (e.g., checking the purity of alcohol or petroleum products).
- Soil science, especially relative density of soil for construction stability.
- Converting relative density values to units like lb/gal for industry-specific requirements.
Solved Examples: Calculating Relative Density
Example 1: If iron has a density of 7800 kg/m3 and water's density is 1000 kg/m3, then iron's relative density is:
- RD = 7800 / 1000 = 7.8
Example 2: The density of copper is 8.96 g/cm3, while oil is 0.91 g/cm3:
- RD = 8.96 / 0.91 ≈ 9.85
Try more calculations with the relative density calculator and explore connections to other measurements and units with conversion of units.
Conclusion: Why Relative Density Matters
Relative density is a fundamental quantity in physics and many scientific and engineering fields. By showing how dense one material is compared to another, it helps determine floating and sinking, purity, safety, and quality. From comparing metals to water, checking liquid mixtures, to analyzing soil stability, mastering relative density—and knowing how to use its formula, units, and methods—is vital in science and industry alike.
Explore more concepts related to relative density, or see how this relates to fundamental concepts in classical physics formulas, unit of density, and fluid friction for further learning.
FAQs on What Is Relative Density? Definition, Formula, and Examples
1. What is relative density?
Relative density is the ratio of the density of a substance to the density of water at 4°C.
- It is a unitless quantity.
- Relative density is also called specific gravity.
- Formula: Relative Density = Density of Substance / Density of Water.
- Water's density at 4°C is 1 g/cm³ or 1000 kg/m³.
Relative density helps compare if a substance will float or sink in water.
2. How is relative density measured?
Relative density is measured by comparing the mass of a given volume of a substance to the mass of the same volume of water.
Steps:
- Take equal volumes of the substance and water.
- Measure their masses using a weighing balance.
- Use the formula: Relative Density = Mass of Substance / Mass of Equal Volume of Water.
- No units are involved as it is a ratio.
Relative density shows if an object is heavier or lighter than water.
3. What is the formula for relative density?
The formula for relative density is:
Relative Density = Density of Substance ÷ Density of Water
- Density formula: Density = Mass / Volume
- For water at 4°C, density = 1 g/cm³
This formula is widely used in science and engineering to determine whether an object will float or sink in water.
4. What is the significance of relative density?
Relative density tells us whether a substance will float or sink in water.
- If relative density < 1: The substance floats.
- If relative density > 1: The substance sinks.
- Used in industries for quality and purity checks.
- Important for understanding buoyancy and displacement.
It is an important property for liquids and solids in physical sciences.
5. Why is relative density a unitless quantity?
Relative density is unitless because it is a ratio of two densities having the same units, which cancel each other.
- Both numerator and denominator are measured in kg/m³ or g/cm³.
- Units get divided out in the calculation.
This makes relative density a pure number, not dependent on any measurement unit.
6. How does the relative density of a liquid affect floating and sinking?
A liquid's relative density determines if objects will float or sink in it.
- Objects with relative density less than the liquid's will float.
- Objects with higher relative density than the liquid will sink.
For example, oil has a lower relative density than water, so it floats on water.
7. What are the applications of relative density?
Relative density has several practical uses in science and industries.
- Checking purity of substances (e.g., gold, milk, petrol).
- Testing whether substances will float or sink.
- Measuring alcohol concentration in beverages.
- Used in hydrometry and shipping to determine buoyancy.
This concept is commonly applied in laboratories and quality control.
8. What instrument is used to measure the relative density of liquids?
A hydrometer is the instrument used to measure the relative density (specific gravity) of liquids.
- It consists of a weighted glass tube that floats in the liquid.
- The reading at the liquid level gives the relative density.
- Commonly used in testing milk, battery acid, and alcohol solutions.
9. What is the relative density of pure water?
The relative density of pure water is always 1 at 4°C.
- This is because water is compared with itself.
- Used as a reference point for measuring the relative density of other substances.
- At other temperatures, the relative density changes slightly due to water's density changes.
10. Can relative density be less than 1? Give an example.
Yes, relative density can be less than 1 if a substance is lighter than water.
Examples:
- Wood (relative density about 0.6-0.8) – floats on water.
- Oil (relative density about 0.9) – floats on water.
Substances with relative density less than 1 always float in water.
11. Why is relative density called specific gravity?
Relative density is also called specific gravity as it measures gravity (weight) relative to water.
- Both terms represent the same property.
- Used interchangeably in science and industry.
- Indicates how much heavier or lighter a substance is, compared to water.
12. How is the relative density of a solid determined by the overflow can method?
The overflow can method is used to determine the relative density of solids that do not dissolve in water.
Steps:
1. Weigh the solid in air (W1).
2. Immerse solid in water and weigh it again (W2).
3. Calculate relative density using:
- Relative Density = Weight in Air / (Weight in Air - Weight in Water)
This method uses Archimedes' principle and is suitable for irregular objects.
