The terms density and relative density are usually used interchangeably especially when it comes to describing the physical characteristics of substances. While they both deal with the amount of matter packed into a given space, their definition, underlying principles, calculations and applications are actually different. Therefore, read the article and learn more about this two concepts.
What is Density?
Density refers to a physical property of a substance that describes how much mass is contained in a given volume. In other words, Density is a way to measure how compact a substance is. It is the object’s mass compared to the object’s volume. The more material is in that volume, the denser it is.
Density affects many aspects of life. How clouds float in the sky, which items sink or float in water, and how gases move in the atmosphere are all due, in part, to density.
Mathematically, density (symbolized by the Greek letter ρ, pronounced “rho”) is calculated using the formula:
Density (ρ) = Mass / Volume
Where:
- Mass is the amount of matter in the substance, measured in kilograms or grams.
- Volume is the amount of space occupied by the substance, usually measured in cubic meters or cubic centimeters.
ρ = m/V
ρ = density, kg/m3, or g/(cm)3
m = mass, in kg or g
V = volume, in m3 or (cm)3
In simpler terms, if you have a certain amount of material and you compress it into a smaller volume, its density increases because the mass is concentrated in a smaller space. On the other hand, if you spread the same amount of material over a larger volume, its density decreases.
The standard unit for density is kilograms per cubic meter, though grams per cubic centimeter can also be used.
Different substances have different densities due to variations in the arrangement and nature of their particles. For example, materials like lead and gold are dense because their atoms are tightly packed, while substances like air and helium are less dense because their particles are more spread out.
Most metals and rocks are very dense. Any item with a lot of mass packed into a small space will be dense.
Density has many practical applications in areas like physics, engineering, chemistry and geology. It helps explain things like buoyancy (why objects float or sink in fluids), or can be used to identify substances (density-based tests). It can also be applied in understanding the composition of materials and the Earth’s structure.
What is Relative Density?
Relative density, also known as specific gravity, is a dimensionless quantity that compares the density of a substance to the density of a reference substance, usually water. In other words, it’s a measure of how dense a substance is relative to another substance. It is usually measured at room temperature (20 Celsius degrees) and standard atmosphere (101.325kPa). It is unitless.
Usually, for liquids, the relative density is measured with respect to water at its densest form at a temperature of 4 0C/39.2 °F. For gaseous substances, relative density is measured with respect to air at room temperature, 20 °C or 68 °F. Specific gravity is another name for relative density.
Mathematically, relative density is calculated by dividing the density of the substance in question by the density of the reference substance.
When determining relative density, the temperature and pressure of the sample and reference should be specified. Usually the pressure is 1 am or 101.325 Pa.
Relative density = Density of substance/Density of reference substance
The basic formula for RD or SG is:
RD = ρsubstance / ρreference
If a difference reference is not identified, it may be assumed to be water at 4 °C.
Different ways to calculate Relative Density:
- Buoyancy Method
- Hydrometer
- Hydrostatic Balance
- Immersed Body Method
- Pycnometer Method
- Air Comparison Pycnometer Method for Solids
- Oscillating Densitometer
Applications of Relative Density
- It is used to determine the buoyancy of objects in a fluid. If the relative density of an object is less than that of the fluid it’s in, it will float; if it’s greater, it will sink.
- It is used to distinguish between different types of metals, minerals, and gemstones based on their densities.
- In industries such as food and beverages, relative density can be used to assess the concentration of solutions and ensure product consistency.
- It is used to understand the composition and properties of different rock and mineral types.
- It helps engineers select appropriate materials for building and construction projects.
Key Differences: Density vs Relative Density
Definition:
- Density is defined as the mass of a substance per unit volume. It is represented by the formula: Density = Mass / Volume.
- Relative density, or specific gravity, is the ratio of the density of a substance to the density of a reference substance (usually water). It is a unitless quantity.
Units
- The units of density depend on the units used for mass and volume (e.g., kg/m³, g/cm³).
- Relative density is a dimensionless quantity and is often given as a pure number without units.
Comparative Nature
- Density provides an absolute measure of how much mass is contained in a given volume of a substance.
- Relative density provides a comparative measure of how dense a substance is compared to another substance, usually water.
Calculation
- Calculated using the formula Density = Mass / Volume, where both mass and volume are the actual values for the substance.
- Calculated using the formula Relative Density = Density of Substance / Density of Reference Substance.
Use Case
- Used to describe the mass-to-volume relationship of a substance and is often used in scientific and engineering calculations.
- Used to compare the densities of different substances, especially in scenarios where the density of one substance needs to be determined relative to a reference substance.
Variation with Temperature
- Density can change with temperature since most substances change in volume as they are heated or cooled. As a result, density can vary at different temperatures.
- Relative density is less affected by temperature variations because it is a ratio of two densities. The reference substance (usually water) also changes density with temperature changes, but the ratio tends to be less influenced by these variations.
Key Takeaways: Density vs Relative Density
| Basis of Comparison | Density | Relative Density |
| Definition | Density is the mass of an object per unit volume. It’s an absolute measure. | Relative density compares the density of a substance to the density of a reference substance. It’s a ratio. |
| Calculation Formula | Mass/Volume | Density of Substance/Density of Reference Substance |
| Units | Common units include kg/m³, g/cm³, lb/ft³, etc. | Since it’s a ratio of densities, it’s a dimensionless quantity (no units). |
| Role | Density provides the actual mass per unit volume of a substance. | Relative density indicates how much denser or less dense a substance is compared to a reference substance. |
| Reference Substance | No specific reference substance is required; density is an absolute measurement. | Relative density always involves a reference substance, often water (with a density of 1 g/cm³ or 1000 kg/m³). |
| Interpretation | Higher density indicates more mass packed in a given volume. | Relative density > 1 implies substance is denser than reference; < 1 implies less dense; = 1 implies equal density. |