Relative Density vs. Specific Gravity: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on January 2, 2024
Relative density is the ratio of the density of a substance to a reference substance, while specific gravity is the ratio of the density of a substance to that of water at 4°C.

Key Differences

Relative density measures how dense a substance is compared to another reference substance, often water. Specific gravity is a special case of relative density, where the reference substance is always water at its densest state (4°C).
In the context of relative density, the reference substance can vary depending on the application. However, for specific gravity, the reference point is universally water at 4°C, making it a more standardized measure.
Relative density is dimensionless, as it's a ratio of densities. Similarly, specific gravity is also dimensionless, as it compares the density of a substance to that of water.
Relative density can be used for solids, liquids, or gases, with the reference substance changing accordingly. In contrast, specific gravity is typically used for liquids and solids, with water being the constant reference.
In calculations, relative density can be adjusted for temperature and pressure variances. Specific gravity, while also affected by temperature and pressure, is generally referenced at a standard condition (water at 4°C).

Comparison Chart

Spelling

"Relative density" has 16 letters.
"Specific gravity" has 16 letters.

Pronunciation

Pronounced as /ˌrɛləˈtɪv ˈdɛnsɪti/.
Pronounced as /spəˈsɪfɪk ˈɡrævɪti/.

Syllables

Five syllables: Re-la-tive-den-si-ty.
Five syllables: Spe-ci-fic-grav-i-ty.

Used as a noun.
Used as a noun.

Usage in a Sentence

"The relative density of iron is higher than wood."
"The specific gravity of mercury is quite high."

Relative Density and Specific Gravity Definitions

Relative Density

A quantity without any units.
Relative density is a dimensionless value.

Specific Gravity

A property used to identify substances.
Specific gravity aids in substance identification.

Relative Density

A measure for comparing densities of two substances.
Relative density helps in understanding buoyancy.

Specific Gravity

A standard measure in scientific studies.
Specific gravity is used in geology for rock classification.

Relative Density

A property of materials comparing to a standard.
The relative density of aluminum is less than steel.

Specific Gravity

Reflects how dense a material is compared to water.
Specific gravity varies widely among minerals.

Relative Density

A concept used in physics to understand substance behavior.
Relative density affects fluid dynamics.

Specific Gravity

Comparison of a substance's density to that of water.
Specific gravity determines if an object will float.

Relative Density

The ratio of the density of a substance to that of a reference substance.
Relative density is crucial in material selection.

Specific Gravity

A unitless measure in fluid mechanics.
Specific gravity is key in hydrometer readings.

FAQs

How are relative density and specific gravity related?

Specific gravity is a type of relative density with water as the reference.

Is specific gravity always measured against water?

Yes, it's always compared to water at 4°C.

What is relative density?

Relative density is the ratio of the density of a substance to that of a reference substance.

What is specific gravity?

Specific gravity is the ratio of the density of a substance to that of water at 4°C.

How does specific gravity affect buoyancy?

It determines whether a substance will float or sink in water.

Can gases have a relative density?

Yes, when compared to a reference gas.

Why use relative density?

Relative density helps compare materials and understand their properties.

What role does specific gravity play in industries?

It's crucial in fields like geology, chemistry, and material science.

Can relative density have units?

No, relative density is a dimensionless quantity.

Is specific gravity used in the food industry?

Yes, for liquids like milk and juices.

Does temperature affect relative density?

Yes, temperature can change the density of the reference and the substance.

Can relative density be more than 1?

Yes, if a substance is denser than the reference.

Is relative density used in geology?

Yes, for comparing mineral densities.

What's the specific gravity of water?

By definition, it's 1 at 4°C.

Does pressure affect relative density?

It can, especially for gases.

Can specific gravity be used in medical diagnostics?

Yes, in analyzing urine and other bodily fluids.

Do relative density and specific gravity have practical applications?

Absolutely, in engineering, science, and everyday life.

Why is specific gravity important in fluid mechanics?

It helps in understanding fluid behavior and properties.

How do you measure specific gravity?

Using a hydrometer or a density meter.

What's an example of a high relative density?

Metals like gold and platinum have high relative densities.