Density Glossary: A–Z Reference of Density & Mass Terms

You are reading a lab protocol, material safety datasheet, or engineering spec and get blindsided by confusing terms like pycnometer, Baume scale, or apparent density. Mixing up particle density with bulk density or confusing viscosity with specific gravity causes costly batch formulation errors and wasted ingredients. Here is the reality: scientific jargon sounds intimidating until you break down the physical definitions. Use this complete A–Z reference to clarify every density, volume, and mass term instantly.

A

Absolute Density The mass per unit volume of a pure substance under specified conditions (temperature, pressure), expressed without reference to another substance. Compare: Specific Gravity.

Apparent Density The density of a porous or granular material calculated using total volume (including pores and void space). Always less than true density. Used for powders, sand, and aggregate materials.

API Gravity A scale developed by the American Petroleum Institute to classify crude oil and petroleum products. Inversely related to density: higher API = lighter fuel. Formula: API = (141.5 / SG) − 131.5. Petrol: ~59.7° API. Diesel: ~37.9° API.

Archimedes' Principle A fundamental law of physics stating that any object immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces. Named after ancient Greek mathematician Archimedes (~287–212 BC). This principle explains why ships float and why less-dense objects rise to the surface.

Aqueous Solution Any liquid in which water is the primary solvent. Dissolved substances (salts, sugars, acids) increase density above pure water's 1.000 kg/L. Example: seawater (1.025 kg/L), honey (1.420 kg/L).

B

Baumé Scale (°Bé) A hydrometer scale used in the food, chemical, and winemaking industries. Two scales exist: one for liquids heavier than water (acids, brines), one for lighter liquids (oils, alcohol). Named after French pharmacist Antoine Baumé (1728–1804).

Brix (°Bx) A scale measuring the concentration of dissolved sugars in a liquid. 1°Bx = 1 gram of sucrose per 100 grams of solution. Used in the food industry (fruit juices, wines, honey). Higher Brix = denser, sweeter liquid. Measured with a refractometer or hydrometer.

Bulk Density The density of a material including all pore spaces and void volume between particles. Used for powders, grains, sand, and aggregate. Bulk density is always lower than particle density. Example: wheat flour bulk density ≈ 0.593 kg/L, but individual starch granules are much denser.

Buoyancy The upward force exerted by a fluid on an object immersed in it. Equals the weight of the fluid displaced. An object floats when buoyancy ≥ its weight. An object sinks when buoyancy < its weight.

C

Centipoise (cP) A unit of dynamic viscosity. Water at 20°C has a viscosity of approximately 1.002 cP. Honey: ~10,000 cP. Not to be confused with density — viscosity measures resistance to flow, not mass per volume.

CGS Units The Centimetre-Gram-Second metric system where density is expressed in grams per cubic centimetre (g/cm³). 1 g/cm³ = 1 kg/L = 1,000 kg/m³.

Coefficient of Thermal Expansion (β) The fractional change in volume per degree change in temperature. Liquids expand as temperature rises, reducing density. Diesel expands at ~0.00095/°C; petrol expands at ~0.00120/°C.

Compressibility The measure of relative volume change in response to pressure. Liquids are nearly incompressible under everyday conditions, but water density increases by ~4.5% at extreme ocean depths (10,000m).

D

Density (ρ) Mass per unit volume of a substance. SI unit: kg/m³. Practical units: kg/L, g/cm³, lb/gal. Formula: $\rho = m \div V$. Core concept explored in our guide on what is density.

Density Gradient A spatial variation in density within a fluid or mixture. In oceans, warmer, fresher water sits on top of colder, saltier water, creating distinct pycnoclines.

Digital Density Meter An instrument that measures fluid density using the oscillating U-tube principle. Highly accurate (to 0.00001 g/cm³), requiring only 1–2 mL of sample. Standard in commercial laboratories.

E

Emulsion A mixture of two or more liquids that are normally immiscible (unmixable). Examples: milk (fat globules dispersed in water), mayonnaise (oil in vinegar/water). Emulsion density depends on the volumetric ratio and individual densities of phases.

Energy Density The amount of energy stored in a given system or substance per unit volume (MJ/L) or per unit mass (MJ/kg). Diesel has a higher volumetric energy density (~35.8 MJ/L) than petrol (~32.0 MJ/L) primarily because of its higher mass density.

F

Fluid Dynamics The branch of physics that studies liquids and gases in motion. Density is a foundational variable in Reynolds number calculations, pipe flow pressure drops, and pump sizing.

G

Gay-Lussac Scale (°GL) A scale indicating alcohol by volume (ABV) as a percentage. 40°GL = 40% ABV. Alcohol has a lower density than water (ethanol = 0.789 kg/L), so alcohol hydrometers measure density to determine alcohol percentage.

Gram per Cubic Centimetre (g/cm³) A common metric unit of density. Numerically identical to kilograms per litre (kg/L). 1 g/cm³ = 1 kg/L = 1,000 kg/m³. Pure water at 4°C = 1.0000 g/cm³.

H

Heavy Water (D₂O) Water containing deuterium (hydrogen-2) instead of protium. Density at 25°C = 1.1044 kg/L (10.7% denser than ordinary water). Used in CANDU nuclear reactors.

Hydrometer A calibrated sealed glass tube weighted at the bottom that floats upright in a liquid. The level at which it floats indicates the liquid's specific gravity or density. Based on Archimedes' principle.

Hydrostatic Pressure The pressure exerted by a fluid at rest due to gravity. Proportional to fluid density, depth, and gravitational acceleration: $P = \rho \times g \times h$. Denser fluids exert higher hydrostatic pressure at the same depth.

I

Ideal Gas Law Equation of state for ideal gases: $PV = nRT$. Gas density is directly proportional to pressure and molecular weight, and inversely proportional to temperature.

Immersion Method A laboratory technique for finding the volume of irregularly shaped solids by measuring the volume of water displaced. Mass divided by displaced volume gives density.

K

Kilogram per Litre (kg/L) The practical metric unit of density most commonly used in everyday cooking, chemistry, and liquid measurement. 1 kg/L = 1 g/cm³ = 1,000 kg/m³.

Kinematic Viscosity Dynamic viscosity divided by density: $\nu = \mu \div \rho$. Measured in centistokes (cSt) or m²/s. Relevant for engine oils and hydraulic fluids.

M

Mass The quantity of matter in an object, measured in kilograms (kg) or grams (g). Independent of gravity. Detailed in our primer on volume vs mass.

Meniscus The curved upper surface of a liquid column. Liquids that wet glass (water) form a concave meniscus (read bottom). Liquids that do not wet glass (mercury) form a convex meniscus (read top). Accurate reading is critical for volumetric glassware.

N

Neutral Buoyancy The condition where an object's average density equals the density of the surrounding fluid, causing it to neither sink nor float. Submarines and scuba divers use buoyancy control to achieve neutral buoyancy.

O

Oscillating U-Tube The measurement principle behind modern digital density meters. A hollow glass U-tube filled with sample fluid is electronically vibrated; the frequency of oscillation changes with fluid mass, determining density to extreme precision.

P

Pycnometer A precision glass flask with a calibrated volume and capillary stopper used to measure liquid density by weighing known volumes with high accuracy. Governed by metrology standards from the BIPM International Vocabulary of Metrology (VIM) and NIST Physical Measurement Laboratory Reference.

R

Refractometer An optical instrument that measures the refractive index of a liquid, which correlates directly with dissolved solids and liquid density (e.g., Brix in juice, salinity in marine aquariums).

Relative Density Another term for Specific Gravity: the ratio of a substance's density to that of a reference substance (water for liquids, air for gases). Defined rigorously in the IUPAC Compendium of Chemical Terminology.

S

Salinity The concentration of dissolved salts in water, typically measured in Practical Salinity Units (PSU) or parts per thousand (ppt). Standard seawater has 35 PSU, resulting in a density of 1.025 kg/L.

Specific Gravity (SG) A dimensionless ratio comparing the density of a substance to pure water at 4°C. An SG of 1.050 means the substance is 5% denser than water. Compare: specific gravity vs density.

Standard Temperature and Pressure (STP) Defined by IUPAC as 0°C (273.15 K) and 1 bar (100 kPa). Standard reference conditions for reporting gas densities.

T

Tare Weight The weight of an empty container. Subtracted from the gross weight (container + substance) to give the net weight of the liquid or powder inside.

Tapped Density The increased bulk density of a granular material or powder after mechanically tapping or vibrating the container to settle particles. Standards maintained by ASTM International Standards Directory.

V

Viscosity A fluid's internal resistance to deformation or shear stress (flow rate). Distinct from density: mercury is very dense (13.5 kg/L) but flows freely (low viscosity), whereas room-temperature honey is less dense (1.42 kg/L) but has very high viscosity.

W

Water (Universal Reference) Pure liquid water at 4°C is the metric reference standard for density, set at exactly 1.000 kg/L. Read our comprehensive water density complete guide.

Frequently Asked Questions About Density Terms

What is the difference between bulk density and true density?

True density measures the mass of a solid divided by volume excluding all pores and air pockets. Bulk density measures granular powders or aggregate as poured, including the air spaces between particles. Bulk density is always lower than true density.

Is viscosity the same thing as density?

No. Density measures how heavy a substance is per unit volume (mass per volume). Viscosity measures a fluid's thickness and resistance to flowing. Mercury is extremely dense but flows like water; motor oil is lighter than water but significantly more viscous.

Why do specific gravity and density share the same number in metric?

Because specific gravity divides a substance's density by water's density at 4°C, which is exactly 1.000 kg/L. Dividing any number by 1.000 leaves the numerical value unchanged, though specific gravity drops the physical units.

What does a hydrometer actually measure?

A hydrometer measures the specific gravity (relative density) of a liquid based on buoyancy. The deeper the weighted glass tube sinks into the liquid, the less dense the liquid is.

What is API gravity used for?

API gravity is a specialized non-linear scale created by the American Petroleum Institute to measure crude oil and petroleum product density. It is inverted: lighter, more valuable fuels have high API degrees, while dense heavy crudes have low API degrees.

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