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Calculate density (ρ=m/V), find mass from density and volume, or find volume from mass and density. Includes reference table of densities for common materials for identification.
Density = mass per unit volume. It determines buoyancy, identifies materials, and converts between mass and volume. Water at 4°C (1.000 g/cm³) is the universal reference.
| Material | Density (g/cm³) | Note |
|---|---|---|
| Hydrogen gas | 0.0000899 | Lightest element |
| Air (sea level) | 0.001225 | Buoyancy reference |
| Gasoline | 0.72 | Floats on water |
| Ice | 0.917 | Less dense than liquid water |
| Water (4°C) | 1.000 | Reference density |
| Seawater | 1.025 | Denser than fresh |
| Aluminum | 2.70 | Lightweight structural metal |
| Iron | 7.87 | Structural metal |
| Copper | 8.96 | Electrical wiring |
| Lead | 11.34 | Radiation shielding |
| Gold | 19.32 | Dense precious metal |
Object floats if average density less than fluid. Ice (0.917) floats on water (1.000). Steel ships float because hull + air gives average density below water. Buoyant force = weight of displaced fluid.
Density is a characteristic property — gold is always 19.32 g/cm³. Measure mass (balance) and volume (water displacement) to identify unknown materials.
Density is one of the most practically useful physical properties. It connects mass (easily measured on a balance) to volume (easily measured or calculated geometrically), identifies materials, predicts buoyancy, and is essential for fluid mechanics, materials selection, and chemical analysis.
Solid density by water displacement: weigh solid in air (mass m), submerge in graduated cylinder and read volume change (volume V). Density = m/V. Liquid density: weigh a specific volume using a volumetric pipette or density bottle (pycnometer). Most accurate method: use an analytical balance and a 25 mL or 50 mL pycnometer. Gas density: measure mass of a known volume of gas at known temperature and pressure, or calculate from molar mass using PV = nRT.
Rock and mineral identification uses density alongside hardness and crystal form. Quartz: 2.65 g/cm³. Feldspar: 2.56-2.76. Magnetite: 5.2. Galena (lead sulfide): 7.6. Gold: 19.32 — this high density allowed early miners to pan for gold (gold sinks to the bottom while lighter rock washes away). Earth's average density is 5.51 g/cm³, suggesting a dense iron-nickel core (12-13 g/cm³) surrounding a silicate mantle (3.3-5.6 g/cm³).
Brix measurement (sugar content in beverages) uses the relationship between dissolved sugar concentration and solution density. Brix refractometer or hydrometer measures density to infer sugar content. Wine must: 1.08-1.10 g/mL (20-25 Brix) before fermentation. After complete fermentation: ~0.995 g/mL (dry wine). Beer: 1.040-1.080 original gravity, ferments to 1.005-1.015 final gravity. Brewers use density measurements (specific gravity) to track fermentation progress and calculate alcohol content.
Density (ρ) is defined as mass per unit volume: ρ = m/V. It is one of the most practically important physical properties of matter, used to identify materials, predict buoyancy, design structures, and convert between mass and volume measurements. The SI unit is kg/m³, but g/cm³ (equivalent to g/mL for liquids) is more commonly used in laboratory chemistry.
Density is an intensive property — it does not change with the amount of substance. A tiny sliver of gold and a gold bar both have density 19.32 g/cm³. This makes density a powerful tool for material identification. If you measure a sample's mass and volume, you can often identify the material by matching its density to reference values.
Solid density by water displacement: weigh solid in air (mass m). Place in graduated cylinder containing known water volume. Record new volume (V₂). Object volume = V₂ - V₁. Density = m/(V₂-V₁). Liquid density: weigh empty pycnometer (glass vessel of precise volume, 10-50 mL), fill with liquid, weigh again. Density = (mass filled - mass empty)/pycnometer volume. Gas density: fill a pre-weighed flask of known volume with gas at measured T and P. Density = (mass filled - mass empty)/volume. Alternatively calculate: d = PM/(RT) using ideal gas law.
An object floats if its average density is less than the fluid it is placed in. Average density is total mass divided by total volume — including any hollow spaces. Ice (0.917 g/cm³) floats on water (1.000 g/cm³) because ice has lower density, despite water expanding when it freezes being seemingly paradoxical. Steel ships float because the ship (steel hull + enclosed air) has average density less than 1.000 g/cm³ — the hollow hull displaces a large volume of water while having relatively little mass. Archimedes discovered that the buoyant force equals the weight of fluid displaced (Archimedes Principle).
Earth average density: 5.51 g/cm³. Continental crust: 2.7 g/cm³ (granite). Oceanic crust: 3.0 g/cm³ (basalt — denser, so it subducts under lighter continental crust at tectonic boundaries). Upper mantle: 3.3-3.5 g/cm³. Lower mantle: 4.0-5.5 g/cm³. Outer core (liquid iron-nickel): 9.9-12.2 g/cm³. Inner core (solid): 12-13 g/cm³. The density gradient from crust to core explains why Earth is layered — denser materials sank to the center during Earth's molten phase early in planetary formation.
Specific gravity (SG = density/water density) measures sugar concentration in brewing and winemaking. Unfermented grape juice (must): SG 1.080-1.110 (8-11% sugar). After complete fermentation: SG ~0.990-0.998 (below water because ethanol density 0.789 g/cm³ is less than water). Brewers use hydrometer readings to calculate: Alcohol By Volume (ABV) = (Original Gravity - Final Gravity) × 131.25. Original Gravity 1.055, Final Gravity 1.010: ABV = (1.055-1.010) × 131.25 = 5.9% ABV. Density measurement is essential quality control in food manufacturing.
Liquid density generally decreases with increasing temperature (thermal expansion). Water unique behavior: density increases from 0°C to 4°C (maximum density at 4°C = 1.000 g/cm³), then decreases above 4°C. This is why ice floats — water expands when freezing, reducing density from 1.000 to 0.917 g/cm³. This anomalous behavior of water is critical to aquatic ecosystems: lakes freeze from the top down, insulating the water below at 4°C, allowing fish and other aquatic organisms to survive winter. Metal densities decrease slightly with temperature due to thermal expansion of the crystal lattice.