What is density and how is it calculated?
Density is mass per unit volume: ρ = m/V. A 250 g object occupying 100 cm³ has a density of 2.5 g/cm³. It's an intrinsic property — a gold coin and a gold bar have the same density — which makes it useful for identifying materials and predicting buoyancy.
How do g/cm³, kg/m³, and lb/ft³ relate?
1 g/cm³ = 1,000 kg/m³ = 62.43 lb/ft³. Water is the anchor point: 1 g/cm³ exactly (at 4°C). Science tables usually quote kg/m³, engineering handbooks lb/ft³, and chemistry g/cm³ — this calculator converts between all three automatically.
Why do things float or sink?
An object floats if its overall density is below the fluid's. Ice (0.917 g/cm³) floats on water; steel (7.85) sinks — yet steel ships float because their overall density, hull air included, is below 1. The float-or-sink indicator here compares your result against water.
Can density identify an unknown material?
It's a strong first clue. Measure mass with a scale and volume by water displacement, then compare: aluminum ≈ 2.7, iron/steel ≈ 7.8, copper ≈ 8.96, silver 10.5, lead 11.34, gold 19.32 g/cm³. Archimedes reportedly exposed a fake gold crown exactly this way — dense metals are hard to counterfeit.
Why does density change with temperature?
Heating almost everything makes it expand while its mass stays the same, so density falls. For solids and liquids the effect is small but real — steel changes roughly 0.03% per 10 degrees Celsius, which matters for precision engineering and for fuel sold by volume. Water is the famous exception: it reaches maximum density at about 4 degrees Celsius rather than at freezing point, and ice is around 8% less dense than the liquid. That anomaly is why ice floats, why lakes freeze from the surface downward, and why aquatic life survives a winter at all. Gases are far more sensitive still, roughly inversely proportional to absolute temperature.
What is specific gravity and how does it differ from density?
Specific gravity is density expressed as a ratio to a reference substance rather than in units — water at 4 degrees Celsius for solids and liquids, air for gases. It is therefore a dimensionless number: a mineral with a specific gravity of 2.7 has a density of 2,700 kg per cubic metre, or 2.7 g per cubic centimetre. The advantage is that it reads the same whichever unit system you work in, which is why hydrometers, brewing and winemaking, battery testing and geological field guides all quote it. Anything with a specific gravity below 1 floats in water; above 1 it sinks.