What is the definition of density and its SI unit?
Density is the mass of a substance per unit volume, symbol ρ (Greek letter rho), definition formula ρ=m/V, which is mass divided by volume. The SI unit is kg/m³ (kilogram per cubic meter). At 4°C pure water density is approximately 1000 kg/m³ = 1 g/cm³ = 1 kg/dm³, which is an easy-to-remember reference value. Density is an intrinsic property of matter and can be used for substance identification, buoyancy calculations, fluid mechanics analysis, etc.
What is the relationship between g/cm³, kg/dm³, kg/L? What is their relation to water?
1 g/cm³ = 1 kg/dm³ = 1 kg/L = 1000 kg/m³, these units have equal numerical values, all equal to the density of pure water at 4°C. This is because 1 cm³ = 1 mL = 1 cc = 10⁻⁶ m³, 1 dm³ = 1 L = 0.001 m³. Therefore water density expressed in different units gives exactly: 1000 kg/m³, 1 g/cm³, 1 kg/dm³, 1 kg/L, very convenient to remember.
Why are g/cm³ commonly used in engineering and material handbooks?
The numerical range of g/cm³ just covers common solid and liquid densities: water 1.0, alcohol 0.79, gasoline 0.72, human body ~1.02, aluminum 2.70, iron 7.87, copper 8.96, lead 11.34, gold 19.32, osmium 22.57, all values between 0.6 and 23, expressible as integers or one or two decimal places, easy to read and remember. When using kg/m³ these numbers are all three digits (water 1000, iron 7870, copper 8960), which is also clear but adds 3 zeros.
How to use lb/ft³, lb/in³, lb/gal?
Imperial density units are still widely used in British and American engineering, aerospace, petroleum, construction industries: 1 lb/ft³ (pound per cubic foot) ≈ 16.02 kg/m³, is a common unit for building material unit weight, air density (air ~0.0765 lb/ft³, water ~62.4 lb/ft³); 1 lb/in³ (pound per cubic inch) ≈ 27680 kg/m³, used for high-strength metals, mechanical engineering stress calculations; lb/USgal (pound per US gallon) and lb/UKgal (pound per UK gallon) are used for liquid fuel, chemical solution transport and mixing.
What are the densities of common substances?
Common material densities (g/cm³, approximate values): gases – air 0.0012, hydrogen 0.00009; liquids – gasoline 0.72, alcohol 0.79, water 1.00, seawater 1.025, milk 1.03, sulfuric acid 1.84, mercury 13.55; metals – magnesium 1.74, aluminum 2.70, titanium 4.51, iron/steel 7.87, copper 8.96, silver 10.49, lead 11.34, gold 19.32, osmium 22.57 (densest in Earth's crust); other – ice 0.92 (floats on water), human body 1.02, glass 2.5, concrete 2.4, wood 0.4~0.8, plastics 0.9~1.4.
What is the difference between density and specific gravity (relative density)?
Specific Gravity (SG) is the ratio of a substance's density to water density (at 4°C), a dimensionless pure number. SG = ρ/ρ_water, therefore the numerical value of SG equals the value in g/cm³ or kg/dm³ (since water = 1 g/cm³). For example iron density 7.87 g/cm³ corresponds to SG 7.87, lead 11.34 corresponds to SG 11.34. SG is widely used in petroleum, chemical, brewing industries; API gravity, alcohol content, Baumé scale are derived indicators of SG.
What determines whether an object floats or sinks in a liquid?
By Archimedes' buoyancy principle: an object floats when its average density < liquid density, sinks when >, suspends when equal. For example: ice density 0.92 g/cm³ < water 1.0, so icebergs float (90% underwater); iron 7.87 is much denser than water so it sinks, but iron ships float because their hollow overall average density < water; human body density ~1.02 is slightly greater than water, sinks after exhaling, can float after inhaling or inflating lungs; Dead Sea has extremely high salt concentration density ~1.24 g/cm³, people can float easily.
Why is gas density expressed in kg/m³ instead of g/cm³?
At standard temperature and pressure (0°C, 1 atm) gas density is much smaller than liquids and solids: air ~1.29 kg/m³ = 0.00129 g/cm³, expressing in g/cm³ would result in many leading zeros (0.00129), inconvenient to read. Using kg/m³ air is 1.29, hydrogen 0.09, oxygen 1.43, carbon dioxide 1.98, chlorine 3.21, values more intuitive. Gas density is closely related to temperature and pressure (ideal gas ρ=PM/RT), temperature and pressure conditions are usually specified.
What is the difference between unit weight, apparent density, true density?
True Density is the density of the material itself (excluding pores), e.g., steel 7.87 g/cm³; Apparent Density is the density of a block including internal closed pores; Bulk Density/Unit Weight is the overall density of bulk/powder materials when naturally piled including voids, e.g., cement bulk density ~1300 kg/m³, dry sand 1600 kg/m³, rice ~600 kg/m³, water is also often expressed in engineering with weight density γ=9.8 kN/m³ (or simplified 10 kN/m³).
What is the engineering significance of metal density differences?
Density directly determines the weight of parts of the same volume, which is key to lightweighting in aerospace, automotive, 3C products: aluminum 2.7 is ~2/3 lighter than steel 7.87, magnesium 1.74 is even lighter but lower strength; titanium 4.5 is 43% lighter than steel but high strength, corrosion resistant, used in aviation/medical; carbon fiber reinforced polymer (CFRP) ~1.6, preferred for high-end aerospace lightweighting; lead 11.34, tungsten 19.3, depleted uranium 19.1 used for counterweights, radiation shielding, armor-piercing projectiles; copper 8.96, silver 10.5 high density good conductivity, used for wires/contacts.
What is the relationship between liquid density and temperature?
Liquid density usually decreases with increasing temperature (thermal expansion and contraction), water has maximum density (1000 kg/m³) at 3.98°C, density is slightly lower above or below this temperature (which is why 4°C water defines the density reference). Fuel density changes significantly with temperature: gasoline density ~0.72~0.78 g/cm³, expands in summer with larger volume for same mass; aircraft fuel is measured by mass (kg or lb) rather than volume (L or gal), because volume changes at different temperatures affect range calculations.
Which density units does this tool support? Why no g/mL, oz/gal?
This tool supports 7 density units: kg/m³, g/cm³, kg/dm³, lb/ft³, lb/in³, lb/USgal, lb/UKgal, covering over 90% of daily use scenarios in physics, engineering, materials, chemical engineering. Note that g/mL has exactly the same numerical value as g/cm³ (1 mL = 1 cm³), kg/L has the same numerical value as kg/dm³, so no need to add duplicates; oz/gal (ounces per gallon) is mainly used for specific water treatment/chemical concentration scenarios, lb/gal already covers major imperial liquid density conversion needs.
