The density of petrol (gasoline) is approximately 0.71–0.77 g/mL (710–770 kg/m³) at room temperature. The commonly used approximate value is 0.72 g/mL or 720 kg/m³. Since petrol's density is less than that of water (1.0 g/mL), petrol floats on water. This difference in density is important in understanding fuel behaviour and safety (petrol fires float on water surfaces).
Density of petrol: approximately 0.71–0.77 g/mL (710–770 kg/m³) at room temperature.
Commonly used value: 0.72 g/mL or 720 kg/m³.
Petrol density < water density (1.00 g/mL) → petrol floats on water.
Petrol is a mixture of C₄–C₁₂ hydrocarbons — density varies by composition.
Petrol fires cannot be extinguished with water — use CO₂ or foam instead.
Petrol and water are immiscible — petrol is non-polar, water is polar.
Diesel density (~0.82–0.85 g/mL) is higher than petrol but still less than water.
Octane rating measures antiknock property of petrol — higher is better.
Petrol (gasoline) density: • Approximate value: 0.71–0.77 g/mL (or g/cm³) • Commonly used value: ~0.72 g/mL or 0.75 g/mL • In SI units: 710–770 kg/m³ • At room temperature (25°C) and standard pressure
Comparison with water: • Water density: 1.00 g/mL at 4°C • Petrol density: ~0.72–0.75 g/mL • Petrol is less dense than water → petrol floats on water
Why petrol density varies: Petrol is a mixture of hydrocarbons (mainly C₄–C₁₂ alkanes, cycloalkanes, and aromatics). The exact composition depends on the crude oil source and refining process, causing slight variation in density.
Note: Diesel density (~0.82–0.85 g/mL) is higher than petrol but still less than water.
Petrol floats on water because: • Density of petrol (~0.72 g/mL) < Density of water (1.00 g/mL) • Objects/liquids less dense than water float on it • The two liquids are immiscible (do not mix) — petrol is non-polar; water is polar
Consequences:
Petrol fires cannot be extinguished with water: • Water sinks below the burning petrol • Petrol continues to float and burn • Correct extinguishant: CO₂ or dry chemical (foam)
Oil spills float on sea/lake surfaces: • Environmental damage — floating layer blocks sunlight and oxygen
Petrol storage: • Petrol tanks store the fuel above any water that may collect
Related concept: Like dissolves like. • Petrol (non-polar) dissolves non-polar substances (grease, wax) • Water (polar) dissolves ionic and polar substances (salt, sugar)
Petrol (gasoline) composition: • Mixture of hydrocarbons: C₄–C₁₂ • Main components: straight-chain and branched alkanes, cycloalkanes, aromatics (benzene, toluene, xylene) • Does not have a fixed formula — it is a mixture
Physical properties of petrol: • Density: ~0.72–0.75 g/mL • Boiling range: 40–200°C (mixture of compounds) • Flashpoint: −43°C (highly flammable) • Colour: clear to pale yellow (dyes added for identification) • Odour: characteristic hydrocarbon smell • Immiscible with water
Chemical properties: • Combustion: CₙH₂ₙ₊₂ + O₂ → CO₂ + H₂O + energy • Octane rating: measure of antiknock property Higher octane → more efficient, less engine knock Regular petrol: octane ~87; premium: ~91–95
The density of petrol (gasoline) is approximately 0.71–0.77 g/mL (710–770 kg/m³) at room temperature. A commonly used approximate value is 0.72 g/mL. Since this is less than the density of water (1.00 g/mL), petrol floats on water.
Petrol floats on water because its density (~0.72 g/mL) is less than the density of water (1.00 g/mL). Less dense liquids float on denser ones. Additionally, petrol and water are immiscible (do not mix) because petrol is non-polar and water is polar.
Water should not be used on petrol fires because petrol is less dense than water. Water sinks beneath the burning petrol, which continues to float and burn on the water surface. Carbon dioxide (CO₂), dry chemical powder, or foam extinguishers are used instead to smother petrol fires.
Petrol is a mixture of hydrocarbons, mainly C₄–C₁₂ alkanes, cycloalkanes, and aromatic compounds (benzene, toluene, xylene). It does not have a single formula — it is a complex mixture obtained by fractional distillation of crude petroleum.
Density of petrol is measured in g/mL (grams per millilitre), g/cm³ (grams per cubic centimetre), or kg/m³ (kilograms per cubic metre). The values are equivalent: 0.72 g/mL = 0.72 g/cm³ = 720 kg/m³.
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