Density is set by temperature, not pressure
Liquid ethylene (C₂H₄) only exists in a narrow window, from its boiling point near −104 °C up to the critical point at 9.2 °C. Across that range the saturated-liquid density falls from roughly 640 to 265 kg/m³. Along the saturation line pressure is fixed by temperature, and compressing the liquid further barely moves it — about 1% per 100 bar.
Saturated liquid density by temperature
| Temperature | Density (kg/m³) | Density (g/cm³) |
|---|---|---|
| −104 °C (boiling point) | 640 | 0.640 |
| −80 °C | 615 | 0.615 |
| −50 °C | 600 | 0.600 |
| −20 °C | 585 | 0.585 |
| 0 °C | 568 | 0.568 |
| +5 °C | 550 | 0.550 |
| +9.2 °C (critical point) | 265 | 0.265 |
To convert, 1 g/cm³ = 1000 kg/m³ = 1 kg/L. The change from the boiling point to 0 °C is gentle and nearly linear; the last few degrees before the critical point are not.
The density crash near the critical point
Well below 9 °C the density changes slowly. In the final degrees before the critical point it collapses as the liquid and vapour phases merge, which is where simple correlations lose accuracy.
Five degrees near freezing costs about 3% of density; the last four degrees to the critical point cost more than half of it.
Reference properties of ethylene
- Molar mass. 28.05 g/mol.
- Critical point. 9.2 °C (282.35 K) and 50.4 bar — the ceiling for any liquid state.
- Boiling point. −104 °C (169.45 K) at 1 atm.
- Purity matters. Dissolved ethane, methane or CO₂ can shift the real density by a few percent from the pure-substance value.
Common questions
What is the density of liquid ethylene?
It depends heavily on temperature. Near its boiling point of -104 C it is about 640 kg/m3; at 0 C it is around 568 kg/m3; and it drops toward 265 kg/m3 as it approaches the critical point at 9.2 C.
Can ethylene be liquid at room temperature?
No. Ethylene stops being a liquid above its critical temperature of 9.2 C. Above that point no amount of pressure will condense it, so any "liquid ethylene" figure applies to cold, pressurised storage only.
Does pressure change the density much?
For the liquid, barely. Going from 1 to 100 bar shifts the density by only about 1 percent, so at fixed temperature you can usually ignore pressure. Temperature is what moves the number.
