Q3(c). Phase diagram of butane and two compressions
You are given the following data for butane: normal melting point = −138 °C; normal boiling point = 0 °C; critical temperature = 152 °C; critical pressure = 38 atm. Assume that the triple point is slightly lower in temperature than the melting point and that the vapour pressure at the triple point is 3 × 10⁻⁵ torr. (i) Sketch a phase diagram for butane. (ii) Butane at 1 atm and 140 °C is compressed to 40 atm. Are two phases present at any time during this process? (iii) Butane at 1 atm and 200 °C is compressed to 40 atm. Are two phases present at any time during this process?
Answer in brief
(ii) Yes: liquid and vapour coexist while the vapour condenses, at about 31 atm. (iii) No: 200 °C is above the critical temperature, so one phase throughout.
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Coversphase diagramphase rulecritical pointtriple pointsupercritical fluidClausius–Clapeyron equation
All parts of Question 3
- 3(a) Collisions of O₂ with the wall10 marks
- 3(b) Droplets made from one mole of water15 marks
- 3(c) Phase diagram of butane and two compressions10 marks
- 3(d) Water condensed while ice melts and warms10 marks
- 3(e) Why crystals have more defects at higher temperature5 marks