After midterm · Session 4 of 6 · 180 minutes
30 September 2026
What happens to gas pressure when the solid inventory runs out?
Write your prediction before opening the lab. By the end, support an answer with one controlled comparison and an independent check.
| Activity | Minutes |
|---|---|
| Opening question and essential concepts | 30 |
| One hand calculation | 20 |
| One lab demonstration | 20 |
| Break | 10 |
| Student pair investigation | 70 |
| Discussion and exit question | 30 |
The 70-minute investigation is student work with instructor support.
| Problem | Equilibrium description |
|---|---|
| SLE | Solute chemical potential equal in liquid and solid |
| Bulk GSE | Chemical potential equal in gas and solid |
| Adsorption | Surface loading at gas/solution chemical potential |
Surface uptake is not the formation of a macroscopic bulk solid phase.
At fixed T, a present bulk solid fixes the ideal-gas partial pressure at its sublimation pressure within the model.
In a rigid vessel, n_g^{sat}=\frac{P_{sub}(T)V}{RT}
If total subliming-species inventory is below this amount, the solid is exhausted and pressure falls below Psub.
Synthetic example: T=300 K, V=1 L, Psub=10 kPa.
n_g^{sat}=10/(8.31446\times300)\approx0.00401\ \mathrm{mol}
With 0.01 mol total, about 0.00599 mol remains solid. With 0.002 mol total, no solid remains and P≈4.99 kPa.
Fixed T,V equilibrium minimizes Helmholtz energy, not Gibbs energy at fixed P.
Compare total inventory with Psub V/(RT). Check gas plus solid moles and distinguish partial pressure from total pressure if an inert gas is present.
In Lab 10 choose the gas–solid vessel mode. Keep T and V fixed and vary the subliming-species inventory across the solid-depletion threshold.
Keep one saved baseline for the pair investigation.
In Lab 10, compare inventories above and below the solid-depletion boundary at fixed T and V. Check the inventory balance and explain the resulting gas pressure.
Use 20 minutes to compare results and discuss unexpected behavior.
What happens to gas pressure when the solid inventory runs out?
Use the final 10 minutes to explain your answer individually and name one assumption that limits it.
In Lab 10, compare inventories above and below the solid-depletion boundary at fixed T and V. Check the inventory balance and explain the resulting gas pressure.
Assessment 4 and independent checkpoint
Retain one study JSON, one comparison and a short explanation. Continue from your classroom case. The hand checkpoint is part of this time budget.
Choose one extension: compare Henry and Langmuir in Lab 11, check a closed adsorption vessel, or explore pure-solid solubility and the eutectic assumptions in Lab 10.
Session appendix · Module reference
Learning path · Offline package
Optional work is additional. Complete the required investigation first.