FeCl2 🔬

Iron (II) chloride (FeCl2), illuminated sideways under the microscope 🔬.
As they form, the small crystals - FeCl2·4H2O - themselves appear colorless here. An apparant yellow color arises at the end because the crystals get covered with a thin layer of yellow solution.
While single microscopic crystals of FeCl2·4H2O appear colorless under the microscope, larger crystals (or large collections of small crystals) of FeCl2·4H2O appear as (faint) green. A similar phenomenon occurs for faint pink MnCl2·4H2O    • LiCl, MnCl2 🔬  .
Progressively, Iron (II) oxidizes to Iron (III), which is visible as deep orange precipitates.

Some solubilities and deliquescent relative humidities:
FeCl2: 68.5 g/100 mL (20 °C) (DRH 60.0%)
FeCl3: 91.2 g/100 mL (25 °C) (DRH 44%)

Salt hydrate pair equilibria:
FeCl2·2H2O = FeCl2 + 2 H2O(g) (RH 4.2%)
FeCl2·4H2O = FeCl2·2H2O + 2 H2O(g) (27.3%)
FeCl3·6H2O = FeCl3 + 6 H2O(g) (RH 4.2%)

Thermochemical data (298.15 K), FeCl2 (dry):
dH_f = -341.833 kJ/mol
dG_f = -302.343 kJ/mol
C_p = 76.663 J / (K mol)

What else do we see? 👀🔬

If you want to see some of my theoretical work in chemistry, see for example:
https://www.pnas.org/doi/10.1073/pnas...
https://arxiv.org/abs/2306.09553

To attend our research seminar on autocatalysis, visit:
https://researchseminars.org/seminar/...