# Why is lithium bromide used in absorption refrigerators?

Lithium bromide refrigerator

Why is lithium bromide used in absorption refrigerators? It is more expensive than sodium chloride. The wikipedia article on lithium bromide says it is extremely hygroscopic. However, it is less soluble in water than NaCl. I am thinking that a salt capable of generating a more concentrated solution would work better in the absorption refrigerator as it would increase the propensity of water to leave the gas phase and enter the solution. Am I wrong about that? Is the affinity of water for lithium bromide so high that it is the most important factor?

Your summary is correct; $\ce{NaCl}$ is more soluble than $\ce{LiCl}$ or $\ce{CaCl2}$, for that matter, but both $\ce{LiCl}$ or $\ce{CaCl2}$ are far more hygroscopic. The explanation of hygroscopicity involves various mechanisms, including adsorption (surface forces) and absorption (requiring transport to the interior of the substance) due to hydrogen bonding, coordinate covalent bonding etc.

So these are distinct, though related, properties: solubility (limit on how much solute the solvent holds) and hygroscopicity (how avidly the solute attaches to the initial molecules of solvent).

For that reason, $\ce{CaCl2}$ is used to absorb water in damp closets, though it will eventually deliquesce into a sodden mess.

BTW, Isaac Asimov wrote of a compound so hygroscopic that it dissolved before it hit water: The Endochronic Properties of Resublimated Thiotimoline, often cited.

• Calcium chloride does itself dissolve before hitting water! Wait long enough, and the sodden mess will be a clean solution. That's slow because the liquid has a much smaller surface. – Volker Siegel Dec 6 '19 at 1:01