# why HI isn't fully miscible with cold water when HF is?

HF is more soluble in water than HI , due to greater hydrogen bonding in HF , accepted but, on the other hand HF is a very weak acid so it's dissociation will be meagre when compared to HI's dissociation , so why is HF and not HI more soluble , am I misinterpreting the concept of solubility ?

• If you know what solubility is, I wonder how you'd suppose to misinterpret it. Also your assumptions about HF are wrong chemistry.stackexchange.com/questions/34818/… Jan 16 '19 at 16:51
• I still could not understand , what I was able to gather from that link is , HF does dissociate but due to high electronegativity diff. thr H+ is not freed resulting in low acid strength , am I right? Jan 16 '19 at 18:37
• This is not off-topic, the author gives a very clear attempt to understand the underlying principles.
– A.K.
Jan 16 '19 at 19:13
• @NutanPrakash Pretty much this. BTW your question may be better worded as why HI isn't fully miscible with cold water when HF is. Jan 16 '19 at 19:27

## 1 Answer

Hydrogen iodide is not listed as miscible with water because it is a gas. Miscibility is a property generally ascribed to pairs of liquids. Hydrogen fluoride is a liquid, and so it can be described as miscible with water. HF is a weak acid, but its high solubility can be explained by the high polarity of the H-F bond and the ability of HF to be both a hydrogen bond donor and accepter.

Hydrogen iodide is a gas at room temperature. It's boiling point is $$-35\ ^\circ\text{C}$$. Just because it is not miscible does not mean that it is not highly soluble. The solubility of HI in water is 245 g per 100 mL. In other words, a saturated solution has more HI by mass than water. HI is a strong acid, and the solubility of the gas is due to the nearly complete ionization of HI. Since HI is a gas, I presume that the solubility limit occurs when the vapor pressure of HI above the solution is high enough that vaporization begins to compete with ionization.

Physical data on HI from Wikipedia

• that means solubility can be attributed to both - high polarity and complete ionization in particular cases.And I chose HI just because I wanted to compare solubilities of species which dissolve due to complete ionization - like strong acids and HF due to hydrogen bonding . Jan 17 '19 at 9:44
• HF is a liquid if your lab is relatively cold (bp ~19 °C). Anyway, the main point is that solubility and ionisation are not strongly related. Jan 17 '19 at 15:15