# How Would A Vast Mixture (Like an Ocean) of Ammonia-Water Behave in Terms of Evaporation and Freezing?

First, I'll go ahead and say I'm not very versed in chemistry. My question comes from my interest in planetary-sciences, specifically the speculation that there are likely exoplanets and moons with surface oceans of water-ammonia. However I can't find much hard info on how a vast body (i.e. ocean) of water-ammonia would behave differently than just water.

In short my question is how might a water-ammonia ocean differ from a "pure" water ocean in terms of:

• Ice(s) formation
• Evaporation
• Changes in water/ammonia ratio based on the above two factors (ice formation and evaporation).

This seems largely like a chemistry question to me. If I'm wrong please let me know which Stack Exchange you think I should post in. :)

Parameters:

• 5% - 40% ammonia.
• Temperature range: -80C to 15C
• Atmospheric Pressure: 0.5 bar to 30 bar.

I've heard that the freezing and boiling points of the mixed liquid would exist on some continuum between the freezing and boiling points of its constituent liquids (ammonia and water), varying depending on the exact mix ratio and of course pressure. Is that correct?

What I'd like to most wrap my head around is whether the water-ammonia mix would behave as a single uniform liquid, freezing and evaporating at a rate determined by the average of the mix's freezing/boiling point, or whether the ammonia molecules would start to evaporate before the water molecules, and the water molecules start to form ice before the ammonia molecules.

If the later behavior is correct I could imagine an ocean of water-ammonia varying substantially in its water/ammonia ratio from pole to equator thanks to the greater heat near the equator and the ammonia molecules evaporating.

Thanks!

• Water/ammonia oceans tend to be subterranean. – Mithoron Jul 16 '19 at 17:43
• They are in the Galilean moons but there is no reason water/ammonia surface oceans can't exists. In fact they are predicted to exist, possibly in some abundance. – n_bandit Jul 16 '19 at 17:46
• The concentrations will definitely vary tremendously over the climate zones. – Karl Jul 16 '19 at 21:08
• astronomy.stackexchange.com/questions/32676/… – Mithoron Jul 18 '19 at 18:04
• en.wikipedia.org/wiki/Uranus#Internal_structure BTW I can't say I'm fully OK with this one, too. – Mithoron Jul 26 '19 at 22:52

Freezing

I've heard that the freezing and boiling points of the mixed liquid would exist on some continuum between the freezing and boiling points of its constituent liquids (ammonia and water), varying depending on the exact mix ratio and of course pressure. Is that correct?

For freezing points, that is incorrect. Pure water freezes at 273 K and pure NaCl (table salt) "freezes" at 1074 K. However, if I dissolve NaCl in water, the freezing point of water will be lowered. In you water/ammonia scenario, water will freeze as pure water, but the freezing point will be lower. Ice formation would increase the concentration of ammonia in solution.

Evaporation

Water and ammonia mix in the liquid state, and everything mixes in the gas state. Evaporation (and condensation) depend on the ratio of components in the liquid state, the partial pressure of components in the gas state (or ratio of components and total pressure), and the temperature. At equilibrium at earth-like temperatures, there would be more ammonia than water in the atmosphere (the boiling point of ammonia is much lower than that of water, so the vapor pressure of ammonia is higher than that of water). A sudden drop in temperature would make it rain ammonia, mixed with a bit of water. So condensation and precipitation would increase the concentration of ammonia in the liquid state, and a sunny day would lower it.

If the later behavior is correct I could imagine an ocean of water-ammonia varying substantially in its water/ammonia ratio from pole to equator thanks to the greater heat near the equator and the ammonia molecules evaporating.

You are making a lot of assumptions here: that the exoplanet rotates fairly quickly around its own axis, that its axis is roughly perpendicular to the star, and that the star is close enough to heat up the planet.

What I'd like to most wrap my head around is whether the water-ammonia mix would behave as a single uniform liquid, freezing and evaporating at a rate determined by the average of the mix's freezing/boiling point, or whether the ammonia molecules would start to evaporate before the water molecules, and the water molecules start to form ice before the ammonia molecules.

No, they would do different things. Pure water at terrestial atmospheric pressure freezes at 0 $$^\circ$$C, while ammonia already boils at -33 $$^\circ$$C. Mixing them will lower the freezing point of water and increase the boiling point of ammonia, so there is no temperature where you would separate them completed by freezing all water and boiling off all ammonia. On the other hand, they don't freeze or boil in lock step. If you lower the temperature, water will form ice and the ammonia concentration in the liquid will increase. If you increase the temperature, more ammonia than water will turn into gas (evaporating or boiling), so the ammonia concentration in the liquid will decrease.

• Thanks for the reply. Your the first person to go over in some detail what you think the behavior would be. I appreciate that! May I ask where your deriving some of this information? Is it a synthesis of info you're already aware of, or are you referencing something? If you do have a reference I'd love to see it so that I could study it myself. Not trying to dispute your points, just hoping to learn more from primary sources. And yes, I was making a bunch of assumptions about the planet, but assumptions that serve my purpose. – n_bandit Jul 18 '19 at 14:50
• – Oh, I see you teach biochemistry. Great! Would it be alright if I asked you a few follow-up questions? I'm really trying to understand how these mixes work, and as I mentioned I'm not terribly versed in chemistry. – n_bandit Jul 18 '19 at 15:08
• @n_bandit Sure. If they questions are substantial, just ask a new question referencing the current one. If they are short and refer directly to my answer, ask them in the comments here, and I or someone else will try to respond. (And I'll incorporate it in the answer if it makes sense.) – Karsten Theis Jul 18 '19 at 15:13
• Can anyone tell me why my question is "too broad" and therefore on hold? How would you recommend rephrasing it without diluting the substance of the inquiry? – n_bandit Jul 20 '19 at 17:57
• @n_bandit RE:references: Phase transitions, freezing point depression and boiling point elevation are concepts you encounter in physical chemistry or general chemistry. Once you know the concepts, you can look up the specific data (boiling point etc) in textbooks, the literature or online databases. – Karsten Theis Jul 20 '19 at 18:02