Reef Chemistry Puzzle #12 Water

[Randy Holmes-Farley]

Reef Chemistry Puzzle 12

1. I am the reason water can be liquid at room temperature
2. I am the reason water ice floats on liquid water

What am I?


Good luck!

Previous Reef Chemistry Puzzle:

Reef Chemistry Puzzle #11 The deeper the smaller

Reef Chemistry Puzzle 11 Three clues today: 1. I get smaller the deeper one goes in the ocean 2. Some reefers prefer me small, and some prefer me larger (compared to most reefers today, Randy prefers larger ones) 3. I can cause a lot of disappointment when buying corals online What am i...

www.reef2reef.com

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[mook1178]

1. Hydrogen bonding caused by the dipolarity of the water molecule having a bend.
2. Ice is less dense than water, but specifically the water molecules push further apart due to rearrangement of the hydrogen bond

 

[rennjidk]

Intermolecular force?

 

[jda]

I am going with magic... or density.

 

[ReefStable]

Density / Force between molecules?

 

[Bob Kukura]

Gravity?

 

[DCR]

Hydrogen bonding

 

[dwest]

H bonding for the win.

 

[Randy Holmes-Farley]

And the answer is:

1. I am the reason water can be liquid at room temperature
2. I am the reason water ice floats on liquid water

What am I?

Hydrogen bonding!

Many folks got it right. Good job.

Answers of forces bet ween molecules is also correct, although not as specific as I was looking for.

Clue 1.

In a rough way, molecules that cannot hydrogen bond have a vapor pressure that relates to their physical size. Larger molecules have more interactions between each other than do smaller ones, and thus the smaller ones are more likely to turn to gas than liquid or solid.

Molecules of similar size to water and that cannot hydrogen bond, such as methane (CH4), hydrogen sulfide (H2S). phosphine (PH3), borane (BH3), silane (SiH4) and hydrogen chloride (HCl) are all gases at room temperature.

Why is water different? It is because liquid and solid phase water molecules interact with one another by hydrogen bonding. That bonding is much stronger than the ways that other molecules interact, with van der walls and dipole interactions.

A hydrogen bond can only exist for any reasonable period of time in liquid or solid water, not in the gas phase. That effect then stabilizes water in those two phases relative to the gas phase, and makes liquid water possible at room temperature.

Here's a drawing that shows hydrogen bonds (the dashed lines) in liquid water:

The strong polar bond between water molecules creates water cohesion. | U.S. Geological Survey

www.usgs.gov

Clue 2.

It is a curiosity that the hydrogen bonds in ice actually make the density of solid water ice to be less than that in liquid water. Thus ice floats. In part, that is because there are more hydrogen bonds in ice (4 per water molecule) than in liquid water (average 3.4 per water molecule). Still, one might expect that more bonding would pull the water molecules closer together and increase the density. But the bonding is very angle/orientation specific, and the pickup of additional hydrogen bonding in ice comes at the price of pushing the water molecules further apart to precisely fit the optimal orientations for hydrogen binding.

This easy to read articles discusses that effect a bit:

https://www.swc.nd.gov/arb/news/atmospheric_reservoir/pdfs/2016_12%20-%20Ice%20Vs%20Water%20--%20Density%20Matters.pdf

Thanks everyone for playing and Happy Reefing!