I suppose I could search and find out how many moles of air are in ice that contains a mole of water. But my Google Fu is not mighty now. Given: When water freezes, it traps air inside, which increases the volume and reduces the density. How much does a volume of ice mass including the air and the water that’s trapped inside?
I didn’t think it was the trapped air, but the molecular structure of water that makes it expand when it freezes. That is is inherently less dense than liquid water.
Northern Piper is right: Perfectly clear ice has no trapped air. It’s less dense than liquid water because the polar molecules spread out and form a crystalline structure which is inherently more spread-out than water molecules are in the liquid form. It would work the same way if water froze in a perfect vacuum.
Ice with some white discoloration has trapped air, which works to reduce density, but clear ice still floats.
I agree with Northern Piper. Although there could some random air trapped in ice, there doesn’t have to be. It is the crystalline structure of the molecules that causes it to increase in volume, not the addition of air.
I won’t accuse you of this fallacy, but just for completeness I’ll say that empty space between atoms at the molecular level is not filled up by air. It’s just empty space.
frozen water is less dense than liquid water. that is why ice floats.
ice with air entrapped is even less dense.
I know that the empty space between molecules is not filled with air. But I thought that in ice, air was trapped. See what happens when these random thoughts pop into my head?
It can be, of course - just look at a typical home freezer ice cube - I think it’s safe to say the properties of ice vary depending on a variety of factors.
The mass of frozen water is exactly the mass of the unfrozen water from which it was derived. If there was air trapped or dissolved in the unfrozen water it will remain trapped or dissolved in the frozen water. The density of frozen water is different from unfrozen water, but its mass remains the same.
Ice can trap air, but it disrupts the crystalline matrix, leading to visible inclusions if the air bubble is big enough. That’s what looks white in home-made ice. It’s also why brine has a lower freezing point than pure water: The salt inhibits the formation of ice crystals by physically getting in the way.
Basically, the space between water molecules in an ice crystal is as “empty” as the concept of “empty space” even makes sense at the quantum level, which is “not very”. It’s just that the lowest-energy configuration available to pure water at that temperature and pressure is “pretty close, but not too close, and arranged in a specific repeating pattern”, and this specific repeating pattern happens to create a solid which is less dense than the liquid form of the same substance. It’s strange, but it happens to be true.
The equilibrium concentration (mole fraction) of air in water @ freezing point (273.15K) at atmospheric pressure is 2.315x10^-5. See page 596 from the NIST database.
Remember you can have more air trapped if you cool very fast / do not allow diffusion.
You do not need Google FU to calculate this, given the mole-fraction above - just use your calculator. Let me know if you run into problems.
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That’s OK. It’s just one of those things that popped into my head when I was reading something elsewhere, and I didn’t think it through before I asked. Had I thought about it, I would have realised this:
Admit it - you chickened out dude :):dubious:
Here’s the calc for the sake of the spirit of SDMB
Density of ice is 0.9167 g/cm3 at 0 deg C (32F) cite
So 1 mol (18g) of ice has a volume of : 19.64 cm3
Volume of air trapped in it at 32F and 1atm = 0.52 cm3
So Volume Fraction of Air in Ice = 2.6% - hope this helps. This assumes equilibrium conditions - and would depend highly on the rate of freezing (that is how quickly you freeze it)
How did you arrive at the assertion I bolded and colored red? You didn’t show any work to arrive at it, and nothing to support the idea that ice always includes trapped air (because it doesn’t).
See my previous post : " The equilibrium concentration (mole fraction) of air in water @ freezing point (273.15K) at atmospheric pressure is 2.315x10^-5. See page 596 from the NIST database. "
Calculate the volume of air when the moles are 2.315x10^-5
Read my post again. I did not assert what you claim. I asserted : 1 mole of liquid water (pure) will have an equilibrium concentration of 2.315x10^-5 moles of air at freezing point and 1 atmosphere. If you freeze the water fast - that will be the moles of air trapped in the water.
And notice that I say that this is dependent on the rate of freezing. If you freeze it very slowly - the air will escape. Depending on minerals and other impurities in the water and depending on you may have less or more air trapped.
The 2.6% by volume gives you a rough estimate of what to expect.
While we’re on the topic: are there other substances which are less dense in solid form than liquid?
Quote from Wikipedia :
“Other substances that expand on freezing are silicon, gallium, germanium, antimony, bismuth, plutonium and also chemical compounds that form spacious crystal lattices with tetrahedral coordination.”
Thanks!
So plutonium “ice” cubes float in liquid plutonium. How cool is that? Next time I have some of each laying around I’ll have to run the experiment. Mind the fumes though.