Sunday, December 14, 2008

What is a liquid?

I recently had dinner with some chemists and after a bit of red wine the question "What is a liquid?" popped up. The reason the question popped up is the gas sulfur hexaflouride. I had never heard of this stuff before but allegedly you can float solid objects on top of it. Intuitively, I would say that if a ship can sail in it it's a liquid but things clearly aren't that simple. There's quite a few videos with this stuff on the web. In this particular video they float an object on some SF6 in a fish tank (I'm assuming the video is not a hoax). So what is the definition of a gas/liquid? Obviously it' a density thing. Did anyone out there pay attention when they had physical chemistry and would they care to explain it to a simple preparative chemist? Also what is the least dense liquid out there? Besides pentane (0.626 g/cm3) I can't come up with much that has a density below 0.7. D!

9 comments:

Mitch said...

I don't think you can compress liquids very easily.

SiO2 lungs said...

HAve you guys idea of the density of liquid nitrogen?? I would expect a lower density than that of pentane at rt...
what about cold acetone (-78C)??

Anonymous said...

Whatever happened to the simple definition of a liquid as something that conforms to the shape of its container?

Chris said...

IIRC, a gas fills all volume available to it (over sufficiently long time), whereas a liquid has a defined volume.

Daniel Sejer said...

A quick Google search gave me this result: The state of matter in which a substance exhibits a characteristic readiness to flow, little or no tendency to disperse, and relatively high incompressibility.
Still a bit to fuzzy for my liking. Surely this has been defined in a highly scientific way using numbers. D!

Daniel Sejer said...

I'been thinking! The volume for 1 mol of an ideal gas is 22.4 liters. Hence, for an ideal gas the concentration is constant. In other words the density of the gas must be directly connected to the mass of the individual gas molecules. The heavier the gas molecules, the denser the gas. Assuming that SF6 behaves like an ideal gas it simply has a very high density alowing it to support light solid objects. D!

milkshake said...

SF6 is not dense enough to support solid objects, its density is only about five times higher than of air. But it can support a hollow container filled with air, (if the container is lightweight enough) If you don't disturb it, the boat will float on the SF6 in the fishtank for awhile, before SF6 mixes up with the air inside the boat.

(You can do similar experiment with air-filled soap bubble floating in a tank filled with CO2).

You are right that when far enough from their critical pressure and boiling point, common gases behave with good approximation like ideal gas, so their density is proportional to the molecular weight.

22.4 L is one mol only at 273K.
The molar volume is 8.314 T/p, at normal pressure 101325 Pa and room temp 300K that gives you about 24L per mol.

So, one liter of air weights about 1.2g and the maximum lifting power you get from a baloon "filled with vacuum" is 1.2 gram per liter. You have to subtract the weight of the gas filling the baloon. So aluminum foil used for boat floating in SF6 has to weight less than 5 grams per liter of the boat volume displacement or it will sink.

Low-density liquids: liquid hydrogen has extremely low density, only 90grams per liter at atmospheric pressure. The shuttle external tank is so ginormous but liquid oxygen is only a minuscule part of it, its sitting on top of this tower of liquid hydrogen.

nitroindole said...

Liquid helium, of course! Wikipedia reference:
The density of liquid helium at its boiling point and 1 atm is approximately 0.125 g/mL.
http://en.wikipedia.org/wiki/Liquid_helium

nitroindole said...

The density of liquid helium at its boiling point and 1 atm is approximately 0.125 g/mL