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57

As other answers have noted, the only gas lighter than helium is hydrogen, which has some flammability issues that make it more difficult to handle safely than helium. Also, in practice, hydrogen is not significantly "lighter" than helium. While the molecular mass (and thus, per the ideal gas law, the density) of hydrogen gas is about half that of helium, ...


48

Actually, hydrogen is the only gas that is lighter than helium. However, it has a very big disadvantage: It is highly flammable. On the other hand, helium is almost completely inert - this is why it is very much safer to use the latter. What might happen when you use hydrogen instead of helium was impressively proven by history when the "Hindenburg" ...


23

Perfluorobutane is inert and has almost twice the density of sulfur hexafluoride. It is non-toxic enough that it is used in fire extinguishers and injected as a contrast agent for ultrasound. Boiling point: $-1.7\ \mathrm{^\circ C}$. Perfluoropentane is similar and rarer but somewhat higher density $(\sim13\ \mathrm{kg/m^3})$ in proportion to its higher ...


22

Actually the densest gas which has an actual use is Tungsten hexafluoride, clocking in at an astounding 13 grams per liter. Tungsten hexafluoride isn't as well known as its lighter cousin, sulfur hexafluoride, because it has a narrow range of uses. It is useful in the field of electronics manufacturing as it can be used to coat circuit boards with Tungsten. ...


19

Boron is a covalent solid with high melting point, like diamond (though not quite), and hence its crystals are hard to make. Unlike diamond crystals, they are not nice and probably wouldn't make a great display. The table on http://periodictable.com/Properties/A/MolarVolume.v.log.html seems to corroborate your findings about boron molar volume being the ...


17

I would guess radon is the densest gas ($9.73\; \text{kg/m}^3$) that is not directly lethal. It is radioactive however, emitting alpha radiation, so you don't want to breath it in. Given that it is much heavier than air, as long as you don't hug the ground it think it would be possible to be in the same room without killing you. From the fully non-lethal ...


16

I didn't know that balloons expanded during the fly because of thermodynamics, and I didn't know how high they can fly, but a rapid search tells that a partially unfilled regular balloon can fly until an altitude of around $\pu{25 km}$. Now, $\pu{25 km}$ means that it reaches the first part of the stratosphere, with temperatures of $\pu{-60 ^\circ C}$, that ...


13

"Added to water" is ambiguous. If the metal objects are shaped and placed such that they float (eg: as a metal boat does, or a light hollow sphere), then they will each displace the same volume of water (the volume of water equivalent to their equal common masses). So long as they float, they will displace the same amount of water no matter their shape. ...


12

Isopentane $\ce{C5H12}$ has the density of $0.6201~\mathrm{g\,cm^{-3}}$ at $20~\mathrm{^\circ C}$ [1, p. 3-330]. References Haynes, W. M.; Lide, D. R.; Bruno, T. J. CRC Handbook of Chemistry and Physics: A Ready-Reference Book of Chemical and Physical Data.; CRC Press, 2017; Vol. 97. ISBN 978-1-4987-5429-3.


11

Without significant mixing, diffusion takes a long time to mix gases. Our understanding of entropy tells us that we will indeed finish with mixed layers, but that doesn't give us a time frame for that mixing, only an outcome. Given a slow but steady production of a dense gas, layers absolutely will form due to density differences. There are plenty of ...


11

The best current non-toxic alternative to Clerici Solution appears to be Sodium Polytungstate (SPT). It also has a neutral (~7) pH, which is important when working with aluminum. By dilution/evaporation, density covers the range $1.1\!\!-\!\!3.1$g/cm$^2$, but above $2.5$g/cm$^2$ it can develop a harmless reversible blue tint when exposed to reduced metals. ...


11

Water reaches its maximum density at about $\pu{4 ^\circ C}$. As its cooled from higher temperatures the density decreases as you would expect as the molecules move more slowly and they can thus "settle" in closer together. But, as it approaches the freezing point, the molecules are moving slowly enough that they start combining together into a more ...


11

The ability of liquid mixtures to separate is not based on density, but on polarity of each liquid. $\ce{CCl_4}$ and $\ce{C_6H_{14}}$ are both non-polar; they will mix well in solution and not separate easily. $\ce{CH_3OH}$ is polar due to presence of a polar alcohol group. Mixtures of non-polar and polar compounds will separate easily. Density comes into ...


10

You're reasoning and your answer ("no") are both correct; this is not a trick question. Let's say we had 8.96 gm of gold and the same amount of copper and dropped them both into separate graduated cylinders that were filled to the 10 ml mark with water. Submerged items displace a volume equal to their own volume. Therefore, this amount of copper should ...


10

Clerici solution is a solution of equal parts of thallium formate and thallium malonate in water, density is 4.25 g/ml, Thoulets solution is solution of potassium-mercury iodide in water with density 3.2 g/ml. Don't even think of using them. Sodium polytungstate $\ce{Na6[H2W12O40]}$ is a relatively safe compound and allows densities up to 3.1 g/ml. $\...


10

Here's the unit cell of $\ce{Li2O}$, which adopts an antifluorite structure. The image is adapted from this webpage; technically it shows the fluorite structure but it doesn't matter. The cation-anion contacts occur along the diagonal of the cube (black dotted line). By Pythagoras' theorem, the length of the black dotted line is $a\sqrt{3}/2$, as I ...


9

Yes, hydrogen is lighter than helium but helium, on the other hand, is an inert gas (very less reactive). Also, hydrogen is highly flammable so that would make it unsafe to play with balloons.


8

The molecule length value you are using is the problem. It is more like 300 pm, which is the van der Vaals diameter and includes regions where electron density is significant. You have calculated an internuclear distance, but the nuclei of one molecule won't come close to those of another molecule because of electron-electron repulsion. This will change ...


8

If the other products aren't significant, I'll recommend the all famous alkali metals' reaction with water: $$\ce{2Li(s) + 2H2O(l) -> 2LiOH(aq) + H2(g)}$$ Hydrogen gas is less dense than air, both at RTP and at STP, because the main components of air (by mole percent) are $\ce{N2}~(78\%)$, $\ce{O2}~(20.95\%)$ and $\ce{Ar}~(0.934\%)$ - all of which have a ...


8

According to Wikipedia, the vapour density of a molecule is "the density of a vapour in relation to that of hydrogen". The density of a gas, $\rho$, is proportional to its molecular mass, $M$: $$\rho = \frac{m}{V} \propto \frac{m}{n} = M$$ where $m$ is the mass of the gas, $V$ is the volume, and $n$ the amount of gas. Therefore, if we denote your compound ...


8

A mole of neutrons in a neutron star would take up about $10^{-20}$ m$^3$. And in a black hole, they would be even smaller.


7

One of the requirements for the liquid base of oilfield drilling fluids is high density. Besides preventing the chips from falling out of suspension, the increased hydrostatic pressure helps prevent the sides of the well from collapsing before a steel casing can be applied. Oilfield drilling fluids are based on alkali metal formate solutions. Sodium, ...


7

The volume is the amount of space in the container, and no matter what temperature, a liter of water at that temperature occupies one liter. What I think you mean to say is, "Does a liter of cold water have more mass than a liter of hot water?" The answer is yes, it does. BTW, an open container (e.g. a beaker or drinking glass), can hold a larger volume of ...


7

The composition of dry air is about $78\%$ $\ce{N2}$, $21\%$ $\ce{O2}$ and $1\%$ $\ce{Ar}$. The molecular weights of these compounds are: $\ce{N2} = \pu{28 g/mol}$ $\ce{O2} = \pu{32 g/mol}$ $\ce{Ar} = \pu{40 g/mol}$ So, the average molecular weight of dry air is given as: $\pu{(0.78 * 28 g/mol) + ( 0.21 * 32 g/mol) + ( 0.01 * 40 g/mol) g/mol = 29 g/mol}$...


7

You are exactly correct that it is a matter of atmospheric pressure decreasing at a rate great enough to overcome the contraction due to decrease in temperature. On a nice, clear, dry 25°C day at sea level, atmospheric pressure decreases by about 12% per km, where the air temperature decreases by about 3% per km. This is very similar to the process that ...


7

It depends, there are different kinds of densities: mass density, particle density, charge density, probability density ... Depending on how your measure your matter, mass, number of particles, charge (e.g. for electrons), you could use the corresponding density. "Matter per unit volume" would only work in a broader sense, since it does not specifically ...


7

You could use tabulated literature data and then interpolate between the data points. For example, the following plot was created using data from NIST Reference Fluid Thermodynamic and Transport Properties Database (REFPROP) – NIST Standard Reference Database 23, Version 9. It shows the liquid saturation line as well as the vapour saturation line. In this ...


6

This is a clever question because the molecular weight of water is only 18.01528 g/mol while the molecular weight of hexane is 86.17536 g/mol. Naturally, you would expect the lower mass to rest on top, but water has a higher density... and we all know that water will sink! But there are other forces at work. So, supposing that you neglect the fact that one ...


6

If you considered the air within a given volume in two rooms, one with moist air and one with dry air, moist air will have some water vapor in place of other molecules that make up air (e.g. oxygen and nitrogen). A Water molecule has a lower mass than either oxygen or nitrogen, thus the volume of moist air will have a lower mass than the volume of dry air ...


6

The maximum density of $\ce{H2O}$ occurs when cooled to about $\pu{4^oC}$. At this point, the thermal motion of the molecules has slowed down enough such that water molecules can start to orient themselves in a manner more resembling that found in the expanded crystalline ice structure, and density thus starts to decrease with decreasing temperature. This ...


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