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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, ...

49

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" ...

32

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 ...

31

There is nothing wrong with your calculations: but some liquids don't play nice Your calculations are, likely, correct. But liquids, unlike ideal gases, don't necessarily obey simple rules when mixed. In particular the two components, ethanol and water, strongly interact with each other when mixed and those interactions mean that the volumes of the pure ...

25

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. ...

21

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 ...

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 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 ...

17

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.

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. ...

13

Since OP is still in the high school, I'll try to explain it simply as possible using mathematical manipulation (hoping OP is more familiar with mathematics than chemistry). Both gold and platinum consist of same crystal packing called body-centered cubic, which is illustrated in following diagram: Crystal studies of both gold and platinum has revealed ...

11

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. $\... 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 ... 11 No, there is no law that requires the density of a mixture to fall between the densities of pure components. It does so most of the time, but the exceptions are not unheard of. Here's one. Water: density$1.00\ \rm{g/cm^3}$Hydrazine:$1.02\ \rm{g/cm^3}$Hydrazine hydrate:$1.03\ \rm{g/cm^3}$So it goes. 11 The density of$\pu{2.17 g/cm³}$refers to the bulk density, i.e. within a crystal of NaCl. In chemical engineering, the terms of powder density, tapped powder density and settled apparent density take into account for the air between the grains of a solid. Especially the later recognizes that there may be a difference between the solid simply poured into a ... 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 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. 9 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 ... 9 According to Hans Jaffe: Eine metallische Verbindung von Lithium mit Ammoniak. Elektrische Leitfähigkeit und galvanomagnetische Effekte. Z. Physik 93, 741–761 (1935) https://doi.org/10.1007/BF01337859 the saturated solution of lithium in liquid ammonia (approx.$\ce{4NH3 \cdot Li}$) has a boiling point above RT, and is significantly less dense (=0.48g/ml) ... 9 If your sample is solid and crystalline, knowledge of the unit cell's dimension and symmetry (e.g., fcc) and density allows you to determine the molecular mass of your compound. In fact, in crystallography, you may determine the macroscopic density of your sample (e.g., find a liquid which i) wets your crystals and ii) lets your crystals float) in first ... 8 One counter-argument: Helium is essentially a "fossil gas", and there's a limited supply of easy-to-get helium (until we get practical fusion reactors running, at least). Hydrogen, on the other hand, is universally available in$\ce{H2O}$and needs only a bit of electricity to break it out. Since helium has important industrial uses other than balloons, I ... 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 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 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 ... 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. 8 Although density may be a relatively easily measurable property of solid materials, it may not suggest the most fundamental relationship between mass and volume for the elements. Molar volume (the volume required to contain$6 \times 10^{23}\$ atoms of each the elements) is pictured in the graph below (WebElements): It is amazing that ~100 elements can be ...

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