Do Gases Get Heavier or Lighter as Their Temperature Rise?

I know that as the temp. in gases change, their weight does, but I'm not sure in what direction.

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7 years ago
Burf is correct.  However, if you're talking about *density*, that decreases as the temperature goes up.  Higher temperature, larger volume (at constant pressure) so mass/volume decreases.  That is the basis for hot-air balloons; the density of the hot air inside the balloon is enough lower than outside the balloon that the balloon can "float" in cold air, so to speak.
NicOmbra (author) 7 years ago
Thank you very much. I've had a lot on my mind lately, when I said weight, I meant density. Sorry for the mix- up.

while mass (weight) does not change the density decreases causing the gases to rise and give the illusion of being lighter
isacco7 years ago
Gases can be compressed and can expand.
PV = nRT is the basic equation of a perfect gas.
P = pression,
V = volume,
T = temperature,
R = constant, typical of each gas,
"n" is the number of gas molecules. This is the "mass" of the matter and cannot change. Thus, in the real world, the WEIGHT cannot change, unless nuclear reactions take place, which transform mass into energy.
As others have stated, weight do not change but DENSITY changes. Density is the ratio mass / volume. Thus density will change if V change. How is the V related to T?
If temperature increases, then the product PV must increase. This can be caused by increase of P or V, or both.
If the gas is contained in a confined system (let say a closed metal bottle), then the volume cannot change and pression will increase.
If the gas is contained in a expandable system (let say a rubber baloon), then the volume can also increase and, up to a limit, pressure will increase only slightly. However, the, as the volume reaches a threshold, the baloon walls are not more elastic. At this point volume do not increase, but pression do. This happens when you inflate a tyre.

lemonie7 years ago
Density decreases with temperature, but for a given quantity the weight (at the same gravitational-point) will be the same.
PV=NRT

L
Alpha29047 years ago
temperature only effects gases by changing its density. Its volume is effected as well BUT NOT THE MASS. en.wikipedia.org/wiki/Gas_laws fallow the link for the gas laws. There are LOTS OF THEM!
7 years ago
fallow: follow (I spelt it wit mah southern axsent.
kelseymh7 years ago
Neither one.  If the gas is enclosed in a fixed volume container (like a tank), then the pressure increases with temperature (Gay-Lussac's Law).  If the gas is enclosed in a fixed pressure container (like a balloon), then the volume increases with temperature (Charles' Law).

So why do balloons rise when you heat them?  Because the volume increases, so the balloon displaces more air.  Since the balloon's weight remains constant, but the larger volume of air weighs more, buoyancy pushes the balloon up.
Burf7 years ago
The weight of any gas does not change with temperature. Its volume changes, but not the weight.