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# Do Gases Get Heavier or Lighter as Their Temperature Rise? Answered

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

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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.

This question has been answered sufficiently, but I should point out that all of the cases listed below are closed systems. That is to say, no gas particles can either leave or enter the system.

There was a comment referring to a tire being inflated, and this requires attention:

Take a fixed enclosure (like a tire), of which will increase in volume to a point, but then no more, upon inflation. Start inflating this tire and you will see that the pressure increases. This is not due to a change in temperature however (much), this is due to increasing the number of particles inside the tire. You are taking air particles from a secondary system. In this case you are increasing the mass of gas within the volume of the tire.

In reality, it is almost impossible to have a true closed system. Gas particle exchange happens an awful lot, even in vacuum chambers. Although the ideal case is very important, and useful to calculate approximations, looking at real world cases is useful in order to understand the limits of these approximations, and when they can be applied.

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

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.

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

L

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!

fallow: follow (I spelt it wit mah southern axsent.

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.

The weight of any gas does not change with temperature. Its volume changes, but not the weight.