# Interesting characteristics of lifting gas (Hydrogen)

#### Willen

Joined Nov 13, 2015
294
If we compressed air, maybe its weight will be increased also. Same way if we compressed hydrogen, will it be lighter more? And can lift things more?

#### Papabravo

Joined Feb 24, 2006
14,252
I don't think there is any evidence to support those hypotheses. The ideal gas law relates pressure, volume and temperature. There is nothing about compression causing a change in weight or mass.

#### djsfantasi

Joined Apr 11, 2010
6,719
If a gas is compressed, then the mass per unit volume is increased. I would think that this would decrease the lifting power of hydrogen or helium.

How is the lifting power generated? Hydrogen is significantly lighter than air, and hence will float in a pool of air, like a cork in water. Compressing it will increase its weight or unit volume making it less lighter than air, thus reducing its lifting power.

#### Papabravo

Joined Feb 24, 2006
14,252
If a gas is compressed, then the mass per unit volume is increased. I would think that this would decrease the lifting power of hydrogen or helium.

How is the lifting power generated? Hydrogen is significantly lighter than air, and hence will float in a pool of air, like a cork in water. Compressing it will increase its weight or unit volume making it less lighter than air, thus reducing its lifting power.
And we know this because the periodic table gives us the weight of a mole of any element. One mole of hydrogen is clearly lighter than a mix of 80% Nitrogen and 20% Oxygen. The remaining components of air don't contribute much to a first order approximation.

#### Willen

Joined Nov 13, 2015
294
Hm...that makes sense. Hydrogen is lighter, because of its less density?

When we made air hot, that also will be light and can lift things (I guess hot air balloon for human lifting use hot air/fire flames, instead of Hydrogen). Can I say "hot air has less density (spread atoms far) than cold so it's lighter"? Or the heat is related to Gravity too?

#### Papabravo

Joined Feb 24, 2006
14,252
Hm...that makes sense. Hydrogen is lighter, because of its less density?

When we made air hot, that also will be light and can lift things (I guess hot air balloon for human lifting use hot air/fire flames, instead of Hydrogen). Can I say "hot air has less density (spread atoms far) than cold so it's lighter"? Or the heat is related to Gravity too?
The ideal gas law is often written as:
$$P \cdot V=nRT$$
where P is pressure, and V is volume.
n is the number of moles of gass, R is the Ideal Gas Constant and T is the absolute temperature in degrees Kelvin (°K)
From this relationship you can see that raising the temperature of a gas causes the product of pressure and volume to increase. If either pressure or volume is constrained in some way then the other quantity must increase with temperature. This is also why airplane wings are more efficient in cold air. Cold air is denser at a given altitude.

The value of R is
$$8.3144598(48) \frac{\text Joule}{\text mol \cdot ^{\circ} K}$$

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#### Willen

Joined Nov 13, 2015
294
Hi, so nice information! Then there's no direct relation of gravity there.

As you said if temperature increased then volume also will be increased which means density becomes less and thinner. Gas is not visible, is it due to its extremely less density?

If a child asked me 'Why I cannot see the air' then what I could say? What I need to understand?

#### Papabravo

Joined Feb 24, 2006
14,252
Hi, so nice information! Then there's no direct relation of gravity there.

As you said if temperature increased then volume also will be increased which means density becomes less and thinner. Gas is not visible, is it due to its extremely less density?

If a child asked me 'Why I cannot see the air' then what I could say? What I need to understand?
At sea level and 15 °C, air has a density of 1.225 kg/m^3, while aluminum has a density of 2.6 x 10^3 kg/m^3. That ratio explains why air is transparent to visible light and a block of aluminum is not.

#### takao21203

Joined Apr 28, 2012
3,695
has not much to do with density but the phase of the element and its general properties.

most gas phase elements are translucent infact all at suitable density.

so you could tell the child, all chemical elements have three phases,
solid, liquid, and gas. Solid and liquid are compareable take similar volume but gas is much much less dense.

#### #12

Joined Nov 30, 2010
18,210
Air, glass, and several plastics are apparently transparent. They are called transparent because light passes through them. As far as I know, compressed gasses are still transparent, or the same color they were at normal atmospheric pressure. I have seen liquid nitrogen and liquid Freon. They are as transparent as water. This seems to say that density is not why some things allow light to pass through them.

It seems that there is no solid, liquid, or gas that is perfectly transparent. The sky is blue. The bottom of the ocean is dark. I think the only thing that is perfectly transparent is vacuum.

#### takao21203

Joined Apr 28, 2012
3,695
Human brains are analogue computers they allow state transitions but no exact state can be reached.

So you can reach approximations to the truth affecting your survival but actually youll never be able to find the total truth.
Youd need a binary computer.

So we say approximately transparent, or transparent for practical purposes.

If your approximation to the truth is better, in some context you may have better survival. If you have too much data and bother details, especially to cover all special cases, your brain becomes clogged.

Its not uncommon for children to think hours about phenomena they cant understand. Some, cant be understood properly until some day like for instane why you dont fall of the earth on the other side.

#### Wendy

Joined Mar 24, 2008
22,156
And yet chlorine is yellow or green (though opaque), and deadly. As with most things there are exceptions.

#### wayneh

Joined Sep 9, 2010
16,399
Another exception: above the critical point, gases are as dense as liquid but remain a gas. For instance, steam and water have the same density at the critical point. But the steam will fill a container while the water behaves normally.

#### WBahn

Joined Mar 31, 2012
26,074
At sea level and 15 °C, air has a density of 1.225 kg/m^3, while aluminum has a density of 2.6 x 10^3 kg/m^3. That ratio explains why air is transparent to visible light and a block of aluminum is not.
So why is glass transparent while chlorine and many other gases are not?

#### WBahn

Joined Mar 31, 2012
26,074
@Willen: Here's an interesting experiment for you. Take a helium-filled balloon with a string and tie it inside your car so that the balloon is tethered in the middle of cabin. Now accelerate from a stop as hard as you can and note which way the balloon moved. Brake hard and make the same note. Make a hard turn and note the motion. Are any of these what you would expect? Can you explain any of them?

#### wayneh

Joined Sep 9, 2010
16,399
You don't even need the helium. If the floor is much colder than the head area, making a sharp turn causes the cold air to slosh to the outside of the turn. It's very noticeable where I live, in cold weather, less so but even in hot weather when you're blasting the A/C.

#### Willen

Joined Nov 13, 2015
294
I saw a TV program there I had seen the experiment. There was hydrogen balloon inside a bus and when bus moved ahead, also balloon moved to front side of the bus, instead of back.

#### GopherT

Joined Nov 23, 2012
8,012
So why is glass transparent while chlorine and many other gases are not?
Because color is a function of allowed transitions of electrons (the molecular equivalent of band gap in a semiconductor material). If an allowed transition (absorption) is in the range of a visible photon, the material will have color. Color can also be caused by fluorescence (emission) from a molecule after it is excited by a non-visible (UV) photon. Chlorine has empty anti-bonding orbitals very close in energy to the non-bonding (lone pairs) and can be excited by low energy of visible light (absorb specific energies of visible light) to give it the unique color. Other molecules like nitrogen require lots of energy to move electrons to the next available empty orbital (in the deep UV).

#### WBahn

Joined Mar 31, 2012
26,074
Because color is a function of allowed transitions of electrons (the molecular equivalent of band gap in a semiconductor material). If an allowed transition (absorption) is in the range of a visible photon, the material will have color. Color can also be caused by fluorescence (emission) from a molecule after it is excited by a non-visible (UV) photon. Chlorine has empty anti-bonding orbitals very close in energy to the non-bonding (lone pairs) and can be excited by low energy of visible light (absorb specific energies of visible light) to give it the unique color.
You didn't read the context very well. Your explanation would be a lot more useful as a response to Papabravo's post.

#### GopherT

Joined Nov 23, 2012
8,012
You didn't read the context very well. Your explanation would be a lot more useful as a response to Papabravo's post.
Because that was so far off I didn't know where to start.