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104 votes
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How are water vapors not visible?

Water vapour is a clear and colourless gas, so it can't be seen by the naked eye. What you see in the photo in your second link is (partially) condensed water vapour, i.e. fog (or mist). Fog contains ...
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92 votes

Why does carbon dioxide not sink in air if other dense gases do?

Gases are all miscible. If initially separate and adjacent, they do not mix instantly, but once mixed (a process that occurs by molecular diffusion and is accelerated by macroscopic stirring or ...
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85 votes
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Why do gases have weight?

Imagine a gas molecule in a closed box bouncing vertically between the top and bottom of the box. Let's suppose the mass of the gas molecule is $m$ and its speed at the top of the box is $v_t$. When ...
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83 votes
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Why don't molecules of a gas settle?

You are used to all collisions being somewhat lossy – that is, when you think of most collisions, a little bit of the kinetic energy is lost at each collision so the particles will slow down. If they ...
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79 votes
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Is there any way for a gas to pass through a solid metal?

Yes, some gases can diffuse into and through metal. It is the bane of the high-vacuum engineer's life. Hydrogen is the worst because it tends to dissociate into atoms at the surface and the nucleus, a ...
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66 votes
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Gas pipes - why does gas burn "outwards?"

A gas flame is essentially a (chemical) reaction front, a (thin) layer in which a hydrocarbon (e.g. methane) is oxidised acc.: $$\text{CH}_4(g) + 2\text{O}_2(g) \to \text{CO}_2(g) + 2\text{H}_2\text{...
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55 votes

Why doesn't hydrogen gas exist in Earth's atmosphere?

The answer to your question comes from Maxwell distribution of speed of the hydrogen molecules. If you take a look at this graph, about the speed of a particle $v$ and the probability of that speed $...
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54 votes

If liquid and gas are both chaotic states of matter, what's the difference between them on the molecular level?

Everything you've said is correct, which is why the conclusion is: there is no fundamental difference! Under the modern classification, they're just the same fluid phase of matter. For example, ...
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52 votes

Gas pipes - why does gas burn "outwards?"

Gert explained why the flame can't travel back into the cylinder (because there's no oxygen). However, that actually doesn't explain why the flame doesn't travel into the burner itself, because ...
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48 votes

Why does hot air rise in a column instead of cold air pressing down?

With the same argument, I could deduce (and I know that it's wrong) that the cold air above is denser, so it will go down, pressing the hot air away sideways. Replace your hot air with a helium ...
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47 votes
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Do air particles "fly"? If not, how do they stay afloat?

I will list your questions and answer them one by one. what if air particles stop moving entirely one day? This scenario is what happens when the temperature is very low. For really no motion at all ...
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45 votes
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Why do gases conduct at low pressure?

At high pressure, the mean free path of electrons is quite low. The electrons don't get enough time to get accelerated. If the electrons don't accelerate for long, they can't gain the high velocity or ...
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45 votes

Why doesn't soda go flat immediately after opening?

When you open the bottle and reduce the pressure you now have a supersaturated solution of carbon dioxide in water so it is energetically favourable for the gas to come out of solution. However for ...
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35 votes

Why doesn't hydrogen gas exist in Earth's atmosphere?

The equilibrium concentration of hydrogen in the atmosphere is about 0.5 ppmv (parts per million per volume) according to Wolfram Alpha (with a tip of the hat to @AccidentalFourierTransform). This is ...
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35 votes
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Why is it so much more energy intensive to compress hydrogen than methane?

The essential reason is that a kilogram of hydrogen contains 8 times as many molecules as a kilogram of methane (because the mass of a hydrogen molecule is about 1/8 of the mass of a methane molecule)...
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35 votes
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What does "rarely" mean in NASA's statement: "technically referred to as an exosphere because it’s so thin, its atoms rarely collide."?

Any given gas molecule goes a long way on average without colliding with another gas molecule. The mean free path for a gas molecule is on the order $$ \lambda\sim \frac{k_BT}{d^2p} $$ where $k_B$ is ...
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34 votes

Would a gas "weigh" less than a liquid if they have the same mass?

First of all, it is impossible to have $1L$ of liquid water in vapor form in a $1L$ container. It is difficult for liquid form and the gaseous form to occupy the same volume. The gas molecules would ...
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34 votes
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Why (does/we assume) gas exert same pressure everywhere in a closed container?

An imbalance of pressure would itself cause an internal flow in the gas. So if the gas has reached equilibrium the pressure must be the same everywhere. The above is for a gas in ordinary ...
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33 votes

Would a gas "weigh" less than a liquid if they have the same mass?

The force the box exerts on the scale will be given by the difference between the force that the gas does downwards on the bottom of the box and the force that it applies upwards to the top of the box....
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  • 4,525
33 votes

If liquid and gas are both chaotic states of matter, what's the difference between them on the molecular level?

The described measurement would allow you to construct the Radial Distribution Function, the probability of finding another particle a distance r from a reference particle usually given as g(r), which ...
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  • 430
32 votes
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Why can our bodies bleed?

The Systolic and Diastolic blood pressure are measured relative to the atmospheric pressure, hence they are "stronger" than atmospheric pressure. The blood pressure measured (Systolic and ...
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29 votes
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Why does the ideal gas law exactly match the van't Hoff law for osmotic pressure?

The law $PV = n RT$ gives the pressure $P$ of $n$ moles of ideal gas in volume $V$. Meanwhile, the law $\Pi V = n R T$ describes the osmotic pressure $\Pi$ due to $n$ moles of solute in volume $V$. ...
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28 votes

Why doesn't hydrogen gas exist in Earth's atmosphere?

The other answers are correct in terms of the principal reason that lighter molecules are much more likely to escape the atmosphere. However, it seems that the premise of the question (and perhaps ...
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  • 5,981
27 votes

If the probability of a gas molecule to have any velocity is zero then how can the molecule have any velocity at all?

I think there is a more intuitive argument: Say your average molecule speed in a gas cloud is $20\:\mathrm{\tfrac{m}s}$. Okay, so set up your apparatus and track one particle. And your apparatus ...
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  • 1,085
25 votes

How are water vapors not visible?

Trouble is caused by definitions of steam and vapour in physics and in common language. Physical definition of water vapour and steam is gaseous phase of water. In common language it is "the white ...
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  • 1,166
24 votes

Why do gases have weight?

Think of the atmosphere as if it were an ocean. You might not think water has weight if you were diving underwater, but obviously when you fill up your cup with water you feel its weight increase. The ...
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  • 341
24 votes

Why are sound waves longitudinal even though they are mechanical energy?

In order for mechanical waves to propagate there needs to be some form of "restoring force" that tries to bring the system back to equilibrium. For longitudinal waves in gases this restoring force is ...
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  • 53.4k
24 votes

Could evaporation of a liquid into a gas be thought of as dissolving the liquid in a gas?

I don't think that's a useful perspective to adopt. Fundamentally, dissolution (or solvation) involves particle of the solute being pulled away from the bulk and surrounded by particles of the ...
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  • 51.9k
23 votes

Why don't molecules of a gas settle?

In fact, particles in a box of gas are slightly denser at the bottom than they are at the top. In general, the probability of finding a particle with a total energy of $E$ is proportional to the ...
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21 votes

Why do gases have weight?

Here is a short answer: Imagine you would have an empty box (i.e. vacuum), that you would put on a weighing scale. It would have some weight. Now, if you would insert some gas into it, the measured ...
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