Hot answers tagged

101

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 tiny, discrete water droplets and light bounces off their surface in random directions, causing the visibility. Water vapour by contrast only contain free ...


84

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 the gas molecule moving upwards hits the top of the box and bounces back the change in momentum is $2mv_t$. If it does this $N$ times a second then the rate of ...


83

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 are subject to gravity, they will eventually settle. By contrast, the collisions between gas molecules are perfectly elastic – for a non-reactive gas (mixture),...


66

A gas flame is essentially a (chemical) reaction front, a (thin) layer in which a hydrocarbon (e.g. methane) is oxidised acc.: $$\text{CH}_4(g) + 2\text{O}_2(g) \to \text{CO}_2(g) + 2\text{H}_2\text{O}(g)$$ This oxidation reaction is colloquially known as burning or combustion. It's obvious from the equation the combustion needs requisite amounts of oxygen,...


55

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 $w$, you can see that there is a non-zero probability that the speed of a certain molecule is greater than the root mean square speed $v_{\mathrm{qm}} $ of that ...


53

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, consider the phase diagram of water. If you take water vapor, slowly heat it up, then pressurize it, and then slowly cool it down, you'll end up with liquid water. ...


52

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 burners (in particular Bunsen/Teclu burners, but hob/camping burners as well) actually pre-mix the gas with air inside the burner tube, but still it only burns ...


47

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 balloon. You can see there's no force on the balloon to push it sideways. The buoyancy forces it to accelerate upward (and some cool air around it to accelerate ...


45

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 kinetic energy that is required to ionize other atoms. While your argument that if there are more atoms, more electrons can be obtained through ionization ...


44

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 the gas to come out of solution you have to form a bubble and the mechanism by which this happens is called nucleation. But there is an energy barrier that ...


35

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 a result of mechanisms of production, and destruction (chemical reactions, escape). You are right that the RMS velocity of hydrogen is less than the escape ...


35

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). If we assume, for the sake of argument, that the compression is isothermal (constant temperature, $T$) the work needed to compress a sample of $N$ molecules ...


34

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 be as close to each other as they were in the liquid form. However, looking at your last paragraph, it can be inferred what you are actually asking for. I'll ...


33

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. Both forces can be written as pressure times area $F = P S$ where $S$ is the area of the top and bottom parts of the box. The difference in pressure is just ...


33

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 has a unique signature for each phase. The plot below from https://en.wikibooks.org/wiki/Molecular_Simulation/Radial_Distribution_Functions shows g(r) for ...


32

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 Diastolic) respectively measure the maximum and minimum pressure between heart beats relative to the atmospheric pressure. Gauge Pressure As John Custer noted in ...


29

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 $V$. These are qualitatively very different situations, but there's a simple fundamental reason that they end up looking the same. Both of these laws are derived ...


28

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 also of some of the answers and comments) is based on an incorrect model of atmospheric escape. Molecules from most parts of the atmosphere would never escape ...


27

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 shows indeed $20.000\:\mathrm{\tfrac{m}s}$. Now we have a small problem. Does it really have a speed of $20\:\mathrm{\tfrac{m}s}$? Afterall, we only measure to 5 ...


24

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 atmosphere is really just a gaseous ocean on top of the surface. In extension, if you were to light a candle on the edge of a building taller than the Earth's ...


24

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 cloud above pot with warm water in it when it is cold there". Take glass kettle and put it on a stove and boil the water in it. You will see bubbles growing in ...


23

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 supplied by pressure in the medium. However, there is no restoring force in a gas for bulk shear movement of the gas particles. Therefore, there is no ...


22

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 Boltzmann factor: $$ P(E) \propto e^{-E/kT}. $$ In particular, the potential energy of a gas molecule is $mgh$, where $h$ is the height above some fixed point (the "...


21

In a gas the molecules move separately. In a liquid they cling together due to van der Waals forces which are strong enough that the vibrating molecules do not completely separate.


20

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 weight would increase exactly by the mass of the gas times gravity. Historically, this is quite an important point when they burned stuff (solid to gas) in a ...


19

Summarizing the question: Start with two large weightless, identical boxes, and $2 \text{kg}$ of liquid water. Put $1 \text{kg}$ of water into each box. Heat one box such that none of the water molecules leave the box, but the water all boils into steam. Then compare the weights of the boxes when placed on a scale. The weight of the boxes will indeed by the ...


17

Because they have mass. And thus when in a gravitational field are accelerated towards other objects with mass, like the Earth.


16

Welcome to the magic of convection cells =) The first thing to remember is that you're working with a large number of gas molecules. The effect theoretically occurs no matter how many particles you have, but the effects are much easier to describe using bulk terms that handle many molecules at once, rather than trying to track each molecule. As BowlOfRed ...


15

You ask why a column of hot air (as in a chimney) rises, given that denser cold air is above it, pushing down. It rises, because denser cold air around the BOTTOM of the chimney is under higher pressure than the cold air at the top of the chimney. The extra pressure due to a chimney-height of cold air is pushing it down. The lesser density of hot air means ...


15

Of course it does. It helps a little bit to compare the ideal gas to a model that does take note of the size of the molecules and the forces the exert on one another. The van der Waals gas has explicit parameters for both behaviors. Compare the equations of state for these two models \begin{align} Pv &= k_B T \tag{ideal gas} \\ \left(P + \frac{a}{v^2}\...


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