Another thread here (Jinx’s) reminds me of a paradox that has puzzled me for a while. Cold liquids hold more dissolved gasses than warm, right? But aren’t the molecules in cold liquids closer together? Shouldn’t there be less room for molecules to dissolve? Or is my original premise wrong? In gasses, the warmer the gas, the more other gasses can dissolve in them, right? Warm air can hold more humidity, for example. I could use some help here. Probably a lot. Dopers?
Cold liquids has slower-moving molecules so the gas atoms don’t get bumped out of the solution.
Warm gas molecules move faster and can keep bumping the water molecules around individually–keeping them ‘disolved’ in the air.
or something.
The explanations use different criteria. That’s what’s confusing. One says it’s because there’s less motion - that’s why it can hold more. The other says it’s because there’s more room - that’s why it can hold more. It feels like they both are probably true, but it’s like using two different arguments to make the same case in two different places. I think they’re probably right, but because the circumstances are different, the explanations have to be different. I just need a little more filled in here.
And then why is it that cold liquids can hold LESS dissolved solids than warm liquids? Something isn’t correct here. Maybe it’s my initial premise?
I have always thought what might be called folk physics on the subject. Dissolving a solid is a little bit (just a little bit) like liquifying it and the higher the termperature the easier that is. I know this is a kind of nonesense (since solubility has little to do with fusion termparature and much more with the chemical bond, but maybe there is some effect. (But why NaCl dissolve slightly more in colder water?) At any rate, with gases it is quite the opposite. Dissolving is a bit like liquifying and to liquify a gas you have to lower the termparature. I actually hold out more hope for that explanation because a gas dissolves until the point that the amount of gas leaving the solution is equal to the amount that is newly dissolving. And I know that the vapor pressure of the gs makes a big difference. That is why so much of the CO2 leaves soda and champaign when you open the bottle.
Pretty soon a real physics person is gona swagger in here and open up a can…but until then:
Remember, 3 phases of matter. Solid (least mobile state of atoms), liquid (moderately mobile atoms) & gas (highly mobile atoms). Generally a solid dissolves faster in a warmer liquid because there is more energy in the warmer liquid to encourage the reaction to break the solid apart and keep the individual atoms in a state of entropy. Heat the solution up further and the the more volatile of the dissolved stuff changes into a gasseous phase and leaves flies free of the liquid solution.
Beer at room temperature is essentially a solution of alcohol & water with a bit o beercrud mixed in. Pop it in the freezer and the energy of the solution decreases allowing the water to “calm down” into a solid phase. Heat it up, and the alcohol will change from a liquid to a gas and leave the solution leaving you water with some beercrud.
The amount of gas a liquid will dissolve depends upon the vapor pressure of said gas, which in turn depends upon the temperature.
To simplify things a bit, suppose we have pure water. That water has a vapor pressure, and that vapor pressure depends upon the temperature of the water. At 25 deg C, the vapor pressure is 3.1690 kPa (101.323 kPa = 1 atm). The vapor pressure increases with temperature up to (and beyond) the boiling point of the liquid. The boiling point is that temperature at which the vapor pressure of the liquid becomes equal to the pressure above the liquid (in most cases, atmosphere). The vapor pressure of water at 100 deg C is 101.32 kPa, and thus 100 deg C is the temperature at which water boils.
So basically as the liquid cools, so does the gas, and thus the vapor pressure of the gas goes down.
Is this what you’re saying? At, say, 25 C, molecules of gas that have dissolved in water will find it relatively difficult to undissolve because the pressure of the air around the water is relatively great and tends to keep the molecules in solution? And at cooler temperatures, the pressure difference is even larger? But as the temperature of the water increases, the difference in the pressure of the air is overcome by the vapor pressure of the dissolved gas and it boils off? If so, what IS this pressure? And where does it come from? And how is it connected to the molecular motion of the liquid? And how is it connected to the space between the molecules in the liquid? Sing: If I only had a brain…
Dude. Didn’t you ever see that film in chemistry class: Mr. Science Man has a this big glass cylinder with a vibrating floor. He throws in some different-sized styrofoam balls. the balls aren’t moving. Dude MSM points and says, “Solid”
Turns it on (adds heat/energy to the system). The little balls go flying around but stay in the cylinder–dude points and says, “Liquid”
Cranks it all the way up to 11 (even more heat/energy) and the balls go shooting out of the cylinder–dude points and says, “Gas”
Demonstrates increasing levels of entropy as energy is added to matter. He then takes a disk and places it over the top of the cylinder and cranks it up to 11–the balls can’t fly out because the lid is holding them in. “Pressure” To drive it home he lowers the disk all the way into the thing where it keeps the balls from moving at all even though the thing is turned all the way up to 11. He then explains the energy is still in the balls, just as water will still have a lot of energy when at 300 degrees but forced into a liquid state by high pressure, but it will take more energy to get those balls moving with enough momentum to push the disk up the cylinder.
A more advanced model uses balls of different mass. The ones with the higher mass tend to remain in the lower phase (liquid or clumped together as a solid) when the less dense ones are dancing happily about in a liquid or gas phase.
The interaction of oxygen or carbon dioxide and cold water is similar–it’s two substances acting according to the pressure and energy around them as opposed to each other directly.
Dude. No, I didn’t.
First off, the vapor pressure of a liquid is the pressure you would have above a liquid at a certain temperature if no other gas was present. Imagine a box that is entirely filled with liquid water. Now suppose we increase the volume of the box, while keeping the temperature of the box constant. Most of the water will remain a liquid, but some of the water will evaporate (or boil) until the pressure above the liquid reaches the vapor pressure of water at that temperature. Putting air (i.e. increasing the pressure of the non-water gas) above the water doesn’t have any effect on the vapor pressure.
The liquid molecules all have some kinetic energy (they’re bouncing around). Some have a little bit more than average, some have less. If a molecule has enough energy, it will leave the liquid phase and join the gas phase. When you increase the temperature, the kinetic energy distribution gets moved such that more molecules have enough energy to leave the liquid phase, which in turn pushes the equilibrium more towards the gas than the liquid.
The space between the water molecules is, to my knowledge, irrelevant to how much of a gas will dissolve in the water.
Praetor has it right. As the temperature increases, the molecules of a gas get more energy. As they get more energy they bounce around at higher rates and escape from solution faster. This is the same thing as saying that the gas has higher vapor pressure. The pressure doesn’t come from anywhere except the occasional molecule that escapes the forces holding it in solution. Liquids and gases have a vapor pressure. Some solids do too.
I don’t know where this notion of space between molecules increasing and decreasing comes from. This may happen to a small degree. If you want to test this get a volume of water and heat it and cool it to see how the volume changes. you will have to watch out for evaporation. If you do it right you should find that water is the most dense (The least amount of space between molecules.) at 4 degrees celcius.
[QUOTE=Christopher]
I don’t know where this notion of space between molecules increasing and decreasing comes from
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I was trying to do a sort of parallel reasoning. Warm air holds more water vapor than cooler air. The explanation that is most often offered is that warm air molecules are farther apart, providing more space for the water molecules. The increased motion of the air molecules and the water molecules is never mentioned in the explanation. From this, I felt there was some sort of paradox in the fact that cold water holds more dissolved gas - but not more dissolved solid - than warmer water, and I was confused. I’m slightly less confused now. Slightly. I believe that the main reason that warm solvents hold more solid solutes than cold is that the molecules of solvent are farther apart, providing more space for the solute’s molecules to occupy. Am I incorrect? If I am not, then I am left with the more or less special case of gasses dissolved in liquids and I’m trying to understand that situation in mechanical terms and my mental model is apparently distorted and/or wrong. Vapor pressure is a concept that I am not conversant with, obviously, and it hasn’t figured in my understanding prior to now.
If cold liquids hold more dissolved gas why does all of this CO2 come out of my Root Beer when I drop my icecream in?
That’s nucleation, an altogether different effect than temperature on the gas solubility. Bubbles of undissolved CO2 gas are formed where there are physical boundaries / imperfections in the system. In this case the imperfections are huge clumps of ice cream that only increase in number as the ice cream melts.
If you pour champagne into a glass with a tiny scratch on the inside, you’ll notice that bubbles appear at the point of imperfection. In fact, in the case of a seemingly perfectly manufactured champagne flute, the areas where the bubbles form are imperfections too tiny to be seen by the naked eye.
Hi Christopher!
Warm air holds more water vapor than cooler air. The explanation that is most often offered is that warm air molecules are farther apart, providing more space for the water molecules. The increased motion of the air molecules and the water molecules is never mentioned in the explanation. From this, I felt there was some sort of paradox in the fact that cold water holds more dissolved gas - but not more dissolved solid - than warmer water, and I was confused.
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I indirectly explained why warm air “holds more water vapor than cooler air” above, but I didn’t exactly spell out why. The reason I put “holds more…” in quotes is because it is irrelevant how much air you have around (i.e. what the air pressure is). At a certain temperature, you’ll get a certain partial pressure of water vapor, and that partial pressure increases as the temperature of the water vapor increases. So it’s not that the air is “holding” the water vapor, it’s more like they’re co-existing in the gas phase. The space between molecules is fairly constant in most liquids (the density of water decreases 0.78% from 4 deg C to 40 deg C) and is so large in most gases that intermolecular forces are usually very small, so the space-between-molecules model really isn’t accurate.
Hi Jake
Ok, then why does my warm soda fizz a lot more when I put in a icecube than when I put in a slice of lemon?
hmmm 13 replies and not one right answer.
What determines whether something dissolves is a balance of two tendencies - enthalpy (heat of reaction) and entropy (“information” content of reaction). If we assume enthalpy and entropy dont change over a limited temperature range then the change in equilibria (e.g. gas solubility) is determined by the enthalpy. Most gases at room temperature give off a little heat when dissolved in water - hence they become less soluble as the temperature rises. (Le Chateleurs Principle)
However as the water gets warmer many gases become more soluble (e.g. He becomes more soluble over 30 C! ). This is due to a change in the clustering behaviuor of water
see example 33 on this link http://www.lsbu.ac.uk/water/explan4.html
The solubility of gases in liquids usually decreases with increasing temperature. Thus, when cold tap water is allowed to come to room temperature in a warm room, bubbles of air escape from the water and cling to the inside wall of the container. If the water is heated to the boiling point, practically all of the dissolved air is driven off. Since the dissolving of a gas in a liquid is exothermic process, this change in solubility is in accord with *Le Chatelier’s * principle.
The dissolving of a solid in a liquid involves both a change in state which is always endothermic, and, in some cases, chemical combination with the solvent, which is likely to be exothermic. Hence the overall process may be exothermic or endothermic; in the great majority of cases, however, a solid dissolves in a liquid with the absorbtion of heat. In accordance with (you guessed it) *Le Chatelier’s * principle, then the solubility of solids usually increases with increasing temperature.
And since I’m sure all of you are beseeched with curiousity: *Le Chatelier’s * principle (from high school chemistry) may be stated as follows: when a system is in equilibrium and one of the factors which determines the equilibrium point is altered, the system always reacts in such a way as to tend to counteract the original alteration.
To clarify some earlier postings here, the solubility of common table salt (NaCl) increases as the water temperature is increased (35.7 parts per 100 parts at 32 degrees F versus 39.8 parts per 100 parts at 212 degrees F) and thus fulfills the general principle discussed above. Calcium sulfate’s (otherwise known as hard scale) solubility, on the other hand, decreases with increasing water temperature (0.298 parts per 100 parts at 68 degrees F versus 0.162 parts per 100 parts at 212 degrees F). Also for the extremely curious among you, consider compounds that exhibit very high solubilities (in water) such as ferric chloride hexahydrate (FeCl3-6H2O), for which 246 parts per 100 parts at 32 degrees F versus being infinitely soluble at 212 degrees F. Don’t play with this at home since it is a salt formed from a relatively weak base (Ferric hydroxide) and a strong acid (sulfuric) thus it hydrolyzes in water to give an extremely acid solution.