Pure Water

[miguel]

I've read that pure water will not boil due to the lack of impurities, but I've also read that pure water will boil, but a higher temperature. What's the deal with this?

Also, except in a laboratory setting, can you have such a thing as pure water? Can you actually buy pure water nowadays?

 

[NeoPTLD]

Originally posted by: miguel
I've read that pure water will not boil due to the lack of impurities, but I've also read that pure water will boil, but a higher temperature. What's the deal with this?

Also, except in a laboratory setting, can you have such a thing as pure water? Can you actually buy pure water nowadays?

Depends on how you define pure. 100% water can't be made. We can get close with 99.9xxxxx%.
You can buy deionized water and distilled water at supermarket or buy some expensive HPLC grade water.

I think you're mixing it up with water in a very smooth container. When you microwave such thing, there is no core for water to start boiling and it becomes superheated. It will boil abruptly when it is disturbed as it is taken out.

 

[miguel]

Thanks for the info. I'll have to look again to see if it's the smooth container I was reading about. What's the reason 100% water can't be made?

 

[NeoPTLD]

Originally posted by: miguel
Thanks for the info. I'll have to look again to see if it's the smooth container I was reading about. What's the reason 100% water can't be made?

physical limitations.

Something will always contaminate anything to varying degrees.

For example, one microscopic dust spec in water or a molecule of nitrogen would destroy water from 100% to 100-n%. If n>0, even if it's 10^-30, 100% can't be maintained.

 

[miguel]

Thanks again. Yes, it was "superheating" that I was reading about before. They used the term "pure" water to mean water by itself. Apparantly, if you add a "contaminant," like coffee or tea, the water would boil in the microwave. Or, like you said, you can use a non-smooth container (I guess that rules out coffee cups).

 

[f95toli]

What you are asking about is actually examples of what is known as "supercritical phases" and is related to the physics of phase transitions.
A very common example is rain with a temperature below 0 degrees Celsius, when it hits the ground it will freeze instantly.

 

if you do try this though, be very careful, as the water can flash boil if you introduce something to do.

 

[NeoPTLD]

If you add apple cider to water boiled in a microwave in a clean glass mug, it will bubble over like soda. Try that with water boiled in pan. It won't happen.

 

[Lynx516]

The reason why yu cannot get pure water or even very pure water easily is because it is a ploar molecule. Because of its polarity it is a very very good solvent and hence it will just disolve anythign

 

[RossGr]

I suppose a industrial DI water system makes about as pure water as you can find. These systems maintain continual circulation through high purity Teflon pipes. It is never allowed to stand stagnate, the resistivity of the water is continually monitored, our system maintains a 10^14Ohms resistance.


Never tried to boil it, so I cannot testify to what happens when it reaches 100C. I do understand that the activity occurring in boiling water is due to particles which act as nucleation points for expanding pockets of air and water. With out the nucleation points water may behave differently then we are accustomed to.

 

[NeoPTLD]

Originally posted by: RossGr
I suppose a industrial DI water system makes about as pure water as you can find. These systems maintain continual circulation through high purity Teflon pipes. It is never allowed to stand stagnate, the resistivity of the water is continually monitored, our system maintains a 10^14Ohms resistance.

Ionically pure and free from particulate matters might not be the samething. We have drinking fountain like thing of deionized water in the lab, but HPLC grade water is like $18 a liter.

 

[element]

Originally posted by: RossGr
I suppose a industrial DI water system makes about as pure water as you can find. These systems maintain continual circulation through high purity Teflon pipes. It is never allowed to stand stagnate, the resistivity of the water is continually monitored, our system maintains a 10^14Ohms resistance.


Never tried to boil it, so I cannot testify to what happens when it reaches 100C. I do understand that the activity occurring in boiling water is due to particles which act as nucleation points for expanding pockets of air and water. With out the nucleation points water may behave differently then we are accustomed to.

Air? Don't you mean water vapor? There is no air usually under water that is boiling. Those bubbles are steam, or water vapor. Which is of course a gas but not air. By air I mean the air we breathe. Until of course they rise to the surface where they meet the atmosphere, shake hands, say "hello" and/or "pwned".

 

[DrPizza]

I'm curious as to the reason the water is continuously circulated in the Teflon pipes (and never allowed to stand stagnate) - does that help maintain purity? If so, I'm a little perplexed as to how.

 

[Snooper]

You can't let the water stand or you will get bacteria and algae growth. Not even pure water can kill off ALL the various bacteria and algae in the water and on the lines. If you allow a "dead head" to stand, life will grow and contaminate the whole dead line. Eventually, it will contaminate your previously clean water lines as well. Then you get the fun joy of a peroxide kill followed up with a LOT of flushes and purges to clean the lines back out. Needless to say, THAT REALLY slows things down (and costs a LOT of money). For some reason, folks tend to go out of their way to prevent that sort of thing from happening....

 

[Amorphus]

tip for the people who microwave water - if you're doing it in a smooth glass container or something with similar texture, put one of those cheap take-out chopsticks in the water: the wood will give something for the bubbles to form on, and the water won't explode on you when you nudge it.

 

[DrPizza]

Originally posted by: Snooper
You can't let the water stand or you will get bacteria and algae growth. Not even pure water can call off ALL the various bacteria and algae in the water and on the lines. If you allow a "dead head" to stand, life will grow and contaminate the whole dead line. Eventually, it will contaminate your previously clean water lines as well. Then you get the fun joy of a peroxide kill followed up with a LOT of flushes and purges to clean the lines back out. Needless to say, THAT REALLY slows things down (and costs a LOT of money). For some reason, folks tend to go out of their way to prevent that sort of thing from happening....

Sorry, but IMHO, if you have particles the size of algae and bacteria in your water, then you're not talking about anything even remotely close to pure water. I'd think that in a discussion of pure water, you've already eliminated the relatively huge particles like algae and bacteria, and are concentrating on removing other impurities such as trace molecules.

 

[f95toli]

Have you ever been in a cleanroom? You will find that many things are very dirty and their might even be som dust on the floor (it is difficult to clean in a cleanroom since you are not allowed to stir up particles). In many processes you do not care about big particles such as bacterias since they are easy to remove when you need to, it is the small stuff which is tricky to control.

 

[Snooper]

The bacteria and algae are not in the water (that is one reason you have to keep circulating the loop: so you can continually filter it). They are (in trace amounts) stuck to the various parts of the piping system. We have DI water in the 12 to 16 MOhm range, but you will STILL get a green line in a couple of months if you let that leg sit stagnate. Just the way it works. Once the line has a large quantity of organics in it, they are constantly releasing spores and other organic junk into the water stream and it is almost impossible to keep the resistivity up without a peroxide kill.

 

[Particle Man]

Removal of ions in water is conducted by reverse osmosis (RO). The pores of a RO membrane are extreme small, almost to the point where water molecules barely fits through (down to angstroms). Almost all of the ions are selectively held back by the RO membrane, which is generally polar polymer (polyamide type). This is the reason why pure water (RO water) from industry is calculated in megaohms. 10 e-14 ohms is still not good enough for some experiments (ie. electrolysis). Research labs are able to get to about 10 e-18 ohms. There is many places on the web that can discuss RO in detail with the different types of filters used for the process.

In addition, the water is purified of almost all organic material, therefore no animal life will be able to be supported in the immediate solution. If the water is exposed to the atmosphere very often, it will collect dust and provide a place for single cell animal growth. However, if it is sealed in a sterile container, I do not see how any single cell animals can grow at all. The reason that so many that these RO systems are "bleached" with peroxides is to get rid of the outside submicron organic buildup that the carbon filters did not catch.

The answer for original question will remain as 100 C for pure water (CRC - Handbook of Chemistry and Physics), if there is some way to breakup the surface tension (deformaties in glass). If the surface tension is not disrupted, this will create the superheated water which will "explode" when disturbed (ie spoon). This has happened when people are microwaving water in a perfectly smooth ceramic mug resulting in 2nd degree burns. On the other hand, when salt is added to water, it will depress the melting point and increase the boiling point (depending as to how much salt is added). This topic is very well explained on many science websites.

 

[NeoPTLD]

Originally posted by: Snooper
The bacteria and algae are not in the water (that is one reason you have to keep circulating the loop: so you can continually filter it). They are (in trace amounts) stuck to the various parts of the piping system. We have DI water in the 12 to 16 MOhm range, but you will STILL get a green line in a couple of months if you let that leg sit stagnate. Just the way it works. Once the line has a large quantity of organics in it, they are constantly releasing spores and other organic junk into the water stream and it is almost impossible to keep the resistivity up without a peroxide kill.

And where does the source of "organics" come from?

Algae, bacteria, etc are all carbon containing creature and they can't multiply if they can't obtain something with carbon in it.

Peroxide flush might kill them, but if they contain carbon, the debris will contain carbon whether dead or alive.

To maintain purity it's got to be kept in quartz glass and evacuated so that it's free from anything but water vapor. Any dissolved gas (O2, N2, CO2, Ar, NH3) is a contaminant. I'm sure some forms an azeotrope such as NH3, so once mixed in, complete removal is tough. Even if it's synthesized from pure hydrogen and oxygen, excess of either will find itself dissolved in the product.

 

[rjain]

The boiling point of water goes up as impurities are added in the same way that the freezing point goes down. Try boiling saturated sugar- or salt-water. The freezing side should be familiar to anyone who has made ice, and is the same reason that you put salt on the roads after a snow or ice storm. The technical term for the phenomenon is "colligative properties".

 

[NeoPTLD]

Originally posted by: rjain
The boiling point of water goes up as impurities are added in the same way that the freezing point goes down. Try boiling saturated sugar- or salt-water. The freezing side should be familiar to anyone who has made ice, and is the same reason that you put salt on the roads after a snow or ice storm. The technical term for the phenomenon is "colligative properties".

Depends on the impurity.

Sometimes a mixture of two liquid have a boiling point lower than either liquids. An example is azeotropic mixture of trichloroethylene and water. Triclene becomes azeotropic and the mixture have a lower boiling point than pure Triclene.

 

[rjain]

Yeah, I think the stuff I was referring to only applies to solids dissolving in liquids.

For liquids in liquids, I think it's a weighted average with a correction for interactions between molecules of the different liquids, as long as the liquids have similar enough transition temperatures and close enough affinity that they don't just transition independently.

Wow, it's been about 6 years since I've studied this stuff. How time flies...

 

[element]

When you test the resistance of water, how far apart do you put the probes?

 

[EmMayEx]

You don't measure the "resistance" of water you measure the "resistivity". Resistivity takes into account how much distance the electrons travel and how much cross sectional area they have to travel in. The units are Ohms * meters^2(cross sectional area) / meters(length). If you divide out the meters the final unit is Ohm-meter (not to be confused with an ohmmeter, which is the instrument used to measure "resistance").

For water you generally immerse two carbon probes (because carbon is fairly inert in aqueous solutions unlike metals which corrode rather rapidly in ultra pure water) and measure the current transmitted when a precise voltage is applied then apply a calibration factor to get Ohm-meters. You get the calibration factor by measuring some conductivity standards.

The conductivity of these is determined from first principles, i.e. if you add X milligrams of salt that dissociates with a dissociation constant of Y you end up with XY positive ions of mass Mp and XY negative ions of mass Mn. You can then use a theoretical equation that calculates conductivity based on ion mobilities as a function of ion concentration and temperature while taking into account the self dissociation of water into OH- and H3O+ ions and calculate a theoretical resistivity for your carefully prepared solution at a given temperature. You make a few of these solutions (or buy NIST certified standard solutions from VWR of Fisher Scientific) then measure them with your probe. With some amount of dilligence you get a straight line that correlates measured current to conductivity in Ohm-meters or Megaohm-meters. 15 Megaohm water means a cubic meter of in a cubic meter box with 1 meter square electrodes on either side has a resistance of 15 Megaohms (ignoring the resistance of the electrodes, wiring and instrumentation).

Max L.