Boiling water

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Here's a simple question for someone. I'm not a scientific person, so be gentle on the explanation.

Anyway, the question is what exactly happens when water boils? How do the bubbles form?

sparkite

oxygen is soluble in water.that means there is oxygen in water.a property of water is that the warmer it is the less soluble oxygen is in it.(oxygen sag curve) so when you heat water the solubility is decreasing and the water can simply not hold anymore oxygen and it is forced out of the water in the form of bubbles which when they hit the surface they turn to steam and flow away in the form of a gas

Exit

ah ok, cheers for that!

sparkite

forgot to mention that the heat has enough energy to break the bonds between the hydrogen and oxygen in water (water is H20) and both H and O are released in the form of bubbles! oh how i do love bubbles!

tricky D

oxygen is soluble in water.that means there is oxygen in water.a property of water is that the warmer it is the less soluble oxygen is in it.(oxygen sag curve) so when you heat water the solubility is decreasing and the water can simply not hold anymore oxygen and it is forced out of the water in the form of bubbles which when they hit the surface they turn to steam and flow away in the form of a gas

Eh, sorry, that's not correct. The Sag Curve is about dissolved oxygen being liberated from water under heat/pressure conditions, not about how H2O molecules change phase from liquid to gas. From the above what you say is that dissolved oxygen turns to steam at the surface. This would involve oxygen reacting with hydrogen (from where?) at the surface and turning into H2O vapour. The reaction between oxygen and hydrogen to produce water is very very volatile and such volatility just is not observed in/on boiling water. Besides what would happen when all the dissolved oxygen is heated off?

Boiling is not a chemical reaction, the individual molecules stay intact, it is merely a phase change - liquid to gas. Chemical reactions may occur, but they are a side show to the phase change. Dissolved impurties can and do indeed form bubbles and float to get released at the surface but that is a charge effect between the impurity and the water, not between the 2 hydrogen and 1 oxygen atoms in the water molecules. If dissolved oxygen was required to boil water then how would you boil water with no impurities/dissolved gases or how would the water continue to boil away after the dissolved oxygen had been liberated. Dissolved impurities and surface defects in the containing vessel do 'help' water to boil by providing a focus for the phase changing and dissolved gas molecules to congregate around due to charge differences. This can be seen in any observed in any boiling saucepan or even beer bubbling in a glass with sanded bottom gives these clues.

forgot to mention that the heat has enough energy to break the bonds between the hydrogen and oxygen in water (water is H20) and both H and O are released in the form of bubbles! oh how i do love bubbles!

Again not correct. That is electrolysis where electrical energy breaks the hydrogen-oxygen bonds and release the gases seperately at respective electrodes i.e. nothing much to do with boiling water.

An old school experiment shows electrolysis and the extreme volatility of the hydrogen - oxygen reaction.

What does happens is that in liquid form the water molecules have a certain attraction which holds them 'together' enough to be liquid. After a certain amount of heating the bonds between molecules (not the bonds between the hydrogen and the oxygen atoms) get weakened and the individual molecules pop off on their own in gas form within the liquid. But the gas molecules require much larger volume and are less dense. The gas molecules are attracted both upwards and to anything with a charge difference (whether due to attraction to impurities, surface effects or charges created due to friction caused by the molecules' own movement). So they gather to produce bubbles which rise and dissipate to air at the surface. If the water is absolutely pure and the vessel has a perfect surface then the gaseous water just finds it more difficult to find gather points, but not much more difficult enough to change the observed boiling point as there is more than enough internal friction between molecules being created.

hth

Capt'n Midnight

Reminder to self to put in a bit about nucleation sites, ie which are the same reason bubbles form at scratches in glasses - an expirement that is best conducted in a pub.

Also must thow in a bit about steam tables / pressure and the supercritical pressure (name?) and sublimation in vacuum and of curse the tripple point- anyone got good links ? (and superheating etc.)

Woden

why does the solubility of oxygen decrease with increasing temp? is it because its a gas or something, just wondering as for a general rule i find the solubility of things increases with temp

Capt'n Midnight

Aren't all gasses less soluble in liquids as the temperature rises ?

Solids would dissolve better since more energy (brownian motion - movement of molecules etc.) available to get them moving - you can get up to 350g of suggar dissolved in water if you raist the temperature.
With a gas this energy would cause it to revert to it's gaseous state.


Bubble size
The hotter the heating element the bigger the bubbles.
http://www.engr.iupui.edu/me/courses/me314lab/lab09.htm

cf: the ascii graph on
http://www.ece.vill.edu/~nick/usenet/00001188

The more nucleation sites - small cracks/discontinutites where bubbles / crystals / raindrops start to form the smaller the bubbles - boiling chips are broken pieces of rough ceramic or similar used to generate lots of small bubbles and disperse bigger bubbles if they form.

Not sure if bigger bubbles are noiser than lots of smaller ones - but kettles are not quite beasts.