Steam in microwave oven ?

[Externet]

Hello all.

While cooking or just boiling water in a glass pot, the water boils limited to 100C;  does the steam above the liquid surface bombarded with microwave energy rises above 100C ?

In other words, what happens exposing steam to microwaves ?

[swansont]

Yes, you could have steam hotter than 100 C if it absorbed microwaves.

[exchemist]

44 minutes ago, Externet said:

Hello all.

While cooking or just boiling water in a glass pot, the water boils limited to 100C;  does the steam above the liquid surface bombarded with microwave energy rises above 100C ?

In other words, what happens exposing steam to microwaves ?

Not sure. If the microwaves are of the right frequency range, they should stimulate rotations of the molecule and cause further heating. But I remember learning that in normal use in a microwave oven it is liquid water they heat, where rotations are quenched by hydrogen bonding, so it may be that the frequency used needs to be different from the one corresponding to the absorption frequency of gas phase rotations.

But perhaps someone else here will know.

[swansont]

3 hours ago, exchemist said:

Not sure. If the microwaves are of the right frequency range, they should stimulate rotations of the molecule and cause further heating. But I remember learning that in normal use in a microwave oven it is liquid water they heat, where rotations are quenched by hydrogen bonding, so it may be that the frequency used needs to be different from the one corresponding to the absorption frequency of gas phase rotations.

But perhaps someone else here will know.

It may not be absorbed efficiently, but adding any energy is going to raise the temperature.

You can also superheat water in a microwave (it’s one of the concerns about putting liquid water in one) in which case you start out above 100 C.

[Externet]

Thank you.

When water already reached boiling in a microwave oven (100C) but the oven is still 'on';  what is the energy doing to the water/pot/steam/air/contents, assuming the air and the pot are transparent unaffected by the microwaves, and walls are reflective ?  What changes that boiling becomes more energetic ?  Water cannot rise more its temperature, what else is happening ?  The boiling bubbles are pure steam, right ?

Edited July 27 by Externet
Exponent zero at temperature to denote degrees became a zero when posted. Corrected.

[exchemist]

3 hours ago, Externet said:

Thank you.

When water already reached boiling in a microwave oven (100C) but the oven is still 'on';  what is the energy doing to the water/pot/steam/air/contents, assuming the air and the pot are transparent unaffected by the microwaves, and walls are reflective ?  What changes that boiling becomes more energetic ?  Water cannot rise more its temperature, what else is happening ?  The boiling bubbles are pure steam, right ?

Same as with a saucepan on the hob: boiling the water, i.e. imparting Latent Heat of Vaporisation, to turn more and more of the liquid to steam, plus, as your original question asked, further heating of the steam. 

5 hours ago, swansont said:

It may not be absorbed efficiently, but adding any energy is going to raise the temperature.

You can also superheat water in a microwave (it’s one of the concerns about putting liquid water in one) in which case you start out above 100 C.

I'm not so sure. True, the rotational spectrum is pretty broad, so there will be some absorption across a wide spectrum:-

However from what I read, microwave ovens are tuned to 2.45GHz, so right up the left hand end of the graph. So maybe they can't heat the vapour, only the liquid.

 

 

[studiot]

A little digging revveals the following:

The most common mw oven uses 2.45 Ghz  or 12 .236 cm

https://www.translatorscafe.com/unit-converter/en-US/calculator/em-spectrum/

 

Absorbtion data reveals

 

Quote

Abstract

The absorption due to uncondensed water vapor in the short microwave region is computed by means of quantum mechanics. The attenuation is attributed to two causes: (a) a single line 𝜆=1.35 cm, and (b) the combined residual effect of all the other lines, whose wave-lengths are too short for resonance. There is a sharp peak in the absorption due to (a) at 1.35 cm, amounting to about 0.2 db/km per gram of H2O per cubic meter. The absorption caused by (b) is inversely proportional to the square of the wave-length. The theory is compared with existing microwave data on damp air, on pure water vapor at low pressures, and on steam. From these data one can determine the precise value of the resonance frequency in (a) and the line-breadth. Until recently these constants could only be roughly estimated from infra-red measurements. On the whole the theory and experiment agree satisfactorily, except that the attenuation due to the residual effect (b) is apparently about four times as large as predicted. Possible causes of this discrepancy are speculated upon—perhaps the Lorentz model of infinitely sharp collisions which we use is too simple. Finally a curve is ïncluded of the predicted absorption in the millimeter region, where water vapor is much more opaque than at centimeter wave-lengths, and new resonances come into play.

• Received 27 December 1946

DOI:https://doi.org/10.1103/PhysRev.71.425

https://journals.aps.org/pr/abstract/10.1103/PhysRev.71.425

 

1.35 cm radiation corresponds to 22.2 Ghz radiation

[John Cuthber]

As far as I can tell from looking at spectra on-line, water vapour does not absorb the 2.4 GHz EM radiation typically used in µwave ovens.
I don't think liquid water has any resonances there either- the heating is dielectric heating.
https://en.wikipedia.org/wiki/Dielectric_heating

Once the food in an oven gets heated to about 100C any additional energy is used to produce steam (and a vanishingly small amount to drive chemical reactions which we call cooking).
Absorption spectra in the vapour phase consist of fairly fine lines. The crude power supplies etc of a microwave oven will mean that the emission is relatively broad. Only a small part of the emission could overlap. So the coupling to the gas would be very poor.

At least some dielectric heating of fats will happen (penetration depths are of the order of 100 mm) and their higher boiling point means that they could (locally) be  heated well above 100C. Steam in contact with them may become superheated.

Edited July 27 by John Cuthber

[Sensei]

15 hours ago, Externet said:

While cooking or just boiling water in a glass pot, the water boils limited to 100C;

The boiling point of compounds depends on atmospheric pressure (without covering the vessel), and from internal pressure (with covering). So, one can exceed 100 C, just by covering the pot.. e.g. making a tea at altitude 500m is slightly different than making a tea at sea level. https://en.wikipedia.org/wiki/High-altitude_cooking

https://en.wikipedia.org/wiki/Pressure_cooking

Adding salt (which is natural for any not distillated not demineralized water), etc. also changes the equation.

https://www.google.com/search?q=salt+water+boiling+point

"The exact temperature depends on the concentration of salt. For a typical saltwater solution (like seawater, which is about 3.5% salt), the boiling point is around 102°C, slightly higher than the 100°C boiling point of pure water."

You also need to break the bonds between the water molecule and the salt ions too.

 

Edited July 27 by Sensei

[John Cuthber]

I live far enough above sea level that the boiling point of pure water would be somewhere near 99.6C
But the things that I microwave are not pure water. 
I gather sea water containing about 3.5 % salt has a (standard, sea level) boiling point of about 102 C because of the salt present in it.

To a good approximation, the boiling point of sea water here would be about 101.6C

And I suspect that the salinity  of "water" in much of my food is about 0.9%
To a fair approximation, the effect of altitude on boiling point will be cancelled our by the presence of dissolved material.

[TheVat]

BP is 96.5 C here, so hard-boiled eggs take a little longer.  The water here is fairly soft.  I would guess that BP comes up a bit as the eggs sit in there for a while and some of the shell calcium dissolves.  Not a lot but it could make a difference.   

[npts2020]

On 7/26/2024 at 4:40 PM, Externet said:

Hello all.

While cooking or just boiling water in a glass pot, the water boils limited to 100C;  does the steam above the liquid surface bombarded with microwave energy rises above 100C ?

In other words, what happens exposing steam to microwaves ?

According to Wikipedia (and engineering schooling I went through many years ago) you have to remove the steam from contact with the water in order keep it superheated (i.e. raise its temperature above the boiling point) beyond a very brief time. I would think any heating above the boiling point will be undetectable by ordinary thermometers by the time you opened the door and tried to measure it.

"Superheated steam and liquid water cannot coexist under thermodynamic equilibrium, as any additional heat simply evaporates more water and the steam will become saturated steam. However, this restriction may be violated temporarily in dynamic (non-equilibrium) situations. To produce superheated steam in a power plant or for processes (such as drying paper) the saturated steam drawn from a boiler is passed through a separate heating device (a superheater) which transfers additional heat to the steam by contact or by radiation"

from https://en.wikipedia.org/wiki/Superheated_steam

[studiot]

8 hours ago, npts2020 said:

Superheated steam and liquid water cannot coexist under thermodynamic equilibrium, as any additional heat simply evaporates more water and the steam will become saturated steam. However, this restriction may be violated temporarily in dynamic (non-equilibrium) situations.

Yes but conditions in a working microwave oven are not those of thermodynamic equilibrium.

[John Cuthber]

10 hours ago, npts2020 said:

To produce superheated steam ...steam ... is passed through a separate heating device (a superheater) which transfers additional heat to the steam by contact or by radiation"

There's not a lot in a microwave oven that's much hotter than boiling water (except maybe oil) to heat by contact and the microwaves are the wrong wavelength to heat steam by radiation..
 

[studiot]

32 minutes ago, John Cuthber said:

There's not a lot in a microwave oven that's much hotter than boiling water (except maybe oil) to heat by contact and the microwaves are the wrong wavelength to heat steam by radiation..
 

As any microwave user knows, sugar also gets hot.  Sogar softens at around 110^oC, is melted around 135^oC and burns at around 170^oC.

[John Cuthber]

Sugar also absorbs water.
The boiling point of a saturated sugar solution is a lot higher than 100C.

So saturated  steam at 100C would condense onto sugar (at 100C) and dissolve it

[studiot]

 

 

https://www.appliancecity.co.uk/news/microwaves/what-should-you-never-put-in-the-microwave/

 

[npts2020]

On 8/4/2024 at 5:03 AM, studiot said:

Yes but conditions in a working microwave oven are not those of thermodynamic equilibrium.

True but the rate at which the system tries to reach that equilibrium is pretty rapid so someone would have to show me it is possible to remove or measure any superheated steam before it returns (for all practical purpose, anyway) to the same temperature as the water it is in contact with.

[dimreepr]

On 8/6/2024 at 4:21 AM, npts2020 said:

True but the rate at which the system tries to reach that equilibrium is pretty rapid so someone would have to show me it is possible to remove or measure any superheated steam before it returns (for all practical purpose, anyway) to the same temperature as the water it is in contact with.

Try putting a pressure cooker into a microwave?

[John Cuthber]

 

23 minutes ago, dimreepr said:

Try putting a pressure cooker into a microwave?

https://www.ebay.co.uk/itm/135122947367?

 

[npts2020]

8 hours ago, dimreepr said:

Try putting a pressure cooker into a microwave?

Pressure only affects the boiling point of any (that I am aware of) fluid, the steam will be hotter than 100C but it will still be saturated and not superheated because the water's boiling point will also be that much hotter.