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Author Topic: Why does water look opaque when it is hot?  (Read 6355 times)

Takashi

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Why does water look opaque when it is hot?
« on: 08/03/2010 10:30:02 »
Takashi asked the Naked Scientists:
   
Dear Naked Scientists:

Hello! My name is Takashi, a medical student in Tokyo, Japan.

I listen to your show every week on podcast.

I started listening last year and soon got addicted to it! It's amazing that you explain everything simply and beautifully. You are my English teachers. Keep up the good work!

I have a question; Why does water look opaque when it is hot and get transparent as it cools down? I noticed numerous tiny bubbles forming in the hot water. Is that because of those bubbles? If so, why do they disappear as water cools down?

By the way, I purchased your book, Naked Science, and it's like salted nuts! Once you start, it's hard to stop reading it.  I'm now looking forward to your next book.

Sincerely,
Takashi

What do you think?
« Last Edit: 08/03/2010 10:30:02 by _system »


 

Offline yor_on

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Why does water look opaque when it is hot?
« Reply #1 on: 09/03/2010 12:51:14 »
The bubbles it is, and the fluids heat :)

Those bubbles are made of air, the hotter the water the less air in the water. Why water behave this way? Well first of all, let us assume that it's not through any breaking of the oxygen/hydrogen bonds that we get those bubbles. If that would be the truth then some of those bubbles would be quite explosive. Which certainly would spice up our cooking :) but also make kitchens a most dangerous place to be in.

So it has to be air existing in the water but not bound into those molecules, right? Let me give you an example, we say that fish breath water, don't we? Well, they do but they don't break the oxygen/hydrogen bonds. Mixed in with the water are gaseous molecules of oxygen and small amounts of carbon dioxide methane etc, and cold water contains more of that dissolved oxygen than hot. The fishes gills will extract this mix out of the water and return the CO2, just as our lungs do.

That's also why the Antarctic water is loosing it's ability to hold CO2. It's getting warmer and so our major heatsinks are loosing their ability to hold those gases. Cold water can hold more of any gas than warmer water. So if the water becomes too warm, like in summer lakes, even if 100% saturated some oxygen levels may become suboptimal for fish, and when thinking of cold waters, perhaps also the same reasoning will go for plankton? But let it suffice for now to say that the bubbles contains an air blend, not steam, nor hydrogen neither pure oxygen.

But why does it make the water milky/cloudy then? One of the main reasons is called refraction, which is the bending of light waves when they encounter a transparent substance like water or glass forcing them to change their speed. And the denser the material the 'slower' and more 'bent' the light from the observers view. In reality light always have the same speed, no matter what material/density it 'passes through' according to main stream physics, but as it gets exchanged into 'new photons' interacting with the material/density, that time will differ with the density (atoms/cm2) in the material/fluid.

If you looked at one of those air bubbles and placed a hair under it the bubble would act just like a diverging lens, the object would be upright but smaller than the object really was, as contrasted to a water drop that instead magnifies your hair. So the light gets scattered, refracted and reflected, and a lesser part of it will get reflected back to your eyes due to those bubbles. The visible part of light that does so will also have a different wavelength due to the temperature of the medium (Water/Water bubbles)that reflected it. And it is that heat difference that will create the slight 'white-grey cloudiness' you observe while looking at the boiling water as the wavelengths will change, getting reflected back to you.

"Both reflectivity and emissivity of all bodies is wavelength dependent. The temperature determines the wavelength distribution of the electromagnetic radiation as limited in intensity by Planck’s law of black-body radiation.

For any body the reflectivity depends on the wavelength distribution of incoming electromagnetic radiation and therefore the temperature of the source of the radiation. The emissivity depends on the wave length distribution and therefore the temperature of the body itself.

For example, fresh snow, which is highly reflective to visible light, (reflectivity about 0.90) appears white due to reflecting sunlight with a peak energy wavelength of about 0.5 micrometres. Its emissivity, however, at a temperature of about -5°C, peak energy wavelength of about 12 micrometres, is 0.99."


As far as I understands it :)
« Last Edit: 09/03/2010 12:52:50 by yor_on »
 

Offline stereologist

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Why does water look opaque when it is hot?
« Reply #2 on: 09/03/2010 20:04:05 »
Watch a bot of water being heated. The first bubbles seen are dissolved air coming out of solution. Watch carefully and observe that these bubbles when rising become larger as they rise. As the bubbles rise they are under less pressure and therefore expand in size. Later on bubbles form that become smaller as they rise. These are collapsing bubbles of water vapor. The liquid above is cooler. The water vapor turns back into a liquid.

Boiling water is turbulent. The churning water allows light through, but does not allow images to be seen through the water. The transparent water becomes translucent - not opaque. The change to translucent is due to the refraction of light at the interfaces between water and water vapor. Light still passes through the water, but images are essentially "jumbled".
 

Offline yor_on

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Why does water look opaque when it is hot?
« Reply #3 on: 09/03/2010 22:22:25 »
Maybe stereologist, but I think it's a combination of both wavelength and refraction? If you take glass that's transparent and crush it you will get a milky white substance reflected back to you and there I would agree to it being only refraction as the temperature would be the same. But when it comes to boiling water you have two factors, don't you? Heat and refraction.
 

Offline stereologist

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Why does water look opaque when it is hot?
« Reply #4 on: 10/03/2010 00:14:05 »
I don't think that the range of boiling water is hot enough to affect the distribution in the optical range. This is well below the heat of a flame. It certainly is a good idea, but my intuition tells me that the bulk of the radiaton from the heat is below the optical part of the spectrum.
 

Offline yor_on

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Why does water look opaque when it is hot?
« Reply #5 on: 10/03/2010 00:38:09 »
Thinking about it again, I think you're right, otherwise I would expect water near boiling to be of a slightly different color, but I'm still not totally convinced though :)

I have this feeling that those bubbles will change the spectrum and that this plays a role? But I can't find anything supporting it when I look around :(
 

Offline stereologist

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Why does water look opaque when it is hot?
« Reply #6 on: 10/03/2010 03:04:46 »
I had not thought about it, but it seems to me that you are correct in that the emitted spectra is dependent on the temperature. My guess is that the effect is likely to be too small to be detected by the eye because the temperature is too low. I'm glad you pointed this out to me. I clearly overlooked this issue.
 

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Why does water look opaque when it is hot?
« Reply #7 on: 10/03/2010 04:54:24 »
Well, I still think you're more right than me ::))
 

Offline yor_on

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Why does water look opaque when it is hot?
« Reply #8 on: 11/03/2010 20:18:07 »
The pressure comes from the liquid (Water) surrounding the bubble Rami. I think you were thinking of the pressure inside the bubble, right? That pressure inside is more or less constant from the time the bubble have formed. At least we can assume so for this question :) although I won't swear to it being impossible for it to be able to get more 'air' squeezed out from the surrounding water molecules.

So if you take a bubble of a certain volume, like a balloon and then dive with it under water you will see the balloon shrink as the surrounding water presses on it. On the other hand, being already under water like some deep sea fishes the air you breath is 'compressed' too. If you would go up from such a depth that air inside your lungs would expand, which is why you learn to let the air constantly out as you go up from f.ex a damaged submarine. Up to a hundred meters depth you can do this without any diving equipment if I remember rightly, as long as you remember to let air out constantly as you go up.

That as the air expands as the pressure becomes less, that is also the reason for diving sickness, air getting so 'compressed' will allow your blood and body to absorb a greater volume, including nitrogen. As nitrogen isn't metabolized by the body it somehow have to be released through your breathing. The longer you are down and the deeper the more nitrogen you will have in your blood and tissue, and if you ascend to fast the nitrogen will get 'stuck' inside you expanding, It will then form bubbles in the blood and body tissues that will give you what's called 'the bends'. And if not breathing out constantly in our free 'ascension' the expanding air can overexpand your lungs tear the tissue and allow air into your bloodstream. Those bubbles travels then to your heart and brain where they can get stuck. But as long as you don't try to keep that air inside you but let its expansion out you will be okay, from that at least. But it should still have to do with how long you've been under too, I think?

 

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Why does water look opaque when it is hot?
« Reply #8 on: 11/03/2010 20:18:07 »

 

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