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Author: Subject: Thought experiment: what happens when you put a blob of water on a surface made of sodium?
Dan Vizine
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[*] posted on 19-2-2017 at 10:44
Thought experiment: what happens when you put a blob of water on a surface made of sodium?


Sometimes, when we think we know a simple piece of data, we may suddenly realize that we don't understand it as well as we think we do when we are called on to make a prediction.

Everybody knows that sodium is a flammable metal and that a blob of sodium on the surface of water catches fire and (often) explodes. Sodium is the limiting reagent and it's molten. OK.

Now, what happens when you put a blob of water on a surface made of sodium?

A lot of things have changed from the previous case, and if you're the type of individual that will try this prediction, you can see what. I don't need to list them all.

It isn't obvious to me what will happen, what do you think?

Will the water blob cause a fire?

Will it just explode without causing fire?

Will it react with all the released energy necessary to start a fire under usual conditions, but not really be able to start a fire because the sodium conducts heat away so readily?






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Dan Vizine
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[*] posted on 19-2-2017 at 15:20


If we accept the currently circulated theory that explosion of sodium is actually a coulombic explosion, it wouldn't really be possible in this mirror case because of two reasons right off the bat. First, water is a poor conductor. Secondly, the water is going to disperse, in some fashion, and very rapidly. Hydrogen and oxygen gas will still be formed in large amounts, but the energy to spark ignition will be shunted into the highly electrically and thermally conductive sodium plane. I think that if you threw a liter of water onto the floor of a skating rink made of sodium, it would just hiss. Absolute worst case would be a tiny flash which ignites the widely dispersed gasses.

At any rate, I'm sure you can guess that I'll be dripping water onto sodium sheets this week.



[Edited on 2/20/2017 by Dan Vizine]





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j_sum1
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[*] posted on 19-2-2017 at 17:11


Skating rink of sodium I think you mean. (Cool image!)

I think it is good ground for some experimentation. I'd be melting some Na in a flat dish under mineral oil and then letting it solidify as a sheet. Then clean off the oil and drop some water.

My suspicion is that you will still get a great leidenfrost effect and the water droplet will skate around pretty well. That, combined with the thermal conductivity of Na will mean that you shouldn't get any melting of the sodium. Like you I am not picking any coulombic explosion either. But I admit to not having thought through that particularly well.

One thing I do know is that for a really common reaction there can be some surprises. I assume that you are familiar with the effects observed when sodium is dropped onto filter paper laying on the surface of water. There are threads on this (Look up transparent sodium). Not saying that you will get the same effects with this geometry but that we might predict a pretty low-key reaction and the actual situation might turn out to be something very different. There is a possibility of initiating a sodium fire for example -- that might happen if the H2 can ignite somehow.

Do you have the ability to video this and post for all to see?
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m1tanker78
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[*] posted on 19-2-2017 at 17:18


At least one SM member will be thrilled that someone created a thought exercise thread AND will be following up with some experiment(s). I don't believe in dropping names so I'll just say that member's user name starts with -a- and ends with -ga-.

So you have a curiosity and a hypothesis. How do you plan to carry out the experiment(s)? If you're taking requests, ( :D ) I'd be curious to know the water:sodium ratio where the sodium explodes (or reacts violently) 50+% of the time.

I did a lot of experiments with sodium a few years back. I don't remember anything immediately spectacular when dropping a few drops of water on a sodium pie at the bottom of a SS trash can. There was a hiss, a few yellow sparks but no ignition. Could be that the steam head diluted the hydrogen gas..?

After the initial hiss and sparks, I could hear a hiss every so often when the newly-formed NaOH would suck moisture out of the air and it would pool up a little. There was also a bit of condensed water running down the inside of the can. After about 15 minutes, the mass began burning slowly at first then accelerated to a 'normal' sodium burn -- billowing clouds of white smoke and all that jazz. I'm sure glad I did this outside!

EDIT:

It's very difficult to make a flat sheet of sodium by casting alone. You might find yourself adding 'tortilla roller' to the experiment item list.

[Edited on 2-20-2017 by m1tanker78]




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JJay
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[*] posted on 19-2-2017 at 19:47


I think it depends on how clean and smooth the surface of the sodium is. If it's crusty and rough and covered with hydroxides, a drop of water might make its way into a tiny cranny and vaporize. Of course, dropping water on a smooth and shiny sodium surface would damage it and eventually could lead to one that is crusty and rough and covered with hydroxides.



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[*] posted on 19-2-2017 at 21:09


Quote: Originally posted by JJay  
I think it depends on how clean and smooth the surface of the sodium is. If it's crusty and rough and covered with hydroxides, a drop of water might make its way into a tiny cranny and vaporize. Of course, dropping water on a smooth and shiny sodium surface would damage it and eventually could lead to one that is crusty and rough and covered with hydroxides.


Good point. And in the context of cleaning up some sodium drums (which, I presume is the thing that has prompted this thought experiment), the presence of hydroxides and their effect is worth considering.


Worthwhile inclusion in this discussion is the fact that a quantity of sodium reacts with approximately 1.3 times its volume of water to produce NaOH.
(Of course NaOH has its own subsequent interaction with water as well.)
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Dan Vizine
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[*] posted on 20-2-2017 at 06:30


m1tanker78,
I agree with the casting comment. I'll probably make a big ball of sodium by hand, do an initial flattening between steel plates in a press. Once I have something reasonably flat I'll transition to a rolling pin. As the surface area to volume ratio increases, self-heating in the atmosphere may be significant

Interesting Na fire fact - an equal weight of hydrocarbon fuel creates 5 or more times the heat energy of burning sodium.

j_sum1,
I will film the experiment. I plan to include photo's in my report.

I expect, truthfully, that the sodium in the drums will consist of conc. NaOH solution floating on top of any possible sodium that remains stuck to the drum bottom.

About a year ago I had used some NaK. The unused material, under a head of xylenes, was stored outside in an upright capped culture tube. Over the summer the xylenes evaporated past the cap and moisture diffused in. Right now, it's clean NaK in the bottom of the tube, with aq. NaOH/KOH floating on top (despite it being heavier). The amount of NaK has remained largely unchanged for many months. So, the question of how much sodium remains is wide open.





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tsathoggua1
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[*] posted on 20-2-2017 at 07:00


Given the reactivity of NaK alloy, that is really surprising that it hasn't reacted.

Unless there is a film of xylenes on top and this is preventing contact of the H2O with the NaK. If there is a bulk quantity of H2O though, floating on top of the NaK, somewhat surprising it hasn't actually exploded, NaK is considerably more reactive than either sodium or potassium on its own, Never worked with it personally but the videos I've seen where NaK is poured into water, the stuff went off with a very impressive kablooey:) Although it wouldn't be surprising to Tsath' if the increased reactivity of NaK is in a large part due to the fact that it a liquid, and as such on NaK being added to H2O (rather than vice versa) it will spread out and expose the whole surface area at once, allowing for a much faster reaction than in the case of a solid.

I'd be curious to see how different the reactivity of NaK coolded to its solidification temperature, reacted, since it wouldn't instantly spread flat due to its fluidity and would, when solid have to first melt via the exothermic reaction of decomposition. Using very cold H2O would be interesting, to record the height of water, and water displacement from two equally sized and shaped portions and containers of water, one with solidified NaK and the other with an equal weight of solidified NaK alloy. Both in the shape of a pellet, short rod, ball etc and another piece hammered flat, but still frozen solid, to allow it to react with a high surface area. As well as if possible recording the time-curve of the temperature of the H2O. Also comparing with potassium and sodium, just molten, solid Na and solid K alone in the same shapes. Especially interesting would be an equal quantity of sodium and of potassium, same shapes, added SEPARATE from each other but at the same time to the H2O.

All in the name of finding out whether it is the physical traits of NaK that make the reaction so violent compared to K or Na, spreading flat and going off with a lound and messy bang:)

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Dan Vizine
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[*] posted on 20-2-2017 at 08:55


tsathoggua1,

That reactivity is why I haven't cleaned this up yet. Amazing amount of crap in there from a dull silver blob under clear hydrocarbons. I just know that trying to pipette or pour will give an explosion.

What I imagine I'll do is wrap the tube in a shop rag, remove the top, then spin rapidly in a circle as I tilt the tube to horizontal. It's the same way I used to coax molten Na or K through metal wool filtering media to make fancy ampoules. When something bad happens, centripetal acceleration assures that all the badness gets flung outward. And, if it shatters, it will be contained.

The bottom 3/4 inch is quite silvery still.

nak.jpg - 670kB

[Edited on 2/20/2017 by Dan Vizine]





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tsathoggua1
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[*] posted on 20-2-2017 at 13:47


Could you not fill the tube with Ar or N2, then use a piece of rag to soak up as much of the H2O? Then using a second, dry rag to soak up the remainder as much as possible, then remove the remainder in vacuo?
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[*] posted on 21-2-2017 at 07:44


I recall reading/seeing somewhere that sodium into water will sometimes catch fire and explode, while water into sodium will always catch fire and might explode. I can't find video evidence though; it's buried by all 'throwing sodium into a pond' videos. I look forward to seeing your results!
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Dan Vizine
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[*] posted on 23-2-2017 at 07:04


Quote: Originally posted by tsathoggua1  
Could you not fill the tube with Ar or N2, then use a piece of rag to soak up as much of the H2O? Then using a second, dry rag to soak up the remainder as much as possible, then remove the remainder in vacuo?


If I had more patience, yes. This doesn't warrant 10 minutes of effort, really. If the tube gets destroyed, no big deal.





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Dan Vizine
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[*] posted on 23-2-2017 at 07:08


Quote: Originally posted by MrHomeScientist  
I recall reading/seeing somewhere that sodium into water will sometimes catch fire and explode, while water into sodium will always catch fire and might explode. I can't find video evidence though; it's buried by all 'throwing sodium into a pond' videos. I look forward to seeing your results!


Thanks, I'm inclined to think that is about right. Now that I know I'm not faced with a 23" inch wide sodium disk in the drum's bottoms (reactor thread). This question has taken on less priority. I'll still try to follow up, but this takes a back seat to what I'm obligated to accomplish by the 2nd week of March.





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