[Truesup]

Hi there,

I was just brainstorming. Do you guys think it is possible to create some sort of tablet/powder/substance that heats up water as you pour it in, and leaves the water still drinkable? A quick search got me here: http://www.freepatentsonline.com/y2010/0074849.html, but I am not aware if water will be drinkable with that compound...

Regards
Lucas

[Archer]

Anything which is a liquid is "drinkable", it might be toxic or corrosive or even cryrogenic (someone drank liquid nitrogen and survived after major surgery, it's in the NJM)

The question is whether the water would still be considered potable under local regulations.

The answer to this is no, it would not be able to be used as potable water. Please see the WHO guide for more information:

http://www.who.int/water_sanitation_health/dwq/en/

[Emphyrio]

As an alternative,  would it be possible to develop some sort of sealed capsule of metal or another material,  that was entirely self contained, had  heat producing chemicals inside, and was cool until activated.  Activation could be achieved by turning a ring, pressing a switch or some other mechanism,  which would cause the chemicals inside the capsule to react and produce heat.  The capsule could then be dropped into water, or another liquid to heat it. It would still be entirely self contained, releasing no exhaust or residue into the water, and would produce heat until the reaction had exhausted itself.  A single capsule would be rated to heat a specified volume of water to a specified temperature. The water would remain untouched.  The capsule could be removed from the water, and the the water used for making tea, coffee, or for any purpose desired.  If the capsule were compromised,  and leaked, it would release a red or yellow dye into the water, indicating the water should be discarded.  Is this device possible?

[Borek]

Sounds doable in principle, but I would start calculating energy density required and looking for reactions that are exothermic enough to work and not make the capsule too large.

Besides, the capsule needs a relatively large surface - otherwise it won't be able to deliver the heat fast enough.

[Enthalpy]

If the reaction creates only H₂O, CO₂, N₂... the water might be considered drinkable. But most other compounds like NO, NO₂... are precluded even in small amount.

For instance ammonium nitrate fails because of minor unwanted products (... and more reasons), but it produces heat by forming H₂O. Any better one, until we have octa-aza-cubane?

Maybe the reaction can take place in a bag removed from water after use like a tea bag? Iron powder might be a source of inspiration. As a hand warmer, is uses table salt to promote corrosion by water and produce heat. Though, salt isn't possible here, and rusting must produce unwanted soluble compounds. Have a metal whose oxides and hydroxides are not soluble at all?

More seducing: an insoluble mineral whose hydration produces heat. Something like dessicated alumina, if it can have a grain size to make hydration fast enough but still remain captive in the bag. Some very porous material, maybe a form of silica?

I'm near the end of my chemical ignorance, so let's hope collective creativity will work.

[Corribus]

The problem isn't just generating the amount of heat to make a difference - though this is a problem, given the enormous amount of heat needed to raise the temperature of water - but also the rate at which the heat is generated. Generate it too fast, and you'd flash boil the water around the sachet (or whatever), which is wasteful. Generate it too slowly, and you'll never heat up the water due to evaporative cooling. You'd need a highly exothermic reaction that has a relatively slow rate, and it must also either not produce toxic byproducts or not produce gas (if the sachet is enclosed). It also has to involve a reasonably small relative amount of material, because it wouldn't make sense to have 100 g of reactants in your sachet just to heat up 100 mL of water, say. Probably you'd want the weight of the reactants to be no more than 5% or so of the weight of the water you're trying to heat up. Oh, and it has to be economically viable.

That's all a pretty tough order to fill. I don't see it as practical, but maybe there's something out there that fits the bill.

[curiouscat]

The problem isn't just generating the amount of heat to make a difference - though this is a problem, given the enormous amount of heat needed to raise the temperature of water - but also the rate at which the heat is generated. Generate it too fast, and you'd flash boil the water around the sachet (or whatever), which is wasteful.

So, say, the exothermic reaction is essentially contained at the center of a satchet of water. \

Now I didn't understand the problem you described of a too-fast reaction. For arguments sake, let's assume we had such a reaction. Even if it flash boiled water next to it, what next? The water must condense thereby transmitting said heat outwards, right?

Unless we are worrying about mechanical integrity i.e. explosions.

[Borek]

Even if it flash boiled water next to it, what next? The water must condense thereby transmitting said heat outwards, right?

I think what Corribus means (and I agree with him) is that a substantial part of heat will be released as a hot steam to the atmosphere and lost, instead of heating up the water.

[billnotgatez]

http://en.wikipedia.org/wiki/Flameless_ration_heater

Maybe you want a FRH (Flameless ration heater) used in MREs (Meals, Ready-to-Eat )?

[Corribus]

What I meant is basically what Borek said. Of course, it depends on how hot you want to heat the water, but let's say your aim is to heat it up to the point where you could brew a cup of tea or coffee - basically, right below boiling. Well, the amount of heat required to raise the temperature of x amount of water to almost boiling from room temperature is certainly enough to heat a fraction of that water to boiling if it's delivered to a small volume of the water all at once. So what you need is the rate of heating of the heating element to be less than the rate of heat transfer from the volume of water directly touching the heating element to the volume of water further away. If it isn't, than you waste a lot of the heat because it goes toward boiling some of the water away - which you don't want.

It's like if you take a red hot iron and plunge it directly in cold water. Some of the water right next to the hot iron will vaporize immediately, because the rate of heat transfer from the iron to the water is faster than the water's ability to dissipate it. Unfortunately water is a fairly decent insulator, so the rate at which your heating element heats has to be reasonable slow, I would think. Obviously, this becomes a bigger issue the larger the volume of water is.

[curiouscat]

Even if it flash boiled water next to it, what next? The water must condense thereby transmitting said heat outwards, right?

I think what Corribus means (and I agree with him) is that a substantial part of heat will be released as a hot steam to the atmosphere and lost, instead of heating up the water.

That happens if the system is open, yes. Maybe it'll work better with a closed system?

[Corribus]

Probably, but that's not how I envisioned the application target. I thought this was intended to be something you or I could carry around and use to heat up soup on a camping trip. But I could be wrong.

[curiouscat]

Probably, but that's not how I envisioned the application target. I thought this was intended to be something you or I could carry around and use to heat up soup on a camping trip. But I could be wrong.

Makes sense.

[billnotgatez]

To quote from the WIKI that I posted earlier

A flameless ration heater, or FRH, is a water-activated exothermic chemical heater included with Meals, Ready-to-Eat (MREs), used to heat the food. US military specifications for the heater required that it be capable of raising the temperature of an 8 ounces (226.8 g) entree by 100 °F (56 °C) in twelve minutes, and that it has no visible flame.

The ration heater contains finely powdered magnesium metal, alloyed with a small amount of iron, and table salt. To activate the reaction, a small amount of water is added, and the boiling point of water is quickly reached as the reaction proceeds

[Enthalpy]

Let's evaluate the heat amount.

A hiker who wants a hot drink (1/3dm³ at +45°C) in a cold weather (+5°C) needs 60kJ plus the losses. Melting snow before (172kJ instead) would have been even better, sure.

A fat tea bag (if this is the method) of dense powder may contain 15g of active material, which must hence provide 4000J/g = 4MJ/kg = 1000cal/g of that one reactant.

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Trying a hydration : Boehmite from alumina http://www.minsocam.org/ammin/AM76/AM76_445.pdf
Al₂O₃ + H₂O -> 2*AlO(OH)
-1676kJ and -286kJ give twice -996kJ, releasing only 30kJ/102g or 300J/g, too litlle.

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But an oxidation is known to suffice.

This one looks not too harmful if it can be made practical:
Si + 2*H₂O -> SiO₂ + 2*H₂
Twice -286kJ give -911kJ, releasing 339kJ/28g or 12000J/g, good.
I didn't check possible hydroxides.

Obscure to me: if some alloying element makes silicon oxidable deply enough, in combination with fine grains - and whether air oxidizes or lights silicon under these conditions.

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The tea bag (in the option where the reaction occurs at water contact) can be stored in a hermetic second bag until use: metal foil, polypropylene...