Hot water boils at a higher temperature

[llewtrah]

The "hot food in refrigerator" thread reminded me of something my mum claimed when I was primary school age (under 12 years). I should not put water from the hot tap into the kettle as it "boiled hotter".

A couple of years later I informed her that water from either tap was still water and still had the same boiling temperature (100 C, not splitting hairs over tiny differences due to impurities) as which point she denied having told me the "boils hotter" thing in the first place, which is not that unusual in itself (the record for mum denying having made a statement stands at less than 1 hour in spite of her own handwriting proving she did state something in the first place).

Has anyone else come across the hot water boils hotter line? I can understand it boiling faster. I can imagine someone thinking it needs to change by a fixed number of degrees rather than having a fixed boiling point. The thing is, in spite of her statement, she already knew water boils at 100 C so it seems she was reciting some bit of lore without thinking about it.

[Ravenhull]

I haven't heard that one, though it's probably in the same zone as water from the hot tap freezes faster UL.

[Tarquin Farquart]

The reason not to use water from the odd tap is that is isn't as free-flowing and pure. Why this would matter if you boiled it I have no idea.

[Richard W]

Originally Posted by Tarquin Farquart

The reason not to use water from the odd tap is that is isn't as free-flowing and pure. Why this would matter if you boiled it I have no idea.

In the past, I think there might have been more risk of contamination from lead pipes and tanks in the hot water supply too, and that wouldn't go away with boiling. Although I'm not sure how true it was.

Originally Posted by Ravenhull

I haven't heard that one, though it's probably in the same zone as water from the hot tap freezes faster UL.

Actually, bizarre as it sounds, it's true that hot water can freeze more quickly than cold water under some circumstances - it's called the Mpemba effect.

[Tarquin Farquart]

Originally Posted by Richard W

In the past, I think there might have been more risk of contamination from lead pipes and tanks in the hot water supply too, and that wouldn't go away with boiling. Although I'm not sure how true it was.

That must have been the origin of that.

[Nick Theodorakis]

The only way it could have a different boiling temperature is if the hot water heater added enough contaminants to raise the boiling point of water. I think that it's unlikely, or at least unlikely to be measurable with a household thermometer, but in any case if that does happen I think you need a new hot water heater.

Nick

[applepwnz]

I've always been told that you shouldn't boil the hot water from the tap because of an increased risk of contaminants/lead, but I still usually use that hot water if I'm cooking up some pasta or something for myself and I've noticed that the hot water will definitely come to a boil faster than cold water would.

Favorite Ramen Flavors: Beef, Chicken, Oriental, Mushroom, Lead???

[Richard W]

Originally Posted by Tarquin Farquart

That must have been the origin of that.

Actually, I should have mentioned that last year my own hot taps suddenly started sputtering and putting out bright red water (rather alarming, as I was running a bath at the time) which was coloured by little red shreds of material; on investigation I discovered that there was a piece of rubber tubing in the feeder tank in the loft, that must have been there as long as I'd had the house, and it had perished sufficiently that a chunk had apparently come off and gone into the boiler, where it was shredding itself in the pipes. I fished the remains of the tube out, but the boiler is sealed so I couldn't get in there, and it took a couple of weeks before all the little bits of rubber had cleared themselves from the tank.

That alone would have put me off using my hot taps for anything food-related.

[Silas Sparkhammer]

Originally Posted by Richard W

. . . Actually, bizarre as it sounds, it's true that hot water can freeze more quickly than cold water under some circumstances - it's called the Mpemba effect.

That is so cool! I'd never known that! Thank you!

(My personal favorite among the proposed explanations would be the dissolved gases argument.)

It is well known that hot water can contain more dissolved solids -- the lime that builds up around the rim of the tap is going to be much more extensive on the hot water tap than on the cold, for those of us whose sinks have separate taps entirely. However, most of these solids are not toxic. A little calcium carbonate isn't gonna hurt you; heck, it's actually probably good for you!

Silas

[jimmy101_again]

Originally Posted by Silas Sparkhammer

It is well known that hot water can contain more dissolved solids -- the lime that builds up around the rim of the tap is going to be much more extensive on the hot water tap than on the cold, for those of us whose sinks have separate taps entirely.

This assumes that the major source of the salts in the hot water is metal dissolved from inside the heater or the pipes between the heater and the tap.

I suspect that 99.9999% of all of the salts in tap water, hot or cold, were in the water to begin with, that is, before they were heated by the hot water heater. It seems to me that it is pretty unlikely that enough stuff could be leached or dissolved out of the pipes to affect the boiling point of water. It generally takes a lot of "stuff" to raise the boiling pint (or lower the freezing) point of water. If sufficient material was regularly being disolved from the pipes by the water passing through it, then the pipes would fail in pretty short order. The health affects of material leaching from the hot water pipes, particular lead pipes, would occur at much much lower levels of material than what is required to raise the boiling point by a measurable amount.

One thing that is different between water that has been heated and water that hasn't been heated is that the water that has been heated will contain less disolved gases if the water was heated in an open container. If you take a pot of water and boil it you remove all the dissolved gases before the water reaches a true boil. There are bubble rising in the water but the water isn't boiling. It would be easy to confuse the degassing process with boiling. Water that has been degases then cooled, then reheated may appear to take longer to boil, but actually the previously unboiled water has started to bubble, but not truely boil, to soon.

I don't believe a typical water heater will degas the supply water since the water healer is a pressurized system. If the water did degas in the heater then the bubbles would still have to be expelled form the heater via the outlet pipe.

[Latiam]

I used to use water from the hot tap whenever I boiled something, because it sped it up. But then my dad said, "I don't like to use water that has been sitting around in a water heater for who knows how long to cook my food," and I decided he was right and stopped. Sigh.

[Silas Sparkhammer]

Originally Posted by jimmy101_again

This assumes that the major source of the salts in the hot water is metal dissolved from inside the heater or the pipes between the heater and the tap.

I suspect that 99.9999% of all of the salts in tap water, hot or cold, were in the water to begin with . . .

Good point. But now that leaves me confused. Because my two-faucet sink does have really significant lime build-up around the hot-water tap. Is this material getting into the water inside the apartment, between the hot-water-heater (department of redundancy) and the tap? Or is the lime already dissolved in the water, and precipitated out by the process of heating-and-then-cooling?

Silas

Not sure where it comes from, but there definitely is a difference. The logic passed around fishkeeping circles is that the hot water sits in the system longer than cold, and that it leaches dissolved salts from the pipes and water heater's tank, which does tend to support my results.

I occasionally test my tap water for hardness (gH and kH) for my more sensitive fish, and there is a difference of about 25 ppm. Straight cold water is soft enough for my kribensis (gH 50 ppm consistently), straight hot water is hard enough for my African cichlids (gH 70-100 ppm, but highly variable), provided I prepare the water before I take a shower - running the hot water seems to narrow the difference, but I've never committed the testing supplies to really prove that.

[Der Induktionator]

The hot water from the sink in the bathyscaphe definitely boils at a higher temperature than the cold water from the sink in the high altitude observatory.

Proof positive.

[Hans Off]

Originally Posted by Silas Sparkhammer

Good point. But now that leaves me confused. Because my two-faucet sink does have really significant lime build-up around the hot-water tap. Is this material getting into the water inside the apartment, between the hot-water-heater (department of redundancy) and the tap? Or is the lime already dissolved in the water, and precipitated out by the process of heating-and-then-cooling?

Silas

We need to think about what is causing the lime to come out of solution.

I don't know but the process may have something to do with the rapid cooling (transfer of energy) that the water goes through as it leaves the mouth of the tap which somehow causes the calcium carbonate to "drop out" of the solution.

It rings a bell from my geology and mineralogy/mineralisation training, but you need someone who actually listened properly to all the complicated chemistry bits to confirm this!

Hans "Wishes he paid more attention in lectures" Off

[jimmy101_again]

Originally Posted by Silas Sparkhammer

Good point. But now that leaves me confused. Because my two-faucet sink does have really significant lime build-up around the hot-water tap. Is this material getting into the water inside the apartment, between the hot-water-heater (department of redundancy) and the tap? Or is the lime already dissolved in the water, and precipitated out by the process of heating-and-then-cooling?

Silas

That's a good point. I would think the simplest explanation is that the hot water tap leaks a bit, or does so on occasion.

There are other posibilities. One characteristic of water mineral deposits is that once formed they are very very slow to redissolve. So, perhaps the small amount of hot water left at the tap when the valve is closed is more likely to evaporate and precipitate salts in a given length of time than is the cold water tap. Once you get a bit of precipitate it's basically permanent and won't redisolve the next time the tap is turned on. Even though it should dissolve it doesn't. This is a classic example of the difference between thermodynamics and kinetics. Just because thermodynamics say something should happen (in this case, the deposits should redissolve) doesn't mean it actually will.

[Silas Sparkhammer]

Originally Posted by jimmy101_again

That's a good point. I would think the simplest explanation is that the hot water tap leaks a bit, or does so on occasion.

Grin! It certainly could be, but I believe that the effect is noticed widely enough to go beyond that kind of contingency.

Actually, I like Hans Off's suggestion, that it might be caused by the very rapid cooling of the water.

There are other posibilities. One characteristic of water mineral deposits is that once formed they are very very slow to redissolve. So, perhaps the small amount of hot water left at the tap when the valve is closed is more likely to evaporate and precipitate salts in a given length of time than is the cold water tap. Once you get a bit of precipitate it's basically permanent and won't redisolve the next time the tap is turned on. Even though it should dissolve it doesn't. This is a classic example of the difference between thermodynamics and kinetics. Just because thermodynamics say something should happen (in this case, the deposits should redissolve) doesn't mean it actually will.

Another notion -- and, again, I do not know and am purely guessing: could it be that a little bit of lime acts as a "seed" for the growth of more lime around it? Does it prefer to accumulate on itself?

I think I need to go and buy "Chemistry for Dummies..."

Silas

[jimmy101_again]

Originally Posted by Silas Sparkhammer

Another notion -- and, again, I do not know and am purely guessing: could it be that a little bit of lime acts as a "seed" for the growth of more lime around it? Does it prefer to accumulate on itself?

A miniscule amount of precipitate does indeed act as a "seed". But it only works when the solution is saturated with salt. It could be that as the small amount of water at the tip of the tap evaporates the presence of a seed makes it more likely to precipitate.

[Troberg]

The "hot food in refrigerator" thread reminded me of something my mum claimed when I was primary school age (under 12 years). I should not put water from the hot tap into the kettle as it "boiled hotter".

A couple of years later I informed her that water from either tap was still water and still had the same boiling temperature

Exactly, water is water, the only difference between the cold and hot tap water is the temperature at which it leaves the tap.

Even if it did boil hotter, that would be a good thing, as it would cook the food faster, much like a pressure cooker, so I don't see why it would have been a problem anyway.

[Der Induktionator]

Originally Posted by jimmy101_again

A miniscule amount of precipitate does indeed act as a "seed". But it only works when the solution is saturated with salt. It could be that as the small amount of water at the tip of the tap evaporates the presence of a seed makes it more likely to precipitate.

I think the 'seed' theory is certainly part of it. Consider also electrostatic effects: mineral (calcium, etc) ions in solution are electrically charged, this means they are susceptible to electric fields, enough to slightly alter where they tend to accumulate, in addition to other factors.

Mineral deposits often form at the point where water drips. Anywhere there is a 'cusp', or pointy shape, any electrostatic field is going to be stronger, or have a sharper gradient. (Example, lightning and other sparks 'prefer' pointy places, like the end of lightning rods or any scratch on the surface of the shiny ball on a vandegraff generator.) This could cause a 'push in the right direction' and make deposits form in a particular point.

In University, we were presented with a design for an electrostatic generator that used dripping water as a way to separate charges. As a drop forms, charges would tend to be separated within the drop, and once the drop falls they are permanently separated.With this design, potentially higher voltages could be generated with other designs, however because charge accumulates just one drop at a time, it was more of a thought experiment than something practical.

In Switzerland, there are big powerful magnets that can be clamped on water pipes to avoid calcium deposits in pipes. I can't begin to guess how they work, but of course any charged particle is affected by a magnetic field, but past that...