Curious tea-kettle behavior

[Cap'n Refsmmat]

I recently received a question by e-mail from my mom:

 

Dear Captain of the Scientific Domain;

 

I have a question that has been puzzling me. I have a stainless steel tea kettle which has a heavy stainless steel bottom. However, one day recently I discovered that I had forgotten it on the burner and the water inside had dried out. When I took it off the burner which was on low, the stainless steel had a yellowish color to it. Ever since then, it seems to superheat water. When I try to pour the water into a tea cup, it makes huffing, puffing and hissing noises (for lack of better descriptive terms), then it won't pour properly. It squirts some out, then it can't pour any, then some more comes out hissing and burbling. At that point, I try to put room temp water into the kettle, and when I do that, the hot water inside the kettle flashes.

 

Why?

 

Curious Tea Sipper

 

The only references online to stainless steel turning yellow mention passivation, which I don't know much about but which doesn't sound like something you can do by leaving the kettle on too long.

 

Any ideas?

[Moontanman]

Is it possible the spout has become partially blocked by minerals that were left behind when it boiled dry? Or could the stainless steel layers separated from each other and left spaces for steam to form and get trapped?

[Iota]

The yellowish colour is just a layer of carbon material that built up when the kettle's metal surface started heating up, due to the water all evaporating away.

 

As for the noises and poor functioning of the kettle... when the water all evaporated out, the kettle itself began to heat up, for longer than it could handle in terms of stress. The expansion of the metal left structural damage, because it was unable to 'shrink' back down to its previous shape.

 

 

At that point, I try to put room temp water into the kettle, and when I do that, the hot water inside the kettle flashes.

 

That's very weird... what exactly does the sender mean when they say, the water inside the kettle 'flashes'? Maybe the metal inside is contaminating the water now.

Edited July 28, 2012 by Iota

[Phi for All]

When I try to pour the water into a tea cup, it makes huffing, puffing and hissing noises (for lack of better descriptive terms), then it won't pour properly. It squirts some out, then it can't pour any, then some more comes out hissing and burbling.

My kettle does this quite often, and I tend to think of it as a kind of vapor lock. Right after boiling, the steam can block the spout causing just a trickle of water to get through, and when you tilt it to get more to come out, the pressure breaks through the steam barrier and boiling water comes gushing out if you aren't careful.

 

I've found that blowing sharply into the spout releases the barrier so the hot water can pour normally, but the steam can easily build up again. I've always thought this was probably just a design flaw, even though this is a fairly popular kettle by a well-known manufacturer. Or it could be the altitude here in Denver. I wonder if Curious Tea Sipper didn't change the shape of his/her kettle just enough to cause these problems. I don't think it has anything to do with superheating the water.

[Cap'n Refsmmat]

Interesting. I'm not sure how steam would block the spout, though. Perhaps there's some complicated effect from deposited minerals. It'd be nice if I were home and could inspect the thing.

[Phi for All]

Interesting. I'm not sure how steam would block the spout, though.

I think it's probably a case of atmospheric pressure and a free surface area for air to flow into the kettle to replace the water going out, similar to why liquid pours in gulps. The water vapor in the kettle acts to narrow the opening even more, possibly causing a greater pressure differential.

 

Perhaps there's some complicated effect from deposited minerals.

My kettle isn't very old and has no visible deposits. And that doesn't explain why the water comes out fine, then stops pouring, then starts coming out again "hissing and burbling".

 

It'd be nice if I were home and could inspect the thing.

Your mother calls you "Captain of the Scientific Domain"?

[imatfaal]

Interesting. I'm not sure how steam would block the spout, though. Perhaps there's some complicated effect from deposited minerals. It'd be nice if I were home and could inspect the thing.

 

 

What's the hardness of the water like back in the realm of the mother of the captain of the scientific domain? I am thinking some form of limescale build up - that was incremental and unnoticed, but when boiled dry and long has changed physically. Rather than a smooth surface, the kettle now has a slightly porous inside base, as the water is poured the very hot base keeps heating these now isolated pockets which boil instantly and thus puff, pop and hiss. Why it should not pour properly I have no idea; Phi's and Iota's (what's with the greek letters - at least Capn's mom tried!) ideas seem reasonable but I have never experienced them.

 

Unless you want to wait to inspect it yourself before possibly removing evidence I would recommend a good dose of kettle descaler - just make sure to rinse very thoroughly afterwards as it can leave a taste in tea even if you follow instructions.

 

 

 

Your mother calls you "Captain of the Scientific Domain"?

 

I always assumed our benevolent admin had chosen his own handle - turns out it is a diminutive of his given name.

[Cap'n Refsmmat]

We have a water softener at home, and she might actually use distilled water for her tea; I can't remember. The water softener would add some sodium to the water, which would be left behind when the water boils away.

 

Your mother calls you "Captain of the Scientific Domain"?

I quoted the email verbatim. The subject line was "Dear Mr. Science Forum Moderator", which meant posting it on SFN was inevitable.

 

Now I have to think up a title for you lot when I send my reply.

 

I think it's probably a case of atmospheric pressure and a free surface area for air to flow into the kettle to replace the water going out, similar to why liquid pours in gulps. The water vapor in the kettle acts to narrow the opening even more, possibly causing a greater pressure differential.

Ah, yes, this makes sense. The steam in the kettle is hot and low-density; once you start pouring and it begins to cool, its pressure will drop, making it difficult for water to escape. You'd have to remove the lid from the kettle to allow the pressure to equalize.

[Phi for All]

I quoted the email verbatim. The subject line was "Dear Mr. Science Forum Moderator", which meant posting it on SFN was inevitable.

Are you sure it wasn't "Dear Mr. Big-Shot Science Forum Administrator Who Should Call His Mother More Often"?

 

Now I have to think up a title for you lot when I send my reply.

How about, "My Personal Army of Research Monkeys"?

 

You'd have to remove the lid from the kettle to allow the pressure to equalize.

Which doesn't work well since the lid is right under your fingers gripping the kettle's handle as you pour.

 

Does the kettle whistle? If so, it has a stopper for the spout that pops up when you pour. Curious Tea Sipper can open stopper and blow into the spout to clear the vapor block. It's not ideal but it saves cleaning up the hissing and burbling boiling water that shoots out.

[Cap'n Refsmmat]

There's no whistle; it's a Turkish-style kettle:

 

 

So you could easily pop the lid off the top and pour. Perhaps she needs to try that.

[michel123456]

Use the scientific method:

_get to your mom a brand new kettle, let her heat the same quantity of water and observe the difference when pouring (this can be disturbing since we are talking about your mom, but you have to eliminate the case of deficiency in 1. human interpretation and 2. human hand stability)

_something else that comes to mind is that some kettles are not made of a solid-block piece of metal but consist of a recipient of thin metal on which a solid thick plate has been collated as bottom. It is the case for most kettles designed for electric heaters (in contrast with kettles used for gas heaters). If this is the case, as I understand from the picture, then maybe the thick bottom has slightly decollate from the recipient. You can check that, when it happens the bottom is not exactly flat anymore and the kettle doesn't stand correctly right up on the electric heater. If you use such a deformed kettle, the heat will not spread evenly into the fluid, the water will boil in a peculiar way and after a longer time.

This has no influence on the pouring though.

Edited July 29, 2012 by michel123456

[Cap'n Refsmmat]

Update: the strange pouring behavior (puffing and hissing) appears to occur even with the lid off. Pouring in room-temperature water causes it to suddenly boil, rather than calming it down as you'd expect.

[Phi for All]

There's no whistle; it's a Turkish-style kettle:

Part of the instructions for the kettle you show says to let the boiling water "settle":

 

Pour 3 cups of water into the larger kettle. Put the Turkish tea leaves and 2 tbsp of water into the teapot and place it on the kettle. Bring the water in the kettle to boil over medium heat. Then turn the heat off. Wait for the water to settle*, then pour half of the boiling water from the kettle over the leaves into the brewer. Let it brew for about 5 minutes**. Then pour the brewed tea into tea glasses using a small tea strainer. Fill in half of the tea glasses with the brewed tea and the rest with the hot water.

Serve Turkish tea with sugar cubes. I like to have my Turkish tea without sugar with just a few drops of lemon juice.

* If you pour boiled water immediately over tea leaves, the tea will lose its vitamins.

** If you extend brewing time, the taste will get bitter. Also freshly brewed Turkish tea should be consumed within half an hour of brewing time.

This recipe produces 4 servings in Turkish tea glasses.

Any kettle is going to have problems if you try to pour while the water is actively bubbling and boiling, trying to force the energetic liquid through the tight spout. This kettle/teapot double-boiler set up has a longer, narrower spout than most kettles I've seen.

 

Pouring in room temp water may force boiling water up the spout, making it seem like it's all suddenly boiling. As far as the change in behavior since the "boil out" episode, are we absolutely sure it's not just your mom paying more attention to normal activity or looking for problems she might expect to find after accidentally mistreating the unit? I remember when a girlfriend of mine boiled out an aluminum tea kettle. When we realized we could smell the metal, she ran into the kitchen and took the kettle off the electric burner. The bottom stayed there, bright orange and practically molten.

 

My point is, you have to stop looking for explanations outside of mainstream scientific knowledge. I'm sure there's a perfectly good reason for this phenomena, and you don't have to reach for magical "superheating" explanations that invoke supernatural Turkish herbal water deities. This is a science forum, you know.