Does a slow boil hold its heat longer?

[Omnis]

Okay, if I have a pot of water and I heat it slowly to a boil, will it take more time for it to come back to room temperature than if I were to just shock it with intense heat to get an instant-boil (both resulting in the same temperature)?

I'm guessing that a slowly-heated pot will transfer the heat to the water better as it will have more time (and therefore the water will take longer to cool), but I was just pondering this situation and thought I'd post about it.

 

[Sureboss]

But then a quickly-heated pot would have heat>time rather than time>heat.

As for what would cool quicker, it'd be identical would it not?

 

[DQuaN]

If you heat the water quickly, a lot of the heat would dissipate into the pan.

If you heat it slowly the pan has time to warm up too.

(That's my theory anyway).

 

[TheCracker]

Like Dunc said, the rate of heat dissipation will be the same as long as the pans are the same temperature and the room's temperature is also the same.

The science bods might know different though.

 

[Diego440]

I know that the contrary is true: if you freeze water in a slow process (i.e. leaving it sitting next to dry ice for a whole night) it'll hold its cold longer than if you freeze it quickly (i.e. immerse it in liquid nitrogen). So I guess it also applies to heating.

 

[Danoff]

I know that the contrary is true: if you freeze water in a slow process (i.e. leaving it sitting next to dry ice for a whole night) it'll hold its cold longer than if you freeze it quickly (i.e. immerse it in liquid nitrogen). So I guess it also applies to heating.

That "so I guess it also applies to heating" bit, is way off. The type of molecular structure that water forms in different kinds of freezing has nothing to do with how that water holds heat.

 

[Diego440]

That "so I guess it also applies to heating" bit, is way off. The type of molecular structure that water forms in different kinds of freezing has nothing to do with how that water holds heat.

Right Mr. Smarty, so does a slow boil hold its heat longer or not?

 

[Danoff]

I doubt it. I think once the water is boiling, it has heated to the temperature evenly. I'm pretty sure I remember something about convection requiring pretty specific circumstances.

If a mass of water is at a particular temperature, it will take a set amount of time to cool down. It's only if the mass of water is unevenly heated that it would take a different amount of time to cool down.

 

[Boundary Layer]

If a mass of water is at a particular temperature, it will take a set amount of time to cool down. It's only if the mass of water is unevenly heated that it would take a different amount of time to cool down.

Greater surface area = greater cooling rate by convection.

If a mass of water is at a particular temperature, with constant surface area, it will take a set amount of time to cool down. It's only if the mass of water is unevenly heated, or the surface area is changed, that it would take a different amount of time to cool down.

 

[Danoff]

Greater surface area = greater cooling rate by convection.

If a mass of water is at a particular temperature, with constant surface area, it will take a set amount of time to cool down. It's only if the mass of water is unevenly heated, or the surface area is changed, that it would take a different amount of time to cool down.

We're talking about convection in both cases, so the surface area should be identical. Also, in both cases, convection will stop pretty darn quickly when the heat is removed.

 

[Philly]

Of course, if the shock of heat came from below, like from a stove, the bottom would heat up faster than the top. Thus the water on bottom would boil, and the heat energy stored there would escape with the water vapor.

If the pan is heated slowly, the top will have more of a chance to heat at the same rate as the bottom. Thus, heat would escape at the same rate per volume but from a larger volume? Maybe this plays no role because they balance out.

What it would come down to in this case is the rate at which the water would cool. A quickly heated pot would contain less energy but would lose it more slowly while a slowly heated pot would be able to trap more heat, but would lose it more quickly.

I don't know if I answered the question, did nothing or made it a million times worse. I know I learned something in writing this post.

 

[Danoff]

Of course, if the shock of heat came from below, like from a stove, the bottom would heat up faster than the top. Thus the water on bottom would boil, and the heat energy stored there would escape with the water vapor.

How exactly does that happen? As water begins to boil, it vaporizes at the bottom of the pot and the trapped air floats to the top and escapes. However, this is not what I would consider to be a "boiling pot of water". What I'd consider to be a boiling pot of water are full-on convection cycles or, what is often referred to as a "rolling boil". If you get water hot enough for convection to occur, it mixes evenly. So if it takes you 20 seconds or 20 minutes for the boil to start, it shouldn't matter.

 

[Boundary Layer]

We're talking about convection in both cases, so the surface area should be identical. Also, in both cases, convection will stop pretty darn quickly when the heat is removed.

I was referring to heat transfer from water surface to air by convection, and what might happen if the boiling water were put into a different size/shape container or if a comparison of the cooling rate were made between different aparatuses. I wasn't talking about convection from the pot/kettle to the water or convection cycles within the water, or anything else.

Sure, that may not have been part of the original question, but it is something that may be done with boiling water that would certainly affect the rate of heat loss.

 

[Philly]

How exactly does that happen? As water begins to boil, it vaporizes at the bottom of the pot and the trapped air floats to the top and escapes. However, this is not what I would consider to be a "boiling pot of water". What I'd consider to be a boiling pot of water are full-on convection cycles or, what is often referred to as a "rolling boil". If you get water hot enough for convection to occur, it mixes evenly. So if it takes you 20 seconds or 20 minutes for the boil to start, it shouldn't matter.

You have a point. Sounds good.