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Name: Julia
Status: student
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Question:
My dad and I were at a bonfire last winter and someone threw a chunk of snow on the fire. Everyone was surprised at how slowly the chunk melted given the size and heat of the bonfire. It came up in conversation today and in looking in your archives we learned that water takes a lot of heat to raise its temperature, and the melting snow probably cooled the fire immediately around the snow area; but the fire was very hot and there is still some debate about the cause. I thought I also read somewhere that any material can only absorb so much heat at any given point in time and any additional heat applied is wasted. This would explain the rate of melting. Is this true and if not what would explain why the chunk of snow melted so slowly?



Replies:
Without some of the specifics, I cannot provide any of the math involved, and I am afraid my own contributions in this case become little more than educated speculation.

First, heat could be transferred into the snow block in two ways. Radiant heat, (basically heat that shines on something, like sunlight), would be less effective against a snow block, as it would tend to reflect much of that heat away. Convection though should still be effective in transferring heat. However, That snow block would be much cooler than the surrounding fire, and could potentially be disrupting the rising hot air with its own cold down draft, thus shielding itself somewhat as it melts.

As for the amount of heat involved, First the fire would have to heat the block of snow up to its melting temperature, followed by a significant amount of energy to transform it from solid to liquid, followed by further heating the water formed to boiling, and another significant hurdle to change it from liquid to vapor. As the fire was trying to boil away the snow, I imagine the water was also dripping into the fire below, not only cooling it, but slowing or extinguishing the fire underneath it.

So my theory on the slowly melting ice is threefold.
1) the fire was cooled,
2) the snow was a poor absorber of the heat, and
3) it still takes a sizable amount of energy to vaporize the ice crystals.

Ryan Belscamper


In the case of the fire and ice, another problem is that the air around the ice is not a very good heat conductor, and has very low heat capacity. To understand heat capacity and conductivity, think of a bucket brigade where the buckets are full of heat. Low heat capacity is like having buckets that are only partly full. Low thermal conductivity is like having the line move slowly or have too few bucket-holders. Because of its low heat capacity and conductivity, air cannot deliver very much heat to the ice even though it is very hot.

Also, not only does water have high heat capacity, but it absorbs much more energy turning from solid to liquid, and again liquid to gas. Think of how cold you feel when you get out of the shower -- that is because the water takes a lot of heat in evaporating off your body. In the case of the ice, this keeps the ice colder for longer (than you might think).

The statement you made about "absorb[ing] so much heat at any given point in time and any additional heat applied is wasted" is basically not correct. While objects cannot absorb an infinite amount of heat, but there is no 'rate maximum' the way you state it. The driving force for heat transfer is temperature. A hotter object will transfer heat to a colder one. The *rate* at which it transfers heat depend on several factors, including temperature, the thermal conductivity of the objects, if they mix (a fluid can mix but a solid cannot), and other factors. I can go into more detail if you like, just reply to this message and let me know what details interest you.

Hope this helps,

Burr Zimmerman



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