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Melting Snow Block in Fire
Name: Julia
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
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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Update: June 2012
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