Submitted by Ok_Kareem_7223 t3_10qviic in askscience
AUniquePerspective t1_j6vcqpa wrote
Reply to comment by wanted_to_upvote in What are the effects of adding rock salt to a cooler full of ice? by Ok_Kareem_7223
It's the transfer of heat that's important.
The goal is to transfer heat away from the fish.
In the cooler, the fish is part of "the environment" that the melting ice will transfer heat from.
The materials that the cooler is made of aren't good for transferring heat and there's not much else in the cooler except for a bit of air. So most of the heat must come from the fish. If the cooler were a hypothetically perfect insulator and a vacuum, then all the heat would come from the fish.
If the cooler were a hypothetically perfect insulator and a vacuum, then the ice would not have anything to get heat from and it could stay the same coldness forever.
It's really intuitive for us to think of mixing temperatures: half a glass of cold water plus half a glass of hot water equals half a glass of warm water. But that's not the concept we're dealing with so try to put it out of your mind.
Don't think of the ice like a bit of a mooch. If it were a mooch, it would take a little bit of of the fish's heat in a similar to the way the two glasses share their heat and both come out warm... or like a friend who goes out for pizza with you but doesn't pay their share but still eats a normal amount of pizza.
Instead, you need to remember that phase change from solid to liquid is really very expensive in terms of the amount of heat it takes. So think of the ice as water that is heavily indebted to it's environment. On its own, water will pay off its big loan slowly as it gradually gathers heat.
And then think of salt as the loan shark who shows up at water's door with a baseball bat and says "Look at you walking around dressed as a solid! You're paying off all that debt now!" Salt forces ice to pay back the solid debt suddenly. So water looks around and there's not much heat in the cooler except for fish.
And fish says, "How much do you owe anyway? I'll give you what I've got since there's no other option." But the amount is way more than the fish expected. But the fish has good credit and pays off the loan in full anyway. But this puts the fish in debt. It now has even less heat than the water. The fish will be colder than freezing. It will be in deep heat debt.
The fish will cool more suddenly and will freeze more deeply with this method (and as a result of the fish cooling more deeply, it will stay cold longer.
It's all counterintuitive unless you get your head around the idea of phase change forcing outsized debt on the fish.
EmeraldHawk t1_j6vodm7 wrote
This is a great explanation that ignores the OP's question.
I will bet anyone that does this experiment 5 bucks that adding the salt actually makes the coldness last less time, because liquid water conducts heat into the walls of the cooler faster than solid ice does. The energy absorbed by the phase change and the fish are distractions from the actual question, which is what lasts longer. And the only thing that matters for this calculation is how fast the heat gets through the walls of the cooler. And the still air around the unmelted chunks of ice is a better insulator than the liquid water.
I completely agree that the salt is better, since cooling the fish down quickly will make the fish stay fresher longer. Again though, this is not what OP asked.
AUniquePerspective t1_j6vreni wrote
The point of my explanation is to point out that freezing the fish quickly and solidly is the motivating reason behind using salt. OP's question is ignored here because it's not relevant and because OP came right out and said they didn't understand.
But I'll repeat the parts you seem to have missed too: The goal is to rapidly transfer a great deal of heat away from the fish. Not just quickly but also to a lower temperature.
In this system, you want to declare the marginal difference of having an internal layer of air immediately adjacent to an insulating material as a defect... but it's simultaneously an advantage with respect to the fish which is not insulated. There's a minor trade off here at best.
The heat debt from the phase change if done using sufficient quantities of ice and salt is overwhelmingly sufficient to fully freeze the fish and keep it frozen for the period of transportation. The marginal loss of heat through the insulated walls of the cooler is small enough to be considered irrelevant.
EmeraldHawk t1_j6vt2dy wrote
Yes, my last paragraph said that adding salt is clearly better. Reread the OP's question, they aren't asking what's better in practice. They want to know what lasts longer.
If we assume a perfectly insulating cooler, both methods last forever and it's a tie.
AUniquePerspective t1_j6vuwxt wrote
They want to know if they have reason to believe fishers. They're missing the point. This isn't a question of total heat within the cooler/ice/water/fish system. The goal is to prevent rotten fish. The fish is the only part of the system the fisher cares about.
wanted_to_upvote t1_j6vu09d wrote
How can you assume a perfectly insulated cooler? No one ever said anything about that. The answer should be about things that really exist.
kuchenrolle t1_j6wgu5r wrote
>No one ever said anything about that.
That's not quite correct. AUniquePerspective, who EmeraldHawk is responding to, introduced that above.
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