Submitted by Ok_Kareem_7223 t3_10qviic in askscience
EmeraldHawk t1_j6vodm7 wrote
Reply to comment by AUniquePerspective in What are the effects of adding rock salt to a cooler full of ice? by Ok_Kareem_7223
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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