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Appaulingly t1_j6u8o9s wrote

Ice in equilibrium with (pure) water will stay at 0 degrees C. No higher and no lower. If you add salt to the water, the equilibrium temperature will decrease. So a brine ice mixture can be lowered below 0 degrees C. This lower temperature system would "stay cool longer" because it is colder.

>It's as if they're saying that by adding salt, they've removed even more energy (heat) from the mass

Melting is an endothermic process. This process will "remove" heat via bond breaking in the ice. So by adding salt to the water and lowering the equilibrium temperature, the system will respond by melting some of the ice. This consumes energy and lowers the temperature until equilibrium temperature is reached.

EDIT: To clarify a misconception, an observed decrease in temperature does not equate to the "removal of energy from the system" (when simply adding salt). A decrease in temperature can occur when there is a transfer of kinetic energy to potential (when ice melts endothermically). Regardless, in the water-ice system the temperature is not actually proportional to kinetic energy. That is only the case in an ideal gas.

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Pornthrowaway78 t1_j6wq1qu wrote

The water in equilibrium with ice will be 0 degrees. The ice could be -20.

The brine mixture can indeed be colder, and the water is the bit that's in contact with the fish, so that's important.

It may be that the very cold ice on top of the water loses more heat to the environment more quickly than an even temperatured slurry. There might be a higher temperature gradient between that ice and outside than between the brine slurry and outside, which may mean slower warming overall for the slurry.

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wanted_to_upvote t1_j6ulgn1 wrote

This lower temperature system would "stay cool longer" because it is colder.

No, it will not. It will absorb environmental heat faster due the higher temperature differential to ambient. The larger temperature difference between the ice-water mixture and the ambient temperature will cause the mixture to absorb heat faster, thus speeding up the warming process.

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AUniquePerspective t1_j6vcqpa wrote

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.

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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.

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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.

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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.

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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.

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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.

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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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FVjake t1_j6upnjf wrote

Wait, so a glass of ice water will warm faster than a glass of water at 50 degrees? That doesn’t seem right. Are we using different metrics? Like, the glass of ice water will absorb much more heat to get to room temperature but certainly it will stay below room temp longer?

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IAMAHEPTH t1_j6uw0an wrote

I think he means if the room is 80F, then a glass of 15C water warming to 20C will take more time than a glass of 5C warming to 10C (same delta T of 5C, but at different heat differentials to the environment)

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Headsanta t1_j6w1phx wrote

The "rate of change" is proportional to the difference in the temperature between the glasses of water and their environment (Newton's Law of Cooling).

This means that the colder glass is changing temperature faster (it's rate of change is larger because its difference is larger).

Think of it like two balls rolling down a hill, where one ball is starting further back but is also steeper.

The ball on the steeper hill will be going faster... but that is no guarantee it will get to the end of the hill faster, because it started further back.

Depending on the exact setup, you could either have the ball on the steeper hill gain enough extra speed to make up for the extra distance it has to travel and win the race. Or it could also lose the race because it started too fast back for the extra speed to be enough.

tl;dr the ice water will be "cooling" faster (the temperature change will be more rapid"). But will it "finish" cooling to room temperature faster? Maybe... need more math.

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RockinRobin-69 t1_j6v2sjw wrote

In this case the colder ice water and the 32/0 degree ice water have the same amount of heat present.

They started in a cooler and the addition of salt made the change in temp happen relatively quickly. It’s colder as the melting ice takes heat energy from the water, cooling the water.

The cooler will have cold on one side and ambient on the other. The colder it is inside the more heat transfer through the walls.

I’m guessing that this is their perception only. It would be an interesting test.

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wanted_to_upvote t1_j6vubki wrote

That is not what is happening at all. They both have same amount of ice. One has salt that improves the transfer of heat from the environment to the ice.

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RockinRobin-69 t1_j6wl776 wrote

Salt lowers the freezing point of water. This causes some of the ice to melt. The melting requires heat energy and this makes the solution drop in temperature.

Take two bowls and put an ice cube in each. Add salt to one. Watch.

Edit cure to cube

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wanted_to_upvote t1_j6ximvl wrote

True, and now the entire surface area of the water is cooling the environment faster at a lower temperature.

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sunsetclimb3r t1_j6v6a4w wrote

So your theory here is, the colder something is, the less fine it stays cold?

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haysoos2 t1_j6vegpc wrote

Salt doesn't actually make the water colder. It just lowers the temperature at which a phase change happens.

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jwm3 t1_j6w7ood wrote

This does actually make the water actually colder though, it decreases in temperature to that new phase change temperature melting some of the ice in the process.

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haysoos2 t1_j6wv8hd wrote

So you're saying salt has the magical ability to violate conservation of energy?

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jwm3 t1_j6wx714 wrote

No, not at all. See "enthalpy of fusion". Ice melting is endothermic, it has to take in heat from the environment to happen. By forcing it to melt earlier it pulls in heat energy and makes it's surroundings cooler than they were before. However this isn't violating the law.of conservation of energy because the act of freezing the water is exothermic, it gave off heat energy when freezing. It would violate the laws of conservation of energy if it didn't make it colder because then it wouldn't be balanced with the exothermic freezing.

Instant cold packs work on the same principle. Also, this is really easy to verify in your kitchen with some salt, ice, and a thermometer.

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Appaulingly t1_j6y813l wrote

Energy is conserved. The thermal, kinetic energy of the ice and water is transferred to the potential energy in the bonds of the ice (to break them).

The temperature of a system is only ever directly related to the total energy of a system for an ideal gas.

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haysoos2 t1_j6yc1fe wrote

But if the ice and the water are all at 0⁰ C, then they're going to stay at 0⁰ C no matter how much salt you pour in.

The only way the water goes below 0⁰ C is if the ice starts below zero.

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Appaulingly t1_j6ycp2e wrote

This is not true. You can do the kitchen experiment yourself.

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haysoos2 t1_j6ydj06 wrote

Which part isn't true?

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Appaulingly t1_j6ygnxs wrote

When you add salt to ice+water the temperature decreases. Your misconception is that somehow energy is being removed from the system.

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haysoos2 t1_j6ypc3p wrote

I am extraordinarily skeptical that the temperature will actually decrease if all of the components are already at 0 C.

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Appaulingly t1_j6yujev wrote

If you had water that was for example at 105C, you're stating it would stay at 105C as it boils? And that the boiling would not lead to a decrease in the temperature?

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lupadim t1_j6wpqkx wrote

Lmaooo this reminds me of that girl in Hell's Kitchen that started a boil with cold water because "I thought cold water was supposed to boil faster" (since more heat transfer).

It doesn't matter that the colder-than-0 mixture absorbs more heat. It will eventually heat until 0c and at that point it'll be at the same starting point as pure 0c water... Except for it also had the extra time before 0c, thus it took longer to heat up. Same reason why starting a boil with hot water will obviously be faster than cold water...

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