What are the effects of adding rock salt to a cooler full of ice?

Submitted by Ok_Kareem_7223 t3_10qviic in askscience

Background: I know some fishermen who do this, because it melts some of the ice, and the resulting liquid in there is as cold as the ice, and it quickly freezes the fish placed in the cooler.

These same fishermen claim that the resulting slurry stays cold much LONGER than just a cooler of ice without the salt. They've done no experiments with timing it, they just make the claim.

I understand the salt melting the ice, and the resulting slurry being partially liquid and the liquid being as cold as the solid. What I don't understand, or even BELIEVE, without some explanation is that he mass would stay cool LONGER in one form or another. It's as if they're saying that by adding salt, they've removed even more energy (heat) from the mass.

Sounds wrong to me. Am I missing something?

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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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No_Masterpiece6568 t1_j6udjvv wrote

By adding salt to the water you are increasing the number of dissolved particles in the water (this is quantitated as the molality of the solution). This decreases the freezing point of water and therefore the temperature of the ice/water mixture because it will always equilibrate at the freezing point of water as long as there is both ice and water present in the mixture. This is known as "freezing point depression".

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SnowboardSyd t1_j6wg2zs wrote

This is the correct answer. It's why magnesium chloride is applied to roads in the winter, to prevent water from freezing by utilizing the same principal. Interesting to note that since mag chloride has an additional chloride atom, it can lower the freezing point by more degrees than rock salt.

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parrotwouldntvoom t1_j6uz3m3 wrote

There are a lot of goofy answers here. Adding ice will not make the system cooler, or the ice cooler, but it will make the water cooler because it will make the ice melt earlier. However, if your ice is at 0C already, adding salt will not make things cooler because just adding salt can't take energy out of the system. In reality, your ice is likely -10 to -20 C, and so adding salt helps.

I can't think of any reason that adding salt would make it stay cold longer. The duration of maintaining cold should be a function of starting temperature and the characteristics of the cooler.

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common_sensei t1_j6v33a6 wrote

The phase change is endothermic, so ice near zero degrees will cool the surrounding ice down as it melts into colder water. You're right that the total energy won't change just by adding salt, but you will reduce thermal energy in the system to gain that potential energy in the liquid.

Your second point is dead on though, if anything, it should warm up faster because there's more temperature differential now.

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Ihaveamodel3 t1_j6wkxp5 wrote

The rate of change will be faster, but will the total time to “warm” be faster? Since the colder one has a larger way to go to “warm”?

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common_sensei t1_j6x18c0 wrote

It's still the same total energy. You'll lose ice getting down to minus whatever degrees, so while you're colder to start, you also have less ice.

Ignoring all the extra stuff that can happen (e.g. condensation on the outside of the colder cooler dumping extra energy into it, or freezing and making an insulating layer), a sealed ice+salt cooler should hit 1 degree Celcius before a sealed cooler with ice alone would.

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parrotwouldntvoom t1_j6w1bu2 wrote

I’m not sure the endothermicity of Salt dissolution is enough to make a noticeable difference in this scenario outside of a lab, but I guess I could look it up.

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Appaulingly t1_j6y9olg wrote

No the melting is endothermic.

>just adding salt can't take energy out of the system

The temperature decreasing does not mean that the total energy of the system has changed. There is an energy transfer between kinetic energy and potential within the system.

Only really in an ideal gas system does the temperature relate to the total energy.

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parrotwouldntvoom t1_j6ygg72 wrote

Melting is endothermic in either the + salt case or the -salt case, so it should be a wash in the final consideration of temperature changes.

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common_sensei t1_j6yu1tm wrote

You said it yourself in your first reply - it makes the ice melt earlier. The relevant concept is Gibbs free energy, where endo/exothermic is only part of the equation.

The only reason ice melts at 0 degrees in pure water is that that's the point where the gain in entropy from turning into a liquid balances out the increase in potential energy from turning into a liquid.

When you add salt to the water, you change the entropy part, making it more entropic to melt, which decreases the equilibrium temperature at which ice turns into water. The ice will melt faster when surrounded by salt, absorbing energy (and quite a bit of it! 334 J/g) until it hits the new depressed equilibrium temperature. Then it'll maintain that temperature by melting slowly, just like ice in pure water.

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PD711 t1_j6vk20o wrote

when i was a kid we got an ice cream maker one year for the 4th. it was a drum filled with salt and ice, and then a second container was put inside the first with the ice and salt surrounding it. and that container you put the milk, cream, sugar etc. and then it mixed the contents until it was ice cream.

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Mitski t1_j6wqgp8 wrote

I remember this as a kid too - we didn’t have the ice cream maker but we used one big coffee can, a smaller can inside and the rock salt ice in the space between. You had to shake it until your arms felt like they would fall off.

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lupadim t1_j6wq40u wrote

1- Your pure ice is not at -10 to -20 C, this is impossible

2- Yes adding salt does cool it

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eclectic_radish t1_j6wusft wrote

How is it impossible for ice to be between -10 and -20? I have my freezer set to -18°C, and everything that has been in there long enough is also -18°C

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Random-Mutant t1_j6wdhe1 wrote

I have just come back from a kitchen experiment.

I added water to an insulated cup of ice, stirred it, waited a minute and took the temperature. 0.5°C, about 33 in those antique numbers.

I added a big whack of salt, stirred again vigorously to dissolve some and measured -2.6°C. I don’t care what that is in non-science units. If you like I can however convert it to Kelvin.

I’m sure I can go colder by using a proper salt solution (heat it to dissolve as much salt as I can) and crushed ice, not cubes, to increase the surface area. I’m guessing I can possibly get to -17° if I try hard enough. Just a hunch.

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BtheChemist t1_j6ub43c wrote

Answer:

Salt acts as an antifreeze, which lowers the freezing temperature of water below 32F / 0C.

This allows the water to get colder.

It works because the salt ions interfere with the hydrogen bonding in water, preventing it from crystallizing into Ice.

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Mackntish t1_j6vmbwq wrote

I feel like there's a lot of silly answers in here. A simple Google search reveals that yes, salt lowers the tempature of ice by melting it. Melting is an endothremic process, and it makes the mixture colder. Which basically means that it releases the cold of the ice into the environment faster.

But! Because the cold is released faster, it means it will warm faster. Because the difference in tempature between the cooler and outside is greater.

So they are half right. It does make it colder, but not for longer.

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wassimu t1_j6vqoz6 wrote

You cannot ”release the cold”. Cold is not a thing. There is only heat. Heat flows along temperature gradients. So you can add heat or lose heat and that’s it. Ice has heat, as does everything in the universe that is not at absolute zero.

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gvilleneuve t1_j6vpt0b wrote

Due to this happening inside a cooler, it will end up being colder longer as well

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Mackntish t1_j6wqz17 wrote

Uh, no, releasing the cold quickly or slowly does not change the total amount of coldness that there is to be released.

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gvilleneuve t1_j6z5k6w wrote

The lower you can get the temp of the cooler, the longer it will take to warm up. Simple as.

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SirReal_Realities t1_j6w5gzm wrote

Coolers keep things cool longer. I can see the salty ice water absorbing heat from the fish faster (thus slightly warming the salty ice water faster than non-salty ice water, but it doesn’t make sense that the contents of the cooler would take less time to reach room temperature. If you start off with a cooler at -15 degrees, it will take longer to reach room temperature than a cooler at 0 degrees; If not, then you don’t have a insulated chest, you have a bucket of ice.

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Knichols2176 t1_j6w6oj8 wrote

The salt makes the water colder than 32F/0C. Once it’s a slurry, it increases temp by heat transferring to it. Without open air pockets, there’s much less surface area to recieve heat transfer. Water becomes denser and won’t accept heat transfer as easily.

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TanteTara t1_j6wn1i1 wrote

Hot water can hold more salt ions than cold water, as you can observe when crystals fall out of a saturated solution while it cools down.

This implies that it takes energy to dissolve the salt crystals.

So, when you add salt to water, it will cool down as it dissolves the salt, making the claim that the system stays cool for longer plausible.

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DoerteEU t1_j6wqa7q wrote

Ever bathed in sea-water below 0°C?

I have. And most coastal ppl in Northern Europe are aware, you cool out much quicker in water than if you' simply cover yourself in Ice or snow. Cooling a solid object quicker, already saves time. But water's also a better insulator than Ice.

Ice melts at >0°C. Saltwater doesn't. So no endothermic reaction like in Ice neither. Best way to cool drinks quickly isn't Ice, but cold water. How do you keep below-zero water from forming ice? Salt!

tl;dr - Pretty common knowledge where I'm from (Ger/DK).

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DrunkenGolfer t1_j6wsp2z wrote

The reason for adding the salt to the ice is the reason we add salt to ice in ice cream makers. The salt causes the ice to melt, which is an endothermic process. This means it needs to get energy from somewhere, and that somewhere in ice cream making is by cooling the cream/custard. In the case of the fishermen, it just means the fish is cooled quickly.

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qwertyuiiop145 t1_j6wzzn1 wrote

Adding salt makes it colder but it would cause the cooler to reach room temperature slightly faster. The rate at which something heats up depends on the difference in temperature between the object and its surroundings—very cold objects heat up quickly at first, then heating slows down as they approach room temperature. Melting ice uses up heat energy to break the bonds holding the ice together as a solid.

When you add salt, the ice absorbs all the heat energy it already has in order to break those bonds, which causes the temperature to drop below the normal freezing point. The resulting water is colder than the ice it came from and the water conducts heat better than ice, so the water warms up quickly until it gets warmer.

When you don’t add salt, the temperature will pause at 32F/0C until all the ice is melted. When the ice absorbs heat energy from its surroundings, that energy goes to changing the ice into water instead of increasing the temperature. The ice will absorb heat at a slower rate than the super cooled salt water because the ice is warmer than the salt water and because ice doesn’t conduct heat as easily as water does.

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incizion t1_j6xu2p1 wrote

>The resulting water is colder than the ice it came from and the water conducts heat better than ice, so the water warms up quickly until it gets warmer.

Thanks for this sentence. I couldn't get my head around how if something was colder it could still reach room temperature fasters than something warmer since the colder thing has to pass the warmer thing's temperature to get there. In my head it was the equivalent of "you can accelerate faster if you start rolling backwards".

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