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FVjake t1_j6upnjf 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

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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