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Why do we salt the ice when making ice cream?

timothyrice.org

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Re: Why do we salt the ice when making ice cream?

#81

Post by eyelidlessness »

lupire wrote: post #80
eyelidlessness wrote: post #74

Earlier quoted context omitted.

> and none of it goes to increasing the temperature You’re clearly more knowledgeable than I am, but this strikes me as probably wrong? The temperature at the surface has raised, which is how some of the surface ice melted. The temperature on average has to raise because thermodynamics. If I understand the article, the core temperature of the remaining ice can decrease and localities can decrease with them. But the e…

At melting, the energy is going into the state change.

And the state change occurs when the portion of solid ice becoming liquid water warms however slightly, even if there’s a cooling side effect on the remaining ice as more energy is dissipated during that warming. Or am I still misunderstanding?

Re: Why do we salt the ice when making ice cream?

#83

Post by ekanes »

topaz0 wrote: post #70
Taniwha wrote: post #64

Earlier quoted context omitted.

yes - then it wont be ice any more - but while there's still ice the temp of the liquid will stay at the equilibrium point - after that it will start to warm

"then it won't be ice anymore". Not so: the freezing point is the temperature at which ice and water are in equilibrium. If you take a bowl of ice to its freezing point it will still be entirely frozen. Then if you continue adding energy, it will remain at its freezing point while progressively more of it melts, i.e. going from 100% ice to 50% ice/50% water to 100% water. All at one temperature. In other words, if yo…

Right on. It’s the same for boiling, isn’t it? The water “can’t” get above 100 degrees C - added energy just goes into making steam. Right?

Re: Why do we salt the ice when making ice cream?

#84

Post by vondur »

ortusdux wrote: post #26

Testing for melting point depression is a common diagnostic method used in chemistry to check for impurities. Pure compounds melt at known temperatures, and typically do so within a narrow range (+/- 0.5 °C). Impurities almost always lower the melting point and widen this band. I spent a lot of my undergrad chem courses packing my products into capillary sized test-tubes and watching them slowly melt. https://www.mt.…

Boy did I hate staring at a mel-temp apparatus...

Re: Why do we salt the ice when making ice cream?

#85

Post by refurb »

It's easier to think of a closed system and what the temperature of water vs. ice would be with just a phase change.

1 kg of ice turning into 1 kg of water requires 333,550 J.

1 kg of water require 4184 J to warm up 1 C.

So ignoring all the physical constraints, if you were to turn 1kg of ice "magically" into liquid water, keeping the total energy of the system the same, you'd end up with 1 kg of water at -80C (yes, I know I'm ignoring entropy).

Re: Why do we salt the ice when making ice cream?

#86

Post by Maursault »

dcow wrote: post #62
fsckboy wrote: post #31

Earlier quoted context omitted.

it's not just the coldness of the ice, salt dissolving in water actually decreases the temperature, it's an endothermic reaction. the salt dissolving into the water brings the water down to 0... omg time for Farenheit to shine... brings the water down to 0F without freezing it (because of the lower equilibrium temp), which is -17.8C (Farenheit uses this endothermic salted water temp as its definition of 0, I think be…

I understand Fahrenheit now.

I thought I did, but just thinking about what temperature is, i.e. the degree of hotness or coldness something is, so it relates to heat, which is the energy transferred from something hot to something cold, so it relates to energy, the more energy something has the hotter it is, the less energy the colder. So I am utterly confused now why wind cools or even can be cold, and wind chill should be a contradiction, and still stuffy air should be cold. Wind has more energy than still air, so wind should be hotter and still air cooler. Wind turbines deployed on a global scale should solve Global Warming even if no energy is generated, because they should slow down the energetic and hot wind, cooling the atmosphere and the planet with it.

Re: Why do we salt the ice when making ice cream?

#87

Post by robotresearcher »

bobthepanda wrote: post #52
foobarian wrote: post #37

Earlier quoted context omitted.

I suppose I could store the salt water solution instead of throwing it away. Assuming I made ice cream often this would be acceptably frugal for me.

Also, if you really want to recover the salt, you’d just have to boil away the water. Alternatively, pickling is a perfectly good use of brine used for frugal purposes (extending the shelf life of produce, eggs, and what have you)

> Also, if you really want to recover the salt, you’d just have to boil away the water.

This is probably the point the poster was making: this requires a lot of energy. There's a lot of NaCl in the world, but getting it out of solution is expensive.

Re: Why do we salt the ice when making ice cream?

#89

Post by majikandy »

tylerhou wrote: post #24
timrice wrote: post #3

Earlier quoted context omitted.

Author here, thank you! It's been bouncing around in the back of my brain for a long time. I couldn't find any clear and concise explanations about what really happens when salt is added to ice, so I did some research and wrote it out myself :D

Here is another way that I thought of it. 1. To make ice cream, we need to cool milk/cream below the freezing point of water (because milk/cream contains water). 2. To cool things, liquids have good thermal conductivity properties, so we would prefer to use a liquid. 3. We need some substance which is still a liquid at slightly below freezing. 4. It happens that salted water has this property and is relatively cheap.

Remember for point 4 it needs something in that salted water that can keep it at the lower temperature.

Re: Why do we salt the ice when making ice cream?

#90

Post by tylerhou »

majikandy wrote: post #89
tylerhou wrote: post #24

Earlier quoted context omitted.

Here is another way that I thought of it. 1. To make ice cream, we need to cool milk/cream below the freezing point of water (because milk/cream contains water). 2. To cool things, liquids have good thermal conductivity properties, so we would prefer to use a liquid. 3. We need some substance which is still a liquid at slightly below freezing. 4. It happens that salted water has this property and is relatively cheap.

Remember for point 4 it needs something in that salted water that can keep it at the lower temperature.

Also, we would like a phase transition slightly below freezing so that it's easier to keep a stable temperature.
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