Keeping things cold

[Buck420]

" The salt we add to the ice creates salt water which is actually colder than ordinary iced water. In other words, salt water has to get colder than 0 degrees Celsius (°C) before it will freeze. In this case, milk is like regular water and freezes when surrounded by colder salt water. This allows the ingredients to mix together to make ice cream.
The salt works by lowering the melting or freezing point of water. The effect is termed 'freezing point depression'.
When you add salt to water, you introduce dissolved foreign particles into the water. The freezing point of water becomes lower as more particles are added until the point where the salt stops dissolving. For a solution of table salt (sodium chloride, NaCl) in water, this temperature is -21°C under controlled lab conditions. In the real world, if you were to add salt to a frozen footpath, sodium chloride can melt ice only down to about -9°C.
Since salt interferes with ice crystals forming in water, adding salt to water effectively lowers its freezing point. This is why the oceans don't freeze, even when the temperature is below 0°C. The ocean, which is about 3.5% salt, freezes at approximately -2°C .
For saltwater that's as saturated as it can possibly get (i.e. there's no way to dissolve any more salt in it no matter how hard you tried), the freezing point is -21.1 °C . This is when the saltwater is 23.3% salt (by weight).
Applications

In colder parts of the world, salt is often used on footpaths and roads to melt ice and snow. Salt is used on roads and walkways because it is inexpensive and readily available.
When ice melts, it absorbs heat energy from its surroundings. When you add salt to ice, the rapid melting of the ice can cool the ice and water to below 0°C. Before refrigeration was invented, ice and salt was used to produce the rapid cooling needed to make ice-cream. "

 

[tkanzler]

It is impossible for the addition of room temperature salt to make ice colder assuming no other input to the system.

In order for ice to melt, heat has to be added to the system, with the system being the blob of ice and the space inside the cooler (and the contents). If the insulation was perfect, and no heat could sneak through it, the ice would absorb heat from the surroundings (air and cooler contents) and melt until equilibrium was reached. The ice would be at 32F, and so would the contents, and no more ice would melt.

Let some heat sneak into the system through the walls of the cooler, or by opening it and letting some warm air in, or by adding a warm object, and more ice melts, absorbing the heat that was added, and eventually restoring equilibrium at 32F (including the melt water, also at 32F).

If you add ice, the freezing point of the water is lowered, at least where the salt is (since salt water has a lower freezing point), and some more ice will melt with the heat to do that coming from the surroundings AND the ice blob itself, until equilibrium is restored. The drop in temperature comes from the phase change of the ice to water, and adding salt just changes the freezing point to a lower temperature.

Since the insulation is not perfect, there is a constant stream of heat leaking into the system, and the ice is constantly melting which absorbs that heat, and the system remains at equilibrium (dynamic), with the ice changing phase at whatever rated matches the heat input to the system. The only difference is the temperature of the ice where the salt has reduced the freezing point.

 

[anchorclanker]

As stated elsewhere about putting ice in a chest freezer, some older/better chest freezers can get down to -15F.

Most of the store bought coolers are Styrofoam filled and its often less than an inch thick. Ive often contemplated making my own cooler using a large Tupperware bin and lining the outside with aluminized pink or blue foam and possibly filling the voids with great stuff. If the cooler had 4 inch walls it would easily have many times the R value of a common store bought cooler. Obviously you give up internal space for greater physical size, but thicker insulation is the only way to keep it cold the longest.

 

[Freedom77]

Seriously? Rock salt is the stuff we put on the roads up here to melt the ice.

In it's purest form it is used to coat the rim of margerita glasses We freeze 1/2 ga plastic milk jugs and when they melt you have fresh water. Freeze everything you can and pack tightly. This will keep thing cold for longer. We have used dry ice in the past but as was said watch out for Co2. Also throw a sleeping back around the cooler.

 

[jibes138]

Huh?

When ice melts, it absorbs heat energy from its surroundings. When you add salt to ice, the rapid melting of the ice can cool the ice and water to below 0°C. Before refrigeration was invented, ice and salt was used to produce the rapid cooling needed to make ice-cream. "[/quote]

Adding higher temperature salt to the system assuming no other exchanges of energy can do nothing but increase the temperature of the system. Why salt works to cool down a surrounding object like ice cream is it lowers the melting point of the ice so the ice melts creating ice water that is colder than 32 degrees which in turn is able to absorb heat energy out of the ice cream and transfer the heat into the ice itself causing more melting. There is no way that melting of ice can cause the ice and resulting water all by itself to be colder. This is thermodynamically impossible. The salt water will be colder than pure ice water but the ice will be warmer than it was plus some heat loss due to entropy. The resulting total system will be warmer than before the addition of the salt.

 

[Ross]

Re: Huh?

Jibes you need to re read my post on the bottom of page 1.

 

[Ross]

Re: Huh?

Some of us are thinking of water and ice as different substances. Ice , water and steam are all water with different levels of energy. Steam is water vapor and comtains 1000 BTU's per pound of energy. Ice is solid crystaline water and requires 140 BTU's per pound to melt it back to a liquid. In humid air if you place 10 pounds of ice in an open container water vapor in the air will condense on the ice giving up its latent heat of vaporization. If you weigh the water after all of the ice has melted you could have a gain of a pound or more. This is one very good reason to control air flow into the icebox.

 

[Jack]

Thanks everyone, great info...
j&r

 

[heritage]

pure water freezes @32 degrees F. concentrated salt water freezes at 0 degrees F. When water changes from a liquid phase to a solid phase it gives up 140 BTU's per pound/ When water changes from a solid phase to a liquid phase it absorbs 140 BTU's Pure water melts and freezes @32 degrees and the salt water melts and freezes at 0 degrees F. This is called the latent heat of fusion. Once the water has frozen it requires 1 BTU per pound to reduce the temperature 1 degree F. The same as for liquid water.
Ask about the effect of condensate on melting ice.

So.... If you you add salt to your ice, the box gets colder, but the ice doesn't last as long. No magic, just a trade off.

 

[Ross]

You got it.

 

[jibes138]

Laws of thermodynamics

Some of us are thinking of water and ice as different substances. Ice , water and steam are all water with different levels of energy. Steam is water vapor and comtains 1000 BTU's per pound of energy. Ice is solid crystaline water and requires 140 BTU's per pound to melt it back to a liquid. In humid air if you place 10 pounds of ice in an open container water vapor in the air will condense on the ice giving up its latent heat of vaporization. If you weigh the water after all of the ice has melted you could have a gain of a pound or more. This is one very good reason to control air flow into the icebox.

Ross, there was a suggestion that the addition of rock salt to ice will lower the temperature of the ice. I repeat my objection to this concept. Lets assume you have a perfectly insulated container filled with air and fresh water ice at an equilibrium temperature of say 25 degrees F. You have rigged up a way to dust the ice with rock salt that is at room temperature of say 72 degrees F without allowing any other disturbance in this closed system. The overall temperature of the system will increase. Inside the box some of the ice will melt but the change in energy of the system will be from the higher temperature salt transferring heat to the colder air and ice until the new combined system reaches a new equilibrium temperature that will be higher than that recorded previously depending on how much rock salt was added. The laws of physics will not allow any other answer to the problem. If you can make a system colder by adding a hotter substance to it then please patent your invention quickly as you are on your way to fame and riches.

 

[Ross]

Re: Laws of thermodynamics

Jibes, I understand your dilemma But we are dealing with less than two pounds of salt per gallon of water. One half pound of ice melting would chill salt two pounds of salt forty degrees. At that point the salt would desolve into the melt water and depress the freezing/thawing point of water/ice. This is not a matter of adding a solid to a liquid. It involves desolving a salt in water. once the salt has desolved in the water there is no way to tell by looking at the water that it has salt in it. I could discuss exothermic reactions and endothermic reactions. When you dissolve sodium hydroxide in water it gets hot. that is exothermic. when you dissolve ammoniun nitrate in water it gets cold that is endothermic. You may have experienced this with a medical cold pack.

 

[anchorclanker]

Ive made ice cream when its been over 100F outside when the salt was just as hot from sitting out on the porch. Ive brought ice home that was melting and have even shined the IR gun at it, often seeing 32F, or taken ice out of the upright freezer thats only at 25 - 20F. Ive also measured the melt water with a calibrated thermometer and have seen it get down as low as 1 degree F. If the physics didnt work the process wouldnt work, and we couldnt make ice cream that way. Your not going to freeze cream at 32F.

 

[jibes138]

Ladies and Gentlemen,
This thread is not about making ice cream. Emperical evidence proves that you can make ice cream. Someone suggested you could make your cooler colder by opening the top and chucking in a handful of rock salt. I am suggesting that in such a closed system all the rock salt will do is make the total contents of the cooler a little warmer. If you want to build a case for heat transfer between the food contents of the cooler and the ice speeding up a little due to a greater delta T between the food and the now colder water surrounding it then I'm all with you, but adding energy to the system will not drive it to a colder state, you need to remove energy to do that. Peace brothers and sisters.

 

[Ross]

Ladies and Gentlemen,
This thread is not about making ice cream. Emperical evidence proves that you can make ice cream. Someone suggested you could make your cooler colder by opening the top and chucking in a handful of rock salt. I am suggesting that in such a closed system all the rock salt will do is make the total contents of the cooler a little warmer. If you want to build a case for heat transfer between the food contents of the cooler and the ice speeding up a little due to a greater delta T between the food and the now colder water surrounding it then I'm all with you, but adding energy to the system will not drive it to a colder state, you need to remove energy to do that. Peace brothers and sisters.

Not only is that not correct it is even wrong.

 

[Fred Villiard]

Jibes, why don't you take a small cooler throw a couple sealed bottles of water in it, cover them with ice and throw a couple of hand full of coarse salt over the ice shut up the cooler and leave it set over night, then see what takes place.

Fred Villiard

 

[justsomeguy]

Jibes, why don't you take a small cooler throw a couple sealed bottles of water in it, cover them with ice and throw a couple of hand full of coarse salt over the ice shut up the cooler and leave it set over night, then see what takes place.

Fred Villiard

That would be the empirical thing to do.

 

[jibes138]

Rock Salt

Ross I think you got me.
But I'll stand by my position that adding energy to the system is a bad thing to do based on the original post which was how to keep the ice from melting longer. In this case the cooler will get colder as the air in the cooler reaches equilibrium with the system, however the resulting melting of the ice will result in less ice which is opposite what was trying to be accomplished.


How does the rock salt work?

Rock salt forces the ice surrounding the can of ice cream mix to melt. The "brine solution" or liquid that forms in the wooden bucket absorbs heat from the mix and gradually lowers the temperature of the mix until it begins to freeze. If there were no salt added to the ice, it would melt at 32 degrees Fahrenheit and eventually the ice water and mix would come to equilibrium at 32 degrees. The ice cream mix, however, does not begin to freeze until its temperature falls below 27 degrees. Therefore, in order to freeze the mix, we need a salt concentration, or a ratio of 5 cups of ice to 1 cups of salt. At this concentration, our brine temperature should remain constant at 8 to 12 degrees F. This will give the rapid cooling and freezing that is essential to making smooth creamy ice cream.

More detailed information provided by David Winer, Bethesda, Maryland For ice to melt into liquid water it must absorb a lot of heat energy. In an ice cream maker this heat needed to melt the ice comes from the mix, and so the mix chills. If the ice were not combined with salt, the mix’s cooling would be slow and would stop when the mix reached the temperature of melting freshwater ice-- at 32 degrees. But salt makes ice melt faster, and drops the temperature to that of freezing salt water. The heat used to melt the ice is drawn much faster from the surroundings, which in this case consist of the melted ice AND the ice cream mix. Since the surroundings lose large quantities of heat to the ice, they cool rapidly and continue to cool until they reach the freezing temperature of melting saltwater ice. This temperature is below the cream mixture's freezing point.

 

[Maine Sail]

not looking for a war either .....and i don't have a scientific explanation for what i described in the use of it ......but i do know that it has worked the way i described.....if one went to the grocery store and looked for ice cream salt you could probably still fine it on the grocers shelf......


regards

woody

We did this on tuna fishing boats all the time, it is called a "brine" and it stays colder significantly longer and will actually freeze the bait not just keep it cool. Big difference without the salt, and lots of it. I don't know that I would want to try that for food as it needs to be part liquid part ice to work best. Sticking your hands in a "brine" hurts it is sooooo cold...

We actually measured a brine solution made of bags of ice from the marina machine and salt. It hit -2F and IIRC was still at roughly 3F the next morning in one of the largest six foot long Igloo's they make. Lots of cold mackerel bait strings....

For us I just turn the fridge dial when the engine is running to chill the holding plate, which is like a huge mechanical block of ice.

 

[anchorclanker]

When I was young I was fascinated by air conditioning. As I studied how it worked it seemed to defy common logic. How can something extremely hot, become instantly cold? Yet thats exactly how it works. Hot liquid freon coming from the condenser under high pressure, is sprayed into the low pressure evaporator through a tiny orifice, an aerosol of sorts, where the liquid freon boils into a gas. Its a scientific fact that all liquids will boil at their boiling point temperature, corrected for pressure. That R12 boils at -21.6F at sea level pressure is a fact. At 100 psi, which is the average low side pressure in an R12 refigerant system, R12 will boil at around 0F.

And how did the Freon become a hot liquid? By compressing the gas through a motor driven compressor to high pressure. We add energy to the system to extract heat. What heat? The heat inside our house, or our car, or inside our refrigerator that were trying to get rid of. That excess heat is drawn into the now cold evaporator and carried away by the freon gas, through the compressor, to escape at the condenser along with some of the heat generated through the process.

Making ice cream uses the same basic science. Because the salt lowers the melting point of water to 0F, the water coming off the ice must be at 0F. And yes, you could get your beer frosty the same way, or outright frozen if you wait too long. Try it sometime.