Milk Jug Ice...add salt

[jakebloleit]

How much salt to add to the water - put 2 lbs of salt in the jug then fill the jug with water.

The saltwater should stay at -6 degree the whole time it is melting, which will allow you to keep frozen food actually frozen. As noted above, you need to have enough salt/ice that it does not all melt the first day. You will certainly go through more jugs of the mix than with water ice.I have

Water weight is 8.34 pounds/gallon. Therefore 1.7 pounds of salt is needed to be added to a gallon of water for a 20% solution. (8.34 x 20% or 0.2 = 1.67) For a quart bottle (1/4 of a gallon) = .42 pounds of salt to be added....

probably a stupid question but… what would the differences be use regular salt or iondized salt?

 

[Meat1nthefreezer]

Howdy Randy,
I might be a little late but just listened to your podcast from a few months ago when you talked meat care and milk jug ice. I've been using frozen jugs just as you described with the exception of using salted water. Salt water freezes at a lower temp and ice stays frozen longer. I use all sizes and really like the small square one liter water bottles.

Steve

Great tip. Thanks.

 

[Ben Sellers]

I feel like the basic premise is ice with no water stops getting colder at freezing like it can’t get warmer than 212 without pressure added. This ice will get as cold as salt water ice, but the salt water ice will melt quicker. The thermal mass is roughly the same for a gallon between the two (for non-scientific purposes). In each you would start with 8ish pounds of -3 (or lower depending on your deep freezer) degree F ice. One would melt faster and the different thermal properties would come into play. I doubt you would be able to tell the real world difference between the two except one costs more and can’t be used to drink when melted.

The main benefit of salt in an ice cream maker is keeping water liquid and so having more surface contact with the inner container.

 

[BucksnDucks]

I feel like the basic premise is ice with no water stops getting colder at freezing like it can’t get warmer than 212 without pressure added. This ice will get as cold as salt water ice, but the salt water ice will melt quicker. The thermal mass is roughly the same for a gallon between the two (for non-scientific purposes). In each you would start with 8ish pounds of -3 (or lower depending on your deep freezer) degree F ice. One would melt faster and the different thermal properties would come into play. I doubt you would be able to tell the real world difference between the two except one costs more and can’t be used to drink when melted.

The main benefit of salt in an ice cream maker is keeping water liquid and so having more surface contact with the inner container.

I don't know the physics of it but I noticed the salted jugs appear to melt faster and colder. By colder I noticed liquids and meats placed next to jugs actually partially froze. Probably not much of a benefit because of faster melt. I still use frozen jugs often in coolers simply because there is not a pool of water to deal with.

 

[R.K.]

I work with molten metal, and freezing/melting is pretty much my entire career. So let's get "hangover technical" this morning.

To clear some things up- it takes energy to raise the temperature of ice/water, and it takes energy to melt ice (breakdown of the crystalline solid). This is known as heat of melting/solidification/fusion, depending on who is grading your homework. The important thing to note is that as you add more energy/heat to a melting substance, the temperature remains the same while it is melting (isothermal process). Meaning from the time you get the first drop of water coming off 50 lb of ice at 32°, that entire mass is going to be 32° until the very last ounce of ice is melted. Only then does the temperature start to increase, because all the energy is going into making the solid a liquid rather than making it hotter.

Frozen salt water has zero advantage over frozen fresh water- almost identical heat capacities. Adding salt to ice (a la ice cream maker) causes the ice to melt by lowering the melting point below 32 degrees. This is where the heat of fusion comes in- in order to get the energy needed to melt, it literally pulls the thermal energy (heat) from everything around it. It's a trade-off, robbing Peter to pay Paul. The melting solid has to have energy from somewhere to increase the entropy of the system, and it steals that energy from particle motion (heat).

The first law of thermodynamics: energy is neither created nor destroyed, it just changes form.

So salting freshwater ice gets you a lot colder at that particular moment, but it does not give you any more longevity in your cold, because the total energy in the system remains constant unless it is lost somewhere (1st law). It's kinda like running your car at 100 mph instead of 55 mph- you only have a certain amount of gas in the tank, and it's going to run out a lot faster doing one over the other.