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My girlfriend will get a refrigerator from the landlord, but right now she hasn't one. But she has a freezer. So we will get ice packs (images: 1, 2, 3) and use a cooling box for the time.

Now I was wondering: Would a box filled with water be just as good as any of the ice packs? What is the part that distinguishes the "cooling effect" of different materials, given that the temperature of the freezer is not changeable? The space the cooling pack may take is limited, the weight is not.

Ideas

From school I vaguely remember that it takes a lot of energy to warm water up / melt ice. So it also cools well. Is that thermal conductivity?

Would a solid block of iron cool better / longer than the same volume of water? Would water with something added (e.g. salt) be better for that?

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There can be differences between ice packs, but in practice the differences seem pretty small, and you'll have a hard time finding hard data on different ice packs' cooling abilities. How much ice/ice packs cool is determined by three things:

First, the specific heat capacity of a material tells you how much energy it takes to raise 1g of the material by 1 degree C. Water has an unusually high specific heat capacity, so it has a lot of "thermal inertia", meaning that cold water can absorb a lot of energy from warmer objects before becoming warm itself. Secondly, the latent heat of a material tells you how much energy it takes to melt 1g of frozen material. Water also has a very high latent heat capacity, so ice at 0 degrees C will absorb a lot of heat before it fully melts and becomes water at 0 degrees C. Finally, since the first two quantities are expressed with per mass units, we can conclude that a more massive object is a better heat sink than a smaller one - two ice packs can absorb twice the energy of one ice pack.

In terms of common substances, water is pretty much as good as it gets for a coolant - it has a very high specific heat capacity in both its solid and liquid forms, and the amount of latent heat absorbed while melting is also very large. A block of iron, on the other hand, absorbs slightly less energy per volume than water (it has 1/10 the heat capacity, but 8x density), and since it undergoes no phase change, it absorbs no latent heat. In total, the cooling capacity of a frozen block of iron will be roughly 20% of the same volume of ice over this temperature range.

The vast majority of the cooling effect of ice comes from melting - it takes only 2.1J to raise 1g of ice from -1C to 0C, but it takes 334J to turn 1g of 0C ice into 1g of 0C water. Keep the cooler packed with as much ice as you can, and replace it once it's melted, and you should have little trouble keeping food cold with a decent cooler. The cold liquid water is better than nothing, but most of its cooling ability was spent when it melted. Ice packs will have pretty much the same result, but may be more convenient since they're reusable and won't soak everything in the cooler, although it won't be quite as obvious when they're fully melted.

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  1. No water in its liquid form will not work as well as ice or ice packs for keeping your cooler cold.
  2. No a block of iron will not work as well as ice or ice packs.
  3. The "hard" packs in your first and last photos are made for the job. There are also soft gel packs that do the same. However the type of gel pack intended for use on muscle injuries are not intended to go in a freezer only an ice tray and may be damaged if you use them for this. Typically they are rated for -4°C or some down to -10°C.
  4. Using ordinary ice works. You can use bags of ice cubes or makeshift ice packs (e.g. freezer bags or plastic water or soda bottles) just make sure the material used is elastic enough as water expands as it freezes so will burst rigid containers.

More Notes:

The store-bought packs with the gel mix will take longer to melt than pure water ice of your ice cubes or makeshift ice packs, so will stay cold longer and will not need to be swopped out as often and is also less likely to burst in the freezer or leak in the cooler.

The inside of your ice box will get cooler and stay cool longer if you use ice or ice packs than if you use a similar amount of liquid water or a block of iron. Reason: The ice absorbs heat from its surroundings, while staying at the same temperature, thus cooling down the inside of the ice box and its contents. The reason it can do this is because it is undergoing a phase change (melting). If you use liquid water or a block of iron, the water or iron will slowly heat up while the surroundings cools down and an equilibrium will be reached that is warmer than the iron or water originally was. Of course once the ice in the ice packs melts, it will do the same, but it will take significantly longer.

You can also cool your drinks and fresh produce down by wrapping in newspaper and placing them in the freezer for a while -not long enough to freeze- before putting them in the cooler. The newspaper insulates somewhat, so the soda cans or whatever you wrap will cool more uniformly and not freeze as fast.

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  • $\begingroup$ "No water in its liquid form will not work as well as ice" - this doesn't make any sense. Water cooled down to -18°C will freeze. $\endgroup$ Jul 7 '20 at 21:40
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    $\begingroup$ @Martin Thoma Yes. In which case it will be ice. Which is the solid form, not the liquid. I'm not sure what the disconnect is here, especially since I have already stated in the answer that if you freeze water in a plastic bag (or other container with enough elasticity) thus turning it into a solid (a.k.a. ice), it will work. $\endgroup$
    – Gwyn
    Jul 7 '20 at 23:12
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The plastic or flexible packs that freeze & are usually colored blue have the same total cooling effect as ice, but they stay cold longer because the water is a gel that doesn't circulate inside the pack much, slowing down heat transfer into the pack. Regular ice of the same weight will have basically the same cooling capacity, but will melt faster and then allow the food to heat up quicker.

Ice is good for keeping things cool because, as you have heard, it takes a lot of energy to melt ice. Ice stays at 32 degrees F (0 degrees C) while it is melting. That means you can never freeze something using only ice since once the other item gets to 32 degrees, there will be no more heat transfer from it to the ice (heat only flows from higher temp to lower). It is the melting action that makes it more effective than a block of iron, for example. Water itself also has a high heat capacity, which means it takes a lot of energy to heat or cool it down.

Salt water just freezes at a lower temperature. This is useful to de-ice your driveway or the road, because at outside temperatures just below freeing, it will make ice melt. It's also why in very cold places, roads are not salted because it's too cold for the ice to melt even at the reduced freezing point. It is also useful to make home-made ice cream (we need the ice temp to be below the freezing point for the ice cream or no heat will flow out of ice cream to let it freeze) or to make your icebox beer very cold.

In summary, your ice box will work fine with ice packs or just bags of ice. It will not keep food frozen, though.

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  • $\begingroup$ What is the property I have to look at if one thing cools better (longer) than something else? Would a 100 cm³ of iron cool longer than 100cm³ of water? $\endgroup$ Jul 7 '20 at 17:34
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    $\begingroup$ You can absolutely freeze something with only ice, it just requires enough ice or low enough temperatures. If you have a chest full of ice at 10F, and put a small cup of water at 33F inside, the water will freeze before the ice melts, and the final overall temperature will be somewhere between 10F and 33F (dependent on the relative quantities of ice and water). Melting ice is only ever at 32F, but ice can certainly be colder than that. $\endgroup$ Jul 7 '20 at 17:35
  • $\begingroup$ @NuclearWang it can be but it typically isn't. $\endgroup$
    – Tiger Guy
    Jul 7 '20 at 17:40
  • $\begingroup$ @MartinThoma, you want to look up the specific heat of an item to tell you which will change temperature more easily . theengineeringmindset.com/specific-heat-capacity-of-materials $\endgroup$
    – Tiger Guy
    Jul 7 '20 at 17:42
  • $\begingroup$ @TigerGuy A freezer should be set at 0F or lower - if anything from your freezer isn't significantly below 32F (ice included), you have a problem. $\endgroup$ Jul 7 '20 at 17:44

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