As someone who loves the outdoors and loves his coolers I have always heard people talk about salt water ice bricks being colder than regular water ice bricks and I could never understand why.
Not understanding the science behind this drove me absolutely mad. But it turns out it's quite simple.
After a lot of research I've finally worked it out and in this article I explain it simply with pictures and graphs. But here's a short explanation of why salt makes ice colder.
Salt makes ice colder because it lowers the melting point of water. A partially-melted ice pack with water and ice will stay at exactly the liquids melting temperature until all ice is gone. Partially melted salt water will stay at 20-28ºF (-7 to -2ºC) whereas partially melted regular water will stay at a warmer 32ºF (0ºC).
Salt makes ice colder because an ice pack with partially melted ice will always be the exact temperature
Why Salt Water Ice Packs Are Colder Than Regular Water Ice Packs? (TL;DR)
When you have a mixture of water and ice the temperature stays exactly the same until all ice is gone. That temperature is exactly the melting point of the ice.
So a water bottle with half melted regular ice will be exactly 0ºC until all ice is melted. If it's almost full of ice with a little bit of water it will be 0ºC. If it's almost full of water with a little bit of ice it's also 0ºC.
However, salt water melts earlier than regular ice (20ºF or -7ºC) so a bottle of partially melted salt water will stay at 20-28ºF or -7 to -2ºC until all the ice is completely gone.
Because ice in a cooler/ice pack almost always has some melted water it'll be at exactly the temperature of it's melting point.
32ºF (0ºC) for regular ice and 20-28ºF (-7 to -2ºC) for salt water ice.
Thus salt water ice is colder.
Salt Water Does Not Freeze Colder Than Regular Water In Your Freezer
This is a common myth where people say because of the lower freezing point salt water will freeze colder than regular water.
This is completely untrue.
If you place both regular water and salt water in a freezer that is 0ºF (-18ºC) then eventually both the salt water and the regular water will go down in temperature to exactly 0ºF (-18ºC) or the temperature of the freezer.
Yes as it freezes the regular water will start to turn into solid ice first, but ultimately they will both reach the same low temperature.
So salt water doesn't freeze colder than regular water, it just starts to melt at a lower temperature and it turns out this is the secret to it being colder than regular water ice.
You can see this in action in my Yeti Ice vs Regular Ice video below:
How Regular and Salt Water Ice Packs Change In Temperature As They Warm Up/Melt
When you have semi-melted ice the ice actually stays at the EXACT SAME temperature (the temperature of it's melting point) until all the ice is melted. This is why salt water ice stays colder than regular ice when it's partially melted.
There is a great discussion on phase change temperatures here but let me explain more simply by looking at how the temperatures of a salt water ice pack and regular water ice pack will change over time.
Start Point 0ºF (-18ºC)
If they are stored in the same freezer both regular ice and salt water ice will start out at the same temperature.
Let's say 0ºF (-18ºC)
They will both be exactly the same temperature, no difference here.
For this example we are going to say the regular ice melts at 32ºF (0ºC) and the salt water ice melts at 20ºF (-7ºC). This is the temperature Engel 20 and 32 ice packs melt at and they are some of the best ice packs out there.
When placed inside a cooler (or outside) both of these blocks of ice will start to warm up.
From 0 to 20ºF (-18 to -7ºC)
From 0 to 20ºF (-18 to -7ºC) both the salt water ice and the regular ice will steadily rise in temperature at almost exactly the same rate.
Still no difference here. Both types of ice are performing exactly the same.
From 20 to 32ºF (-7 to 0ºC)
Once we hit 20ºF (-7ºC) our salt water ice starts to melt and this is where the magic (well science) happens to make salt water ice colder than regular ice.
See when you have a mixture of liquid and ice the temperature will remain constant.
In this case the semi-melted salt water ice will remain at a constant 20ºF (-7ºC) until ALL of the ice has melted.
So once your salt water ice hits 20ºF (-7ºC) it will stay that exact same temperature until all of the ice has turned to liquid, then it will start to warm up.
However, because regular ice doesn't melt until 32ºF (0ºC) from 20 to 30ºF (-7 to 0ºC) it will continue to rise in temperature.
Once it hits 32ºF (0ºC) the now semi-melted regular water ice will stay at exactly 32ºF (0ºC) until all the ice is completely melted. Once all the ice is melted then it will begin to warm up.
Because ice packs spend the majority of their time in a semi-frozen state salt water ice packs will be locked in at 20ºF (-7ºC) until all the ice has melted whereas regular ice packs will be locked in at 32ºF (0ºC) until all the ice has melted.
At its core, this is why salt water ice packs are colder than regular ice packs.
Over 32ºF (0ºC)
Once all the ice is melted from both the regular water and the salt water they will both increase in temperature at the same rate.
In fact, you'll find that salt water and regular water ice packs will ultimately reach room temperature around the same time.
No difference here.
It's Only During The Partially Melted Phase Salt Water Ice Is Colder Than Regular Ice
As we've learned it's only during the partially melted phase that salt ice is colder than regular ice.
- When put in the freezer both salt water ice and regular ice will reach the exact same temperature. Salt water ice does not freeze colder than regular ice
- Both salt water ice and regular ice will rise in temperature at the same rate until 20ºF (-7ºC)
- A mixture of partially melted salt water ice and the liquid salt water will stay at 20ºF (-7ºC) until all the ice is gone
- A mixture of partially melted regular ice and regular water will stay at 32ºF (0ºC) until all ice is gone
- Once all ice is gone both salt water ice and regular ice will increase in temperature at the same rate
- Both salt water ice and regular ice will reach room temperature around the same time.
Both Ice Packs Will Reach Room Temperature Around The Same Time
This is interesting, because while salt water seems to stay colder for longer (during that semi-melted state) once melted it'll then start to rise in temperature again.
In theory the salt water ice pack should all completely melt before the regular water ice pack. It'll then start to climb in temperature and reach 32ºF (0ºC) just as the last of the regular water ice melts.
Now that both the salt water and regular water are completely melted they'll both rise in temperature at a constant rate. Reaching room temperature at the same time.