Does a bottle of salt water stop water from freezing for our cats?
I have seen several explanations why it should work like: since saltwater freeze at a lower temperature it works as a “heating unit” for the surrounding water. The fatal error in this explanation is that the saltwater has mainly the same temperature as the water outside the bottle. It just not freeze until later…that does not mean it generates heat.
There is however a physical phenomenon that might make it work. When salt water is turning into crystals when freezing, it is an exothermic reaction meaning heat is created. The freezing process of the surrounding sweet water is also an exothermic reaction. So the difference between these two processes might “generate” heat. But is that difference big enough to make any difference at all? And is the exothermic reaction bigger in saltwater than in sweet water?
The answer is sadly no and I explain why below.
Lots of tests has been performed
I have set up about 15 tests proving it doesn´t work. I have used both small bowls and buckets. I have had different salinity. I have let the bottle swim freely and steady “planted” at the bottom of the bowl.
I have I have used cold water and hot water. swapped the position on my two test bowls/buckets to eliminate eventual local differences like more or less wind for example.
It hasn´t worked at all.
The "successful" experiment
Last night however we managed to catch the moment when the salt water started to freeze and generate heat.
My boyfriend came to my home with a professional thermometer with certificate, that means it is very exact. (My own simple thermometer did never work when trying to catch that moment since it is far too incorrect.)
19:00
Test started and I used two buckets with.+2 degrees Celsius water and two bottle of water at +57 degrees Celsius, one with lots of salt in. There was so much salt that some didn’t solve despite the warm water. It was about –6 degrees Celsius cold outdoors.
20.00
There was already a thin "lid" of ice in both buckets.
2.2 degrees Celsius in bucket with salt water and 2,7 in the one with a bottle with sweet water.
We measured right beside the bottles to catch any local effect from the exothermic reaction in the bottle.
I have no idea why there was a difference except for that I had noticed that left bucket always freeze first no matter what kind of bottle it contained (about ten min before right bucket). I think its because more or less air circulating at that bucket. In this test the left bucket contained the salt water bottle
22:30
+0.8 degrees Celsius at the bucket with salt water bottle and -1.2 at the other, that means the bucket with a bottle of saltwater is 2.0 degrees WARMER!
We measured right beside the bottles to catch any local effect from the exothermic reaction in the bottle and this time there was one.
The difference was 2 degrees in favour for the bucket with the saltwater bottle. BUT – the water 10 centimeters away from the bottle was just as cold in both buckets and both buckets had thick lids of ice.
Conclusion
In all my tests the effect of this exothermic reaction has been far too small to make any difference at all.
No matter if the bowl/bucket contained a bottle of salt water or sweet water, the one to the left froze more quickly because of some local phenomenon, I think it was more or less windy there.
The answer why the exothermic reaction makes no difference is probably:
- it is too small to make any difference. Its too small to melt what already is frozen or warm up water below 0 degrees Celsius
- The difference between the energy released from sweet water freezing and saltwater freezing is about the same. Salt water has to generate much more to make any difference.
- It starts up far too late – the sweet water is already frozen when the salt water turns into crystal and "generate" heat.
Motion
I have deliberately NOT set up a test with the bottle getting touched regularly sinc I need my sleep and have no cats to feed outdoors at the moment. But I am sure that those who have had successful tests with a saltwater bottle would be just as sucessful with sweet water in the bottle or a ball. Just anything stiring around the water will stop it from freezing for a while. The wind or curious animals will make whatever you chose to move.
Copilot about freezing sweet water and saltwater
Here are the key differences between the freezing processes of pure water and saltwater:
Pure Water Freezing
1. **Freezing Point**: Pure water freezes at 0°C (32°F).
2. **Process**: As the temperature drops to 0°C, water molecules slow down and arrange themselves into a crystalline structure, forming ice.
3. **Energy Release**: The latent heat of fusion is released, which is about 334 J/g.
Saltwater Freezing
1. **Freezing Point Depression**: Saltwater has a lower freezing point than pure water. The more salt present, the lower the freezing point. For example, seawater typically freezes around -2°C (28.4°F).
2. **Process**:
- **Initial Cooling**: As the temperature drops, water molecules begin to slow down.
- **Formation of Ice Crystals**: Ice crystals start to form, but the salt ions interfere with the formation of the crystalline structure, requiring a lower temperature to freeze.
- **Brine Rejection**: As ice forms, it excludes the salt, creating pockets of highly concentrated brine. This brine can remain liquid at lower temperatures.
3. **Energy Release**: Similar to pure water, the latent heat of fusion is released, but the presence of salt means the process occurs at a lower temperature.
##Summary
- **Freezing Point**: Pure water freezes at 0°C, while saltwater freezes at a lower temperature.
- **Crystalline Structure**: Salt ions disrupt the formation of ice crystals, requiring colder temperatures.
- **Brine Rejection**: Saltwater freezing involves the exclusion of salt, creating brine pockets.