You may have heard that adding salt to water makes it colder. But does this almost mean it freezes faster?
The opposite is actually true:
Adding salt to water makes it freeze slower, as the freezing point is lower than regular water. This is known as “freezing point depression”.
Adding salt does decrease the starting temperature of when water will freeze, meaning the water takes longer before it starts to turn into sold ice.
While freshwasher begins to turn into ice at 32ºF/0ºC saltwater will stay completely liquid at this temperature. Temperatures need to drop below 16ºF/-9ºC for the saltwater to turn into ice.
Given you home freezer is around 0ºF/-18ºC both freshwater and saltwater will eventually freeze inside your freeze. However, the freshwater will turn solid first and it will take longer for the saltwater to full freeze.
You can see this in action in the experiment below:
Why Does Salt Make Ice Freeze Faster?
Salt doesn't make ice freeze faster, but it can make iced drinks colder more quickly.
Salt lowers the freezing point of ice, so it actually freezes slower than freshwater.
As mentioned above, saltwater won't start to freeze until it reaches (16ºF or -9ºC). So, a puddle of regular water in your driveway will start freezing over when the temperature hits 32ºF (0ºC).
If you threw salt in that puddle, it would remain in a liquid state until the outdoor temperature hit the new freezing point.
However, adding salt to the ice will make your drinks colder faster by lowering the temperature of the ice. Learn more about the 3 ways salt makes ice colder.
But that doesn't mean you should add saltwater cubes to your drinks!
Instead, fill a cooler or bucket with salty ice and pop in your beverages for a few minutes. The salted ice will absorb the heat from your drinks, so they're ready to enjoy in a few minutes. Here's exactly how quickly drinks cool down in a cooler.
The same approach is taken in making ice cream at home.
If you've tried to make homemade ice cream to beat the heat, you've likely seen the instructions to add ice and salt to an exterior bag and shake it up.
If you don't add the salt, your ice cream will likely be a semi-liquid slop.
As mentioned before, sugar also disrupts liquid molecules from bonding. The freezing point of ice cream is around -3 °C (27 °F), colder than the average block of ice.
However, by adding salt the temperature of the ice can drop as low as 1ºF/-17ºC, more than cold enough to freeze your ice cream.
Does Salt Prevent Ice From Melting?
The myth that salt prevents ice from melting stems from the recommendation to add it to water to keep your camping cooler cold for longer.
As mentioned previously, the ice will melt faster, but the water and partially melted ice will remain colder for longer. So too will your camping snacks and delicious canned beverages.
The contradiction to this rule — because there's always a contradiction in complex scientific processes — is ice floating in water.
If ice is floating in salt water, it will melt slower than ice sitting in fresh water. This effect is related to displacement and density.
You can see it in action in the video below:
Cold water is denser than warm water. So, as the cold water from the melting ice sinks, it pushes warmer water up.
In fresh water, the cold water moves away from the ice, and the warm water moves toward the ice, causing it to melt faster.
Saltwater is denser than freshwater, which changes the effects. When the freshwater ice starts to melt, it can't sink below the saltwater. So, the cold water sits on top of the salt water, insulating the ice and keeping it colder for longer.
This effect is why icebergs last longer in salt water compared to fresh water. It's also why ice will cool a diet soda faster than a full-sugar soda.
This is a fun experiment to try at home. Add food coloring to two ice cubes, then float one in saltwater and one in freshwater.
In the freshwater, you'll see the dye sink to the bottom as the ice melts. In the saltwater, you'll see the layer of food coloring on top. Read more on how salt can make ice melt slower.