Why does rock salt make ice colder?

In this brief guide, we are going to answer the question “why does rock salt make ice colder” with an in-depth analysis of why rock salt makes ice colder. Moreover, we are going to discuss why salt is added to icy sidewalks in winter.

So without much ado, let’s dive in and figure out more about it.

Why does rock salt make ice colder?

Rock salt has the chemical name, Sodium Chloride that comprises a positively charged sodium particle and negatively charged chlorine atom bound together by the electrostatic force of attraction into a grid structure. At the point when you put the salt in water, what happens is that the polar water particles break the design and encompass the individual particles, which scatter into the solution. 

What happens is that the salt when it gets dissolved in the thin layer of water on the surface of ice breaks down into sodium and chloride ions that get in the way of the water molecules and hinder them from forming the rigid structure (ice). 

At the point when the temperature goes down, the particles meddle with the capacity of the water atoms to shape a crystal structure (formation of ice), and the combination won’t transform into solid (ice) until you bring down the temperature underneath the edge of the freezing point of pure water. Hence the freezing point decreases (otherwise the melting and freezing point of pure water is 32 degrees Fahrenheit).

Thus the more salt we add, the more will be sodium and chloride ions that will get in way of the water molecules and won’t let them bind together to form the rigid structure (ice).

The new edge of freezing point relies upon the amount of salt that is added to the ice, yet the most reduced it can go is – 21.1 degrees Fahrenheit or – 5.98 degrees Fahrenheit. This phenomenon takes place at the saturation point, and after this point, the addition of more salt won’t bring about any change in the freezing point of the ice.

Why is salt added to icy sidewalks in winter?

At the point when salt is added to ice on sidewalks, what it does is that breaks up in a thin layer of the water that is available on the outside of the ice. At the point when the salt blends in with this dainty water layer, it decreases its freezing point. Subsequently, this layer has a lower edge of freezing over than that of the unadulterated water as the water changes into ice at 32 degrees Fahrenheit while this salt blended water has a lower melting point than 32 degrees Fahrenheit.

So the ice that interacts with this saltwater liquefies, this softened ice disintegrates more salt in it, which will bring about melting of more ice and this cycle simply proceeds like this. 

In this way the higher is the amount of salt added to ice, the lower will be the freezing point. But it is worth mentioning that as there is a cutoff point of how much salt can be disintegrated in water so once this cutoff is reached, there won’t be more decrease in the freezing point. 

It is likewise worth referencing that once the temperature of this salty ice diminishes to minus zero degrees Fahrenheit, the augmentation of salt won’t have any more effect on the melting of ice.

So salt lowers the freezing point of the water. This is evident from the fact that the seawater that has a high amount of salt present in it has a freezing point of 28.4 degrees Fahrenheit while on the other hand, the freshwater has a freezing point of 32 degrees Fahrenheit. Thus the addition of the salt lowers the freezing point.

So to understand the chemistry lying behind this very process, you have to understand that the water and ice are in the state of dynamic equilibrium which means that the molecules of water are changing their state from liquid to solid or vice versa. Thus there is a continuous interchange between the liquid and the solid phase. 

So when the temperature is high more of the molecules enter the liquid state from the solid-state while at the lower temperature, more of the molecules enter the solid phase from the liquid phase. Now when it comes to the freezing point, it is the point where both of these rates are equal which means that the number of molecules that are entering the liquid phase is equal to the number of molecules entering the solid phase.

So when salt which is ionic in nature is added to this equation, the rate of the detachment of molecules from the ice (leaving the solid phase) remains the same but the rate at which the molecules are attaching to the ice (entering the solid phase) decreases (thus the ice can’t solidify the layer of water in contact with it at 32 degrees Fahrenheit anymore). This whole process is known as the freezing point depression.

This is the very reason that in winters, salt is added to the ice present on the sidewalks to melt it. 


In this brief guide, we answered the question “why does ice get colder with salt” with an in-depth analysis of why ice gets colder when salt is added to it. Moreover, we discussed whether or not ice melts slower in saltwater.





Mahnoor Asghar is a Clinical Nutritionist with a bachelor's degree in Nutrition and Dietetics. She is compassionate and dedicated to playing her part in the well-being of the masses. She wants to play a fruitful role in creating nutrition and health-related awareness among the general public. Additionally, she has a keen eye for detail and loves to create content related to food, nutrition, health, and wellness.

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