What does adding salt to ice do?

In this brief guide, we are going to answer the question “what does adding salt to ice do” with an in-depth analysis of the effect of the addition of salt on ice. Moreover, we are going to discuss how salt lowers the freezing point and the reason why the ice melts slower in the saltwater.

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

What does adding salt to ice do?

When salt is added to ice, what it does is that dissolves in a thin layer of the water that is present on the surface of the ice. When the salt mixes with this thin water layer, it decreases its freezing point. Thus this layer has a lower freezing point than that of the pure water as the water changes into ice at 32 degrees Fahrenheit while this salt mixed water has a lower freezing point than this.

So the ice that comes in contact with this saltwater melts, this melted ice dissolves more salt in it, which will result in melting of more ice and this process just continues like this. Thus the higher is the quantity of salt added to ice, the lower will be the freezing point. But as there is a limit (saturation) of how much salt can be dissolved in water so once this limit is reached, there would be no more lowering of freezing point. It is also worth mentioning that once the temperature of this salty ice decreases to minus zero degrees Fahrenheit, the addition of salt won’t have any more impact on the melting of ice.

How does salt lower the freezing point?

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.

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). 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).

Hence the melting point decreases (otherwise the melting and freezing point of pure water is 32 degrees Fahrenheit).

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

Why does ice melt slower in saltwater?

So the ice melts slower in the saltwater. The possible reason behind this fact is the difference in densities of the water. The ice that comes in contact with the saltwater melts and the subsequent melted ice water has a lower density than that of the saltwater and therefore it forms a layer on the surface of the saltwater. As the ice is surrounded more by this melted ice water rather than the salt water, therefore it melts slowly as compared to if it was put in freshwater.  

Conclusion

In this brief guide, we answered the question “what does adding salt to ice do” with an in-depth analysis of the effect of the addition of salt on ice. Moreover, we discussed how salt lowers the freezing point and the reason why the ice melts slower in the saltwater.

Citations

https://www.scientificamerican.com/article/why-do-we-put-salt-on-icy/

https://www.britannica.com/story/why-does-salt-melt-ice

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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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