Suppose you are living in a very cold area where it is often seen the snowfall in winter. Then you might have seen that salt is being sprinkled on frozen sidewalks and gutters in winter. But have you wondered why They sprinkled salt on the ice?
The Answer is Salt Helps to increase the speed of melting down the ice by Lowering the freezing point of the water, this phenomenon is known as “Freezing point depression”. Now let’s discuss in more detail how this physical change happens.
How Does Water React In Cold Winter? (Overview)
Did you know that every year More than 20 million tons of salt are used to melt snow and ice in the cold northern regions? But do you know how does salt do it?
Before we dwell on that first we need to understand a little bit about H2O in the winter.
As we all know 32°F or 0°C is the freezing point of the water. That means When the temperature drops to 32°F or below, the hydrogen bond between the water molecules strengthens at this point. And because of that, the water molecules arrange themselves into a crystalline structure, which is how liquid water becomes solid ice.
Similarly, When the Temperature rises above 32°F, Ice starts to turn to liquid as the Hydrogen bond between H2O molecules loosens and the structure becomes more fluid.
However, not all the water converts into ice at once. Some of the water molecules attach themselves to the ice and freeze, while some of the ice molecules attach themselves to the liquid water and melt. This inter-conversion rate is constant so that we have as many water molecules turning into ice as we have ice molecules turning into water.
At this temperature (32°F), your icy road generally has a thin layer of water on top of the ice, and the ice molecules and water molecules interact with each other. This water is constantly melting some of the ice, while the ice beneath it is freezing some of the water.
At this temperature, the exchange rate is pretty constant, meaning the amount of water and the amount of ice stays the same. If it gets colder, more water becomes ice. If it gets warmer, more ice becomes water.
How Does Salt Melt Ice Faster?
Road Salt, or Sodium Chloride, Melt Ice essentially by lowering the freezing point of the water, and that causes the ice to melt even when the temperature is below the normal freezing point of 32°F or 0°C.
When salt is applied, it dissolves into separate sodium and chloride ions that disrupt the bonds between water molecules. As the ions loosen hydrogen bonds, the ice melts into water.
But as we know Ice is the solid form of water then how does salt dissolve into the water? Because salt is only able to dissolve themselves when the liquid water is present.
As we discussed above there is always some amount of liquid water present on the surface this is true for any frozen material including ice. However, this level decreases as the temperature goes below the freezing point.
So, when you put salt on the surface of the ice, it mixes with the liquid water present on the surface of the Ice and breaks into its elements Sodium (Na+) and (Cl–) ions. This reaction would not have been possible if there was no liquid water present along with the ice.
Now these free ions occupy the space between the liquid water molecules and prevent them from coming close together to form solid ice. This lowers the freezing point of the water.
But as we discussed above in the normal condition there is a constant interconversion rate, meaning the amount of water and the amount of ice stay the same.
However, when salt is added to the surface of Ice this changes this interconversion rate. How?
Let’s understand this with a simple example.
At The same temperature of 32°F, there are 100 molecules of liquid water present on the surface of the ice. Now what happens in the constant interconversion rates when these 100 molecules are frozen down into the ice at the same time there are also 100 molecules of the ice melting down into the water. So, this rate is constant under normal conditions.
Now, we added salt to the surface of the Ice, and salt was mixed with the 100 molecules of liquid water that were present on the surface of the ice.
Now what happened here is that 100 molecules of water are not able to freeze at the same speed as before and the rate of freezing is disturbed but the rate of melting down is constant so when the 100 molecules of ice melt into liquid it also comes in contact with the salt that break into more and more ions, continuing the melting. So, ice begins to melt before the water can freeze.
This is how salt helps to melt down the ice on the road even below the freezing point of the water. But there is also a limitation for salt as well because as the temperature falls below the freezing point salt loses its efficiency in melting down the ice.
When Does Regular Salt Stop Melting Ice?
Research confirmed that normally a 10% solution of Normal table salt can lower the freezing point of the water to 21oF (-6oC). This means as long as the temperature is above the 21oF (-6oC) the ice will be melted in a 10% salt water solution.
But, if the temperature of the surroundings falls below the level of 21oF (-6oC), you would need to add more salt to further lower the freezing point of the water.
However, sodium chloride becomes much less effective when the pavement temperature drops below 15 degrees. At low temperatures, it’s best to switch to salt formulated for lower temperatures that contain magnesium chloride (MgCl2) or calcium chloride (CaCl2).
MgCl2 and CaCl2 are made up of 3 ions, while sodium chloride (NaCl) is made up of 2 ions. More ions mean more melting power since there are more ions to disrupt bonds between water molecules.
Apart from salts, other substances such as sugar and alcohol can produce the same effect, as anything disrupting the normal structure of water and ice would prevent it from freezing. However, less abundance and a higher cost restrict their use as de-icing agents.