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Why Does Salt Melt Ice?

Why Does Salt Melt Ice?

Do you want to know how and why salt melts ice? It is this property of salt which makes it a popular choice for melting ice from the sidewalks during winters.
Rohini Mohan
The freezing point for water is 32º F (i.e. 0º C), but salt reduces the water's freezing point when mixed with it. This is why salt is often used in cold areas so as to make the snow melt faster. Simply put, even if the weather is freezing cold, the snow melts when combined with salt. Let's have a look at the effect of salt on ice, in detail.
Basically, salt has a peculiar tendency of absorbing moisture, and therefore, it absorbs most of the water present on the surface of ice and snow. This water is usually not visible to the naked eye. Once the salt absorbs moisture, there isn't sufficient water left which can be turned into ice. Whatever moisture has been absorbed will take a longer time to freeze over, since its freezing point has now been altered or brought down. Let us now find out how salt manages to melt ice faster?
Why Does Salt Melt Ice and Snow?
This phenomenon is scientifically known as the Freezing Point Depression. Salt being an impurity or a foreign particle dissolves in water, which thereby reduces the freezing point of water. Salt stops dissolving into water when it reaches its saturation point. Another reason as to why salt displays these peculiar tendencies, is because salt being a solvent as well, has solutes or dissolved particles within it. This changes its state according to the substance in which it is dissolved.
Since NaCl has a 1 ion each for each molecule of sodium and chloride, its tendency to bring down the freezing point of water is limited. Thus, the salt is able to melt ice down to around -9º C or 15º F. The reason being that the more the number of ions per molecule, the higher the substance's potency to bring down the freezing point of water. Nonetheless, salt is preferred over other matters simply because it is not harmful, nether is it toxic and nor it is expensive. It is an easily available as well as an extremely affordable natural substance, and therefore is preferred for preventing snow on sidewalks and roads from freezing over.
Experiment
There is a simple experiment which you can try at home. Take three steel containers A, B and C. Fill A with simple water, fill B with water mixed with lots of salt so as to make a saturated solution and finally fill C with saturated sugar solution mixed with water. Now place all the three containers into the freezer and leave it be for about 5 hours. When you finally check the results you will find the following results:
  • Container A holding normal water would freeze as usual, like how water always does.
  • Container B holding the saturated salt solution would have a slushy solution of ice.
  • While container C holding saturated sugar solution would remain inert and would be simply liquid with no ice crystals.
Of course the pace at which salt melts ice would depend on several factors such as the amount of ice available to the ratio of salt poured on it and most importantly the temperature outside. If the temperature is below the freezing point of water, then salt will have absolutely no effect on the ice and it will simply leave a powdery residue behind. For example, salt will stay inert in places such as Antarctica, which has an annual temperature which is far below the freezing point of water.
It is because of this ability that salt is used during winters to keep the sidewalks frost-free. This helps prevent people from falling and it also prevents vehicles from skidding on roads, and therefore, protects both life and property.