When it’s winter and the roads and highways are covered with ice, we often see that salt is used to dissolve the ice. Sugar can also be used, but it is not that effective. So, how does salt work and why does salt melt ice faster than sugar is still not a simple matter to understand easily.
Ice is melted faster using salt than sugar because of the colligative property. The colligative property of salt determines how fast or slow the ice will melt.
What is Colligative Property:
Colligative property depends on the number of dissolved particles in a solution but not upon the solute’s identity.
Salt Or sugar decreases the freezing point of water. It binds with the molecules of water and creates more space between them. This helps to overcome the electrostatic forces, which binds them into a solid structure. The same story occurs for any kind of substance, which is water dissolving.
For pure water and salty water, the freezing point of salty water is less. This is due to the presence of salt dissolved in the water. Salinity in the water resists freezing, which is why it is used on icy roads in winter.
What is the freezing & freezing point?
The molecules move freely and quickly in warm water. It makes the liquids fluid and is able to change into any shape of pot or container you pour them into. But as soon as the water starts to freeze, the movement of the molecules slows down. When the water turns into ice, the molecules stick them together in a process. It is called freezing.
Salt acts to decrease the freezing or melting point of water in order to melt down the ice quickly. Ice melts and becomes liquid when its surrounding temperature is equal to or more than 32degrees Fahrenheit. This temperature, 32degree Fahrenheit, is known as the Freezing point.
What happens when a substance is added to the water:
The freezing point of pure water is 32 degrees Fahrenheit, and it is constant. Two things happen when a substance is added to the water:
- Reduce the freezing temperature of the water.
- Differs the freezing temperature of the water
The freezing temperature varies continuously due to the addition of different types of substances. When sugar is added with water, the melting rate of water changes, and when salt is added with water, the melting rate also changes but differently.
At low temperatures, sugar does melt ice, but it does not work as efficiently as salt does.
Does salt lower the freezing point more than sugar?
Salt and sugar both are water dissolving elements. Both can be used to lower the freezing point of water. But salt melts the ice faster than sugar.
Yes, salt lowers the freezing point more than sugar. We can understand this through a small experiment.
Take three empty bottles. Fill one with tap water, another with a saturated solution of salt, and the other with a saturated solution of sugar. Put all the bottles in a deep freezer. After some time, take the bottles out of the freezer. You will see that the bottle filled with tap water freezes as usual. The bottle filled with sugar water has frozen patches, but the bottle filled with salt doesn’t freeze at all. This experiment easily proves that salt melts ice faster than sugar.
This overall experiment demonstrates the Freezing Point Depression.
Freezing Point Depression of Salt vs. Sugar:
The temperature at which a liquid is converted to solid is called the freezing point. Freezing Point depression is an observation where a pure substance, i.e., water, has a specific freezing/melting point, but with the addition of an impurity (salt or sugar), the freezing point gets lower than before. If the amount of impurity is high, the melting/freezing point will be lower.
Since Freezing or Melting Point Depression is a colligative property, for the colligative property of solution, the type of solute doesn’t matter. The thing which matters most is the number of molecules of solute. If you make two solutions using salt and sugar and compare between them, the solution made with salt will be able to lower the freezing point more than that of the sugar solution. This is because there are more molecules in One gram of salt than in One gram of sugar.
The salt has the ability to lower the melting point more than that of the sugar. This is because the chemical bond of salt is in ionic form, and the chemical bond of sugar is in covalent form. As the salt is bonded in ionic form, its ions are able to dissociate in solution fully.
Why does salt melt ice faster than sugar?
Salt functions to decrease the freezing or melting point of water. The salt gets to mix with the crystals of ice, and thus interfering with the liquid water over the melting ice makes the melting process of ice quick. Ice melts and becomes liquid when its surrounding temperature is equal to or more than 32 degrees Fahrenheit (0degrees C). This temperature, 32degree Fahrenheit (0degrees C), is known as the freezing point. When salt and water are added together, the freezing point drops down.
When salt is left on ice crystals, the water molecules of liquid that have not turned into ice dissolve a little amount of salt. Due to this, a saltwater solution is formed called “Brine.” The brine is a solution that prevents the freezing of liquid water molecules. As the ice starts turning into liquid form, the water molecules get free from the ice, accelerate their speed, and dissolve more of the salt. Salt lowers the freezing point of water(32defree F) after continuously getting dissolved. When more than 10% salt solution is added, the melting point drops to 20degree Fahrenheit. When more than 20% solution is added, the melting point comes to 2degre Fahrenheit. But, if the temperature is below 15degre Fahrenheit, the salt might not perform its job. Water is a must required for the salt to melt ice. If the water is itself frozen in solid form, the salt cannot perform its job.
Salt will always melt the ice faster. But sugar also is not that slow in melting ices. Sugar is capable of melting ice by getting dissolved in water but not as efficient as salt. Sugar usually melts ice by lowering the melting and freezing points of water, the same as salt does—sugar interfaces with the water molecules by getting dissolved on ice. The water molecules bind the crystals of ices in order to freeze. Before the water molecules get into touch with the crystals of frosts, the presence of sugar requires a much lower temperature. As said earlier, ice can be melted with sugar and can lower its freezing point, but it is not efficient as salt. In the salt, there are ions like sodium and chloride. The salt breaks down into sodium and chloride ions & it will add the two components to the solution, which will provide more interference and prevent the water molecules from freezing.
How does salt melt ice?
The actual reason why & how salt melts ice is that the freezing point of the solution of water and salt dissolved is lower than that of pure water. When salt is added to ice, its temperature drops significantly. It first dissolves the coated part that is present on the surface of the liquid water, and therefore, the freezing point gets lower and comes below the temperature of the ices. The salty water thus melts the ice that comes in contact with it and creates more liquid water. This liquid water dissolves more salt, and more ice melts, and the process continues. If the concentration of the salt is higher, its freezing point will be lower. Water which contains the maximum amount of salt dissolved, the freezing point of that water is Zero degree Fahrenheit. So, if the temperature is under zero degrees Fahrenheit, the ice on the roads and sidewalk will not melt.
How does sugar melt ice?
It is not that accurate to say sugar melts ice. Sugar, actually by lowering the melting and freezing points, keeps the water in the liquid state even when the temperature is very cold. Sugar does this by providing space among the water molecules and decreasing their attraction towards each other. At 30degrees Fahrenheit, if you put some sugar on ice, it will easily melt. But if the temperature moves below, the ice will eventually freeze. The melting or freezing point is less than that of pure water but will be more if you throw some salt on the ice crystal. Sugar is capable of melting ice by getting dissolved but not as efficient as salt. As said earlier, ice can be melted with sugar and lower its freezing point, but it is not efficient as salt. This is because there are sodium and chloride ions in salt.
Both salt and sugar melts ice and causes freezing point depression. But If anyone asks you, Why does salt melt ice faster than sugar? Then simply, you would answer that ice is melted faster using salt than that of sugar because of the colligative property of salt that determines how fast or slow the ice will melt. Salt is more efficient as salt breaks down into sodium and chloride and dissolves its two components in water by creating more interference in preventing water from freezing.
Salt can melt ice more effectively than sugar. In almost all the North American and European countries, when their roads, highways, foot walks get covered with ice, they use salt to melt those ice faster. This gives them a good result and the use of salt for this purpose has increased nowadays.