Salt melts sea ice because the salt ions interfere with the hydrogen bonds in the ice, disrupting its crystalline structure and lowering its freezing point.
Here's a more detailed explanation:
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Freezing Point Depression: Salt dissolved in water lowers the freezing point of the solution compared to pure water. This is a colligative property, meaning it depends on the concentration of solute (salt) particles, not their identity.
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Disruption of Hydrogen Bonds: Ice is formed by water molecules arranged in a highly ordered crystalline structure held together by hydrogen bonds. When salt (like sodium chloride, NaCl) dissolves in the water that's starting to freeze, it dissociates into sodium ions (Na+) and chloride ions (Cl-).
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Hydration of Ions: These ions are attracted to the polar water molecules. As the provided reference mentions, the ions on the surface of the salt get hydrated by water molecules in the semi-liquid layer of the ice.
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Weakening of Ice Structure: The hydration of the ions interferes with the formation of strong hydrogen bonds between water molecules. This disrupts the ice crystal structure, making it more difficult for the water to freeze and easier for the ice to melt.
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Surface Layer Effect: The presence of salt ions requires the water molecules to re-orient themselves to accommodate the ions. This change modifies the thickness of the surface layer of the ice (the interface between the ice and liquid water), influencing the melting process. This helps more of the ice break up to ensure appropriate thickness of the ice layer, which depends on temperature.
In essence, salt doesn't "burn" through ice. Instead, it lowers the temperature at which the ice can exist as a solid. If the ambient temperature is above this newly lowered freezing point, the ice melts.
