Salt melts ice primarily through a process called freezing point depression. This means that the salt lowers the temperature at which water freezes.
Understanding Freezing Point Depression
The presence of salt in water disrupts the formation of ice crystals. Pure water freezes at 0°C (32°F). However, adding salt interferes with the water molecules' ability to form the ordered crystalline structure of ice. As the reference states, "the salt makes it harder for the water molecules to bond together in their rigid structure". This disruption requires a lower temperature for the water to freeze.
Essentially, the salt ions (sodium and chloride in the case of table salt) get in the way of the water molecules as they try to arrange themselves into a solid (ice) structure. This means the water needs to be colder than 0°C to freeze. If the ice is already at a temperature above the lowered freezing point caused by the salt, it will begin to melt.
How Salt Works in Detail:
- Dissolution: When salt (NaCl) is added to ice, it dissolves into its constituent ions, Na⁺ and Cl⁻.
- Disruption of Crystal Lattice: These ions interfere with the hydrogen bonds between water molecules, preventing the formation of a stable ice crystal lattice.
- Lowered Freezing Point: This disruption lowers the freezing point of the water, causing the ice to melt, even if the surrounding temperature remains below 0°C (but above the new, lower freezing point).
Practical Example:
Think of de-icing roads in winter. Spreading salt on icy roads lowers the freezing point of the water, allowing the ice to melt even if the air temperature is slightly below freezing.
