Final answer:
Adding salt to ice results in freezing point depression, which lowers the temperature at which ice can melt. This process alters the balance of energy needed for water molecules to escape the ice lattice and become fluid. The effectiveness of salt in melting ice is temperature-dependent and decreases with significantly lower temperatures.
Step-by-step explanation:
Understanding How Salt Melt Ice
Adding salt to ice causes a phenomenon known as freezing point depression, which lowers the temperature at which ice can melt. When salt is spread on ice, it dissolves into the liquid water that's always present on the surface, even when the ice is below 0°C. This solution has a lower freezing point than pure water, meaning that the ice will melt at a lower temperature. Moreover, as the solubilities of salts decrease with decreasing temperature, there's a limit to how much the freezing point can be depressed, regardless of the amount of salt used. If the temperature is significantly below the effective threshold for the salt to work (-35°C for example), then the salt will not cause the ice to melt until the ambient temperature rises.
When heat is removed from water at its melting point, the water begins to freeze, changing from a liquid to a solid phase. This process releases energy equal to the heat of fusion. When liquid water freezes into ice, the physical composition remains the same, hence it's a physical change. Interestingly, water is among the rare substances whose crystalline solid form (ice) is less dense than its liquid form, which relates to the unique properties of water's phase diagram. These properties influence how ice melts, specifically how pressure can affect the melting point of ice.
In essence, when you add salt to ice, it interferes with the freezing process by lowering the temperature at which water can re-freeze, causing the ice to melt more readily. The heat absorbed in the process of melting ice without salt is referred to as the latent heat of fusion, and the presence of salt changes the balance of energy required for water molecules to break free from the ice structure and become liquid water.