Here's why:

* Lower freezing point depression: Calcium chloride has a lower freezing point depression than sodium chloride. This means that it can lower the freezing point of water to a greater extent, making it more effective at melting ice at lower temperatures.

* More ions released: Calcium chloride dissociates into three ions in solution (one calcium ion and two chloride ions), while sodium chloride dissociates into only two ions (one sodium ion and one chloride ion). The greater number of ions in solution from calcium chloride leads to a greater freezing point depression.

* Hygroscopic nature: Calcium chloride is highly hygroscopic, meaning it readily absorbs moisture from the air. This further contributes to its effectiveness in melting ice by providing more water molecules to dissolve and lower the freezing point.

In practical terms, calcium chloride is often used in road de-icing applications because it can be effective at lower temperatures than sodium chloride.

However, it's important to note that:

* Calcium chloride can be more corrosive to concrete and other materials.

* Both calcium chloride and sodium chloride can damage vegetation and soil.

* Using excessive amounts of either salt can negatively impact the environment.