* Impurities: Rock salt typically contains impurities like magnesium chloride and calcium chloride. These impurities lower the freezing point of water even more effectively than pure sodium chloride (table salt).

* Particle Size: Rock salt often has larger, coarser particles. This allows it to dissolve more slowly, providing a longer-lasting melting effect. Table salt dissolves more quickly, leading to a shorter-term impact.

In essence, rock salt is more effective at melting ice due to its chemical composition and larger particle size.