A student at a temperature of 0 degrees C adds the 25.0 g of NaCl crystals to a mixture of 25 mL of ice water in a coffee cup calorimeter. He stirs the ice water salt mixture for a few minutes with at thermometer and observes that some of the ice has melted and the temperature are now -18 degrees C. Explain why the ice melted and the drop in temperature of the mixture.