Step 1: Think in terms of escaping tendency of water molecules. In pure water all the surface sites are occupied by water molecules, giving a certain vapour pressure. In salt water, dissolved Na+ and Cl- ions occupy part of the surface and hold water back, so fewer molecules escape and the vapour pressure drops.
Step 2: For boiling, the solution's vapour pressure has to be raised until it equals atmospheric pressure. Since it started lower, extra heating is needed, so the boiling temperature is higher than that of water. This elevation is proportional to the molal concentration of particles.
Step 3: For freezing, ice and liquid must have equal vapour pressure. The solute lowers only the liquid's vapour pressure, so equality is reached at a temperature below 0 degree C, meaning the solution freezes lower than water.
Step 4: Because NaCl splits into two ions per formula unit, the effective particle count nearly doubles \( (i \approx 2) \), strengthening both the elevation of boiling point and the depression of freezing point.
Result: Salt solution boils above 100 degree C and freezes below 0 degree C.