What experimental evidence supports the hypothesis that impurities lower the melting point while increasing the boiling point of a substance?

The hypothesis that impurities lower the melting point while increasing the boiling point of a substance is supported by various experimental evidence. Here's a step-by-step explanation:

1. Melting Point Depression: When a pure substance melts, its solid and liquid phases are in equilibrium at the melting point. However, when an impurity is added, it disrupts this equilibrium. The impurity particles interfere with the orderly arrangement of the substance's particles, preventing them from forming a solid structure. This means the substance can't solidify at its usual temperature, so the melting point is lowered. This phenomenon is known as 'melting point depression'.

   Experimental Evidence: A common experiment to demonstrate this involves comparing the melting points of pure ice and a salt-ice mixture. Pure ice melts at 0°C, but if you add salt (the impurity), the mixture melts at a lower temperature. This is why salt is used to melt ice on roads in winter.

2. Boiling Point Elevation: When a substance boils, its liquid and gas phases are in equilibrium at the boiling point. Adding an impurity increases the boiling point because the impurity particles disrupt the equilibrium between the liquid and gas phases. The impurity particles make it harder for the substance's particles to escape into the gas phase, which is what happens when a substance boils. This means more energy (a higher temperature) is needed for the substance to boil. This phenomenon is known as 'boiling point elevation'.

   Experimental Evidence: A common experiment to demonstrate this involves comparing the boiling points of pure water and a salt-water solution. Pure water boils at 100°C at sea level, but if you add salt (the impurity), the solution boils at a higher temperature. This is why chefs add salt to water when cooking – it makes the water hotter, so the food cooks faster.