An unknown compound (x) melts at 60°C and is mixed with a compound (y) that melts at 45°C and is soluble in x. Will the Melting point for the mixture start below 45°?

I think so, because if you look at ice and salt for an example, salt has a way higher melting point than water does, but it lowers the melting point of water anyway. This is the opposite of the question though. Y is soluble in X and has a lower melting point, but salt has a higher melting point and is soluble in water. I don't know if this is a good way to think about the question.

The melting point of a mixture is lower than the melting point of either pure compound.

To determine whether the melting point of the mixture will start below 45°C, we can consider the principles of colligative properties.

Colligative properties are characteristics of a solution that depend on the number of solute particles and not their specific identities. One important colligative property is the depression of the freezing point.

When a solute dissolves in a solvent, it disrupts the regular arrangement of solvent particles, making it more difficult for the solvent to freeze. This leads to a decrease in the freezing (melting) point of the solvent. The greater the concentration of the solute, the greater the depression of the freezing point.

In the given situation, compound y is soluble in compound x. This means that when they are mixed, compound x acts as the solvent and compound y acts as the solute. Since compound y has a lower melting point than compound x, it will depress the melting point of compound x.

Therefore, when the two compounds are mixed, the mixture's melting point will be lower than the melting point of compound x (60°C). However, it is uncertain specifically how low the melting point of the mixture will go without additional information on the relative concentrations of the compounds.