When calculating the change in boiling point or freezing point, what factor is usually considered?

The change in boiling point or freezing point of a solution is primarily influenced by the number of solute particles present in the solution, which directly relates to the colligative properties of the solvent. Colligative properties depend on the ratio of solute particles to solvent molecules rather than the identity of the solute.

When a solute is added to a solvent, the addition of solute particles disrupts the solvent's ability to form a stable structure, which alters its physical properties, such as boiling and freezing points. For example, in freezing point depression, the presence of solute particles interferes with the formation of the solid phase of the solvent, requiring a lower temperature to reach freezing. Conversely, in boiling point elevation, solute particles increase the energy required for the solvent to transition into a gas, thus raising the boiling point.

The number of solute particles is crucial as it provides a measure of the effect the solute has on the solvent's properties. This concept is fundamental in determining changes in boiling point and freezing point because it underscores that the greater the number of dissolved particles, the more significant the effect on the physical properties of the solvent.