Did you know that 1 + 1 is not always 2?
When 250 mL of water is added to 250 mL of water or when 250 mL of alcohol is added to 250 mL of alcohol, the final volume will always be 500 mL, as expected. However, when the same water is added to an equal volume of alcohol, the final volume is about 10% less than the original volume of the two liquids. How?
When two liquids are physically mixed, the “vanishing volume” or “appearing volume” depends on whether the intermolecular interactions attract or repel. Which may cause contraction or expansion, respectively in the arrangement of the solvent molecules in the mixture versus the pure substances. The strength of the interactions determine the extent of contraction or expansion. The molecular interactions and arrangements in the mixture determines the outcome.