Why don't ethanol/water mixtures have additive volumes?

Ethanol/water mixture don't have additive volumes for several reasons. When ethanol and water are combined, the smaller ethanol molecules fill the empty spaces between the larger water molecules. Another reason is because water molecules are known for forming strong hydrogen bonds due to the polarity of the water molecule. The oxygen atom in water is more electronegative than the hydrogen atoms, creating a partial negative charge on the oxygen and partial positive charges on the hydrogens. This leads to the formation of hydrogen bonds between water molecules. Additionally, water's dipole moment is primarily due to its strong hydrogen bonding, while ethanol has a dipole moment arising from the difference in electronegativity between carbon and oxygen. When these polar molecules are mixed, dipole–dipole interactions occur between water and ethanol molecules, further influencing their arrangement. These forces lead to a non-additive process, where the combined volume of water and ethanol is less than the sum of their individual volumes.