why are volumes of different liquids not additive? c?

Question

based on our experiment, combining 2 mL water & 6mL acetone will not gain 8mL liquid

Answer 1

Liquids which are soluble in each other will go into solution when mixed together. Solutions of two liquids Always occupy less space than the sum of both volumes. This is because the act of dissolving species A in species B means that species A will be fitting within the gaps between the molecules of species B.
This is going to sound terrible but thats really only partially true. For one example species A ends up fitting 50% of itself between species B's. So it ends up expanding the B-B by 50% of species A size. Thats still less volume that it would have been. If species A and B were each similar volume, then there will be a 25% reduction from the expected additive volume.
Species B is ---- B____B____B
Species A is ---- A____A____A
solutions
expectation-B____B____B____A____A____A
solution is---B___A___B___A___B___A

So you can see the distance between B's actually expands from the just B's liquid, but you still fit more in less space

Answer 2

Volume is not additive because it depends on the interactions of the liquids' molecules. Each component's contribution to the volume is given by a quantity called molar partial volume. (It's the same with any other extensive thermodynamic properties, such as enthalpy or Gibbs free energy). A molar partial quantity is defined as the derivative of the quantity with respect to the moles of the component given that the composition asides that component remains constant.

BTW, since molar partial volumes can be either possitive or negative, the resultant volume of the solution can be higher or lower than the volumes of the isolated liquids.

Answer 3

In some cases two liqids can mix forming a new solution which has a different density and the volume can vary