Question

What two liquids when mixed occupy less combined volume than the sum of their individual volumes (at room temp, sea-level pressure)? I've heard that mixing 1 unit volume of water with one unit volume of antifreeze occupies less than 2 units of combined volume. Is this true? What is the shrinkage?

Answer 1

If you are talking about ideal solution, The total volume always equals the summed volumes of the pure liquids, but ideal behavior neglects several important facts about real molecules,

two liquids may have may different density,concentration, and may also experience stabilizing or destabilizing interactions with each other at at room temp and sea-level pressure.

If they are interact with each other then there is a slight probably to reduce the volume because some energy transfer between them according to their actual nature.mostly Homogeneous mixture have the total sum of volume after the addition due to the complete Miscibility  

Mixing 1 unit volume of water with one unit volume of antifreeze occupies less than 2 units of combined volume? For that i want to say to you that when we are talking about antifreeze water when it will come at room temprature after addition of normal water, The volume will be the almost around 2 and if you are ignoring human error then It would be perfect 2. (energy Conservation law_Thermodynamics)

Feel Free to ask in case you have any query

Hope you Understood well!