You're right John! But these were two solutions....that was the sad part...

There is however a phenomenon in solution chemistry called "non additive volumes." It's really pretty neat to see...you take 20 mL of water and 20 mL of an alcohol (say methanol) and pour them together. You would expect to end up with 40 mL of solution...but you don't...it's less than 40 mL...the water and alcohol interact in a way that pulls the molecules together and reduces the volume. That particular student that I was talking about was not wondering if non-additive volumes was going to be a problem....

Katie