Answer:
This reaction could be an endothermic reaction reaction. In these reactions, heat is created, eventually being released. If the two liquids created a strong enough heat, some of the liquid could have evaporated and escaped as vapor, effectively decreasing the mass of the liquid mix. This would mean that the mass is either vapor in the air or has collected into droplets, most likely near the container of liquid
Hope this helps!
(note: I am NOT a chemist or exceptionally good at chemistry, so take that for what its worth)