A scientist measures the mass of two liquids before and after combining
them. The mass after combining the
liquids is less than the sum of the masses
before. Where is the missing mass? What
type of change occurred? How do you
know?

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)

your answer would be a. 12.5 kg