During a lab activity, students prepared two solutions separately at the same temperature. later they mixed the solutions and the temperature dropped. why?

The answer is Energy was absorbed during the chemical reaction.

When energy is released from the chemical reaction, it is expected that solution will become warm and that the temperature will increase. But, in this example, the temperature decreased. This means that the energy was not released, but on the contrary, the energy was absorbed. When energy is absorbed, the temperature must decrease.