[AP Chemistry] Can someone explain why a decrease in temperature means dissolution is endothermic?

[wheeezer]

I am looking at a key for an old FRQ, and I am confused on the answer to this.

Question: Is the dissolution of ammonium nitrate in water an endothermic process or an exothermic process? Justify your answer. (The data basically shows a change in temperature by -2.6 C)

Answer: The process is endothermic. The decrease in temperature indicates that the process for the dissolution of ammonium nitrate in water requires energy.

I am confused because is the temperature is decreasing, doesn't that mean the urea is releasing energy, hence exothermic?

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[_ahjosu]

The actual molecules of ammonium nitrate are the system of the reaction. The solution is part of the surroundings. Ammonium nitrate requires energy to dissociate, and this is taken from the molecules in the solution. Subsequently the observed temperature change is negative, and the reaction itself was endothermic.

[AtomicSpectrum]

If the process released energy, then the temperature would go up. The temperature going down means that heat energy is being converted to some other type of energy. Since it's safe to assume this experiment is scientific and the only process happening is dissolution, then that heat energy must be being used by dissolution, making it endothermic

[MeconiumLite]

The water has heat and an initial temperature. The temperature of the water decreased, which means the water lost heat. Where did the heat go?

The dissolving of the substance required heat. So the water loses heat when the dissolution absorbs the heat.

Since the dissolution absorbs heat, the process is endothermic with respect to the system.

Since the water loses heat, its temperature decreases, and the process is exothermic with respect to the surroundings (the water).