Two changes occurred in the process of dissolution. One is that molecules (or ions) of solute diffuse into water to absorb heat; The other is that molecules (ions) of solute interact with water molecules to generate hydrated molecules (or hydrated ions), and this process releases heat. Different solutes, the two processes absorb or release different heat, which makes the temperature of the solution change. When the oil droplet solute is dissolved, the heat absorbed in the diffusion process is less than the heat released in the hydration process, indicating that the temperature of the solution increases; On the contrary, the temperature of the solution decreases.

The temperature of NaCl solution in aqueous solution has little change;

NH4NO3 absorbs heat when it is dissolved in water;

When NaOH is dissolved, it releases heat.