If an exothermic reaction occurs in a reaction mixture, then it means heat will be liberated into the surroundings. This is due to the fact that product's energy must be always less than that of reactant's energy for an exothermic reaction to occur. Thus, enthalpy change is negative for such reactions. In these kinds of reactions, heat is liberated into the surroundings. Thus, the surrounding become hotter. Thus, the surrounding's temperature gets increased in case of exothermic reactions.
For ex- condensation of water vapour into rain