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· Energy is drawn in from the external environment, causing its surroundings to loose heat, or “cool down.”

Excited but a bit confused, Sam and Julie run to their chemistry teacher. Sam asks, “Teacher, why did my flask turn cold after adding the salt to water, while Julie’s flask turned hot?”The teacher replies: “That’s because you were given two different salts. One of your salts generated an endothermic reaction with water, while the other salt generated an exothermic reaction with water. Let me first reveal the identity of your salts: Salt A is ammonium nitrate (\text{NH}4\text{NO}3NH4NO3N, H, 4, N, O, 3) and Salt B is calcium chloride (\text{Ca}\text{Cl}2CaCl2C, a, C, l, 2).”Now, Sam and Julie are curious about the difference between an endothermic and an exothermic reaction.Consider the reaction mixture—salt plus water—as the system and the flask as the surrounding.In Sam’s case, when ammonium nitrate was dissolved in water, the system absorbed heat from the surrounding, the flask, and thus the flask felt cold. This is an example of an endothermic reaction. In Julie’s case, when calcium chloride was dissolved in water, the system released heat into the surroundings, the flask, and thus the flask felt hot. This is an example of an exothermic reaction.The reaction going on in Sam’s flask can be represented as:

NH4NO3 (s) + heat ---> NH4+ (aq) + NO3- (aq)

NH4NO3 (s) + heat —> NH4+ (aq) + NO3- (aq)You can see, heat is absorbed during the above reaction, lowering the temperature of the reaction mixture, and thus the reaction flask feels cold.The reaction going on in Julie’s flask can be represented as:

CaCl2 (s) + 2(H2O) ---> Ca(OH)2 (aq) + 2 HCl (g) + heat