Which of the following is an exothermic reaction?

a. dissolving ammonium nitrate in water to cool the water

b. condensation

c. melting ice

d. dissolving sugar in water

The correct answer is Option B.

Explanation:

Exothermic reaction is defined as the reaction in which heat is released to the surroundings from the system. The total enthalpy change of the reaction comes out to be negative.

Endothermic reactions are the ones where heat is absorbed from the surrounding by the system. The total enthalpy change of this reaction comes out to be positive.

For the given options:

Option A: When ammonium nitrate is added in the water to cool it, the heat is getting absorbed. Then only the water is getting cooled. Thus, it is considered as an endothermic reaction.

Option B: Condensation is a process in which gaseous state changes into liquid state. The temperature of the system gets reduced and thus heat is getting released in this process. Thus, it is considered as an exothermic reaction.

Option C: Melting is a physical process in which solid state is converted into liquid state. To do this, heat must be provided to separate the particles from one another. Thus, it is considered as an endothermic reaction.

Option D: When sugar is dissolved in water, heat is provided so that it gets completely dissolved. Thus, it is considered as an endothermic reaction.

Hence, the correct answer is Option B.

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