Which of the following is an exothermic reaction?
a) melting ice
b) iron rusting
c) dissolving sugar in water
d) dissolving ammonium nitrate in water

The exothermic reaction among the choices is B. iron rusting. An exothermic reaction is a reaction that releases heat. Rust is formed from the reaction of iron with oxygen. This reaction releases heat, hence, an exothermic reaction.

Further Explanation:

A. melting ice is an endothermic process. An endothermic process requires the absorption of energy from the surroundings. Heat is needed to change ice to liquid water. That is why, when ice is removed from the freezer and left on a warmer place it melts. The heat from the surroundings is absorbed by the ice causing it to change to liquid phase.

B. iron rusting is exothermic. Heat is released when oxygen and iron react to form rust.

C. dissolving sugar in water is an endothermic process. Heat is needed to break up the intermolecular forces of attraction holding the sugar molecules together. Also, adding heat makes the process of dissolution faster

D. dissolving ammonium nitrate in water is an endothermic process. A characteristic of endothermic reactions is that it turns the container of a substance colder. As the substance absorbs heat from the surroundings, the temperature of the surroundings, including the container, decreases. In exothermic reactions, heat is released to the surroundings making the container feel hotter during and after the reaction.

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Keywords: exothermic reaction, endothermic reaction