Answer:
The example of endothermic reaction is when a flask turns cold in a reaction.
Step-by-step explanation:
The example of endothermic reaction is "when a flask turns cold in a reaction". In endothermic reaction, energy is absorbed from the surroundings, it decreases in temperature. If a flask turns cold in a reaction, it indicates that the reaction absorbed heat from the flask and from the surrounding, makes it an endothermic reaction.
In contrast, sodium mixed with water and the combustion of methane are both exothermic reactions because they release heat and raise the temperature of the surroundings. The condensation of steam is a physical change, not a chemical reaction, and does not involve the absorption or release of heat.