Final answer:
A chemical reaction occurs within a system, which is the specific part of the universe under study, and is surrounded by the rest of the universe, together referred to as the surroundings.
Step-by-step explanation:
A chemical reaction occurs within a focused finite part of the universe, an environment called a system. To study the flow of energy during a chemical reaction, we define system as the small, well-defined part of the universe in which we are interested, such as the particular chemicals that are reacting. The surroundings are everything else, including the container in which the chemical reaction is being carried out and the rest of the universe.
During a reaction, energy can be transferred from the system to the surroundings or vice versa, in accordance with the law of conservation of energy, which states that energy is neither created nor destroyed, ensuring that the total energy of the universe is conserved. For example, in exothermic and endothermic processes, energy is exchanged between the system and its surroundings.