Waves that are activated from an outside source are considered which type of reaction?

• A. Endothermic
• B. Exothermic
• C. Catalytic
• D. Isothermal

Answer: (A)

When energy comes from outside the system to drive a reaction, the process is endothermic. The external energy source—like light waves, heat, or other forms of energy—adds energy to the reactants so they can overcome the activation energy barrier and form products. Because energy is being absorbed, the products end up at a higher energy state than the reactants, and you may notice the surroundings feel cooler as energy is drawn in.

Exothermic reactions, by contrast, release energy to the surroundings, warming them. A catalyst speeds up reactions without changing whether energy is absorbed or released, so it doesn’t define the energy flow. Isothermal simply means the temperature stays the same, which can occur with either endothermic or exothermic processes depending on heat transfer.