Which of the following is a characteristic feature of an exothermic reaction?

Introduction / Context:
Chemical reactions are often classified as exothermic or endothermic based on how they exchange energy with their surroundings. Exothermic reactions release energy, usually in the form of heat, while endothermic reactions absorb energy. Recognising whether heat is released or absorbed is a fundamental part of understanding reaction energetics, fuel combustion, and many everyday processes like burning, neutralisation, and respiration.

Given Data / Assumptions:
• The question specifically asks about a characteristic of exothermic reactions. • We consider simple chemical and physical examples, such as combustion and neutralisation. • The primary focus is on heat exchange, not detailed thermodynamic equations.

Concept / Approach:
An exothermic reaction is one in which the energy of the products is lower than the energy of the reactants, with the difference released to the surroundings, typically as heat. In energy diagrams, the energy level drops from reactants to products, and the environment receives the released energy. This often leads to a temperature rise in the surroundings or the reaction mixture. Common examples include burning of fuels, many oxidation reactions, and neutralisation of acids and bases. Therefore, the central characteristic is the release of heat.

Step-by-Step Solution:
Step 1: Recall the definition: exothermic reactions release energy to the surroundings. Step 2: Identify that this energy is usually observed as heat, sometimes accompanied by light (as in flames). Step 3: Compare with endothermic reactions, which absorb heat and often cause cooling of the surroundings. Step 4: Look at the options and find the statement that best matches the idea of releasing heat energy. Step 5: Conclude that "Release of heat" is the correct characteristic feature of exothermic reactions.

Verification / Alternative check:
Consider the combustion of methane: CH4 + 2O2 → CO2 + 2H2O + energy. This reaction is strongly exothermic and is used in domestic gas burners. The flame and hot cooking vessel are direct evidence that heat is being released. Similarly, mixing a strong acid with a strong base causes the solution to warm up, showing that neutralisation is exothermic. These everyday examples confirm that exothermic reactions are associated with heat release.

Why Other Options Are Wrong:
Option b (Absorption of heat): This describes endothermic reactions, such as melting of ice or photosynthesis, not exothermic ones. Option c (No change in temperature at all): In real systems, exothermic reactions usually cause an observable temperature rise unless heat is immediately removed; no change in temperature is not their defining feature. Option d (None of the options is correct): This is incorrect because option a accurately describes exothermic reactions. Option e (Formation of only cold products): Products of exothermic reactions can be hot or cold depending on conditions; the key point is energy release, not that products are always "cold."

Common Pitfalls:
Students sometimes mix up exothermic and endothermic terms because both involve heat change. A simple memory aid is: "Exo" means "out," so energy goes out to the surroundings in exothermic reactions. "Endo" means "in," so energy goes into the system in endothermic reactions. Also remember that exothermic reactions often feel hot to the touch because they warm the surroundings.

Final Answer:
A characteristic of an exothermic reaction is the Release of heat.