When dilute sulphuric acid is added to lime water, the reaction mixture becomes hot. Which of the following best explains this phenomenon?

Correct Option

The correct option is

It is a neutralization reaction which is exothermic in nature.

Explanation

The reaction between an acid and a base to form salt and water is known as a neutralization reaction. These reactions are chemical changes and are generally accompanied by the evolution of heat.

Detailed Analysis

• Reactants: Lime water contains calcium hydroxide [Ca(OH)₂], which is a base. Dilute sulphuric acid [H₂SO₄] is a strong acid.
• The Reaction: When mixed, they undergo a neutralization reaction:
  Ca(OH)₂ (aq) + H₂SO₄ (aq) → CaSO₄ (s) + 2H₂O (l)
• Exothermic Nature: Neutralization involves the combination of hydrogen ions (H⁺) from the acid and hydroxide ions (OH⁻) from the base to form water molecules. This bond formation process releases a significant amount of energy in the form of heat. Consequently, the temperature of the reaction mixture rises.
• Analysis of Incorrect Options:
  • It is a physical change involving the release of latent heat.: This is incorrect because the reaction involves the formation of new substances (salt and water), making it a chemical change, not a physical one.
  • Lime water naturally releases heat when agitated.: Agitation alone does not generate significant heat; the heat is a result of chemical bond formation.
  • Sulphuric acid absorbs heat from the surrounding air.: The acid does not absorb heat from the surroundings; rather, the system releases heat generated by the internal chemical potential energy change.

Key Takeaway

Neutralization reactions are always exothermic because the formation of O-H bonds in water molecules releases energy, causing the reaction mixture to become hot.