Which one among the following substances evolves heat when dissolved in water?

The dissolution of calcium oxide (CaO), also known as quicklime, in water is a highly exothermic combination reaction. When CaO reacts with water, it forms slaked lime (calcium hydroxide) and releases a significant amount of heat [2]. This process, known as lime slaking, is so energetic that it can cause the water to boil or melt plastic containers. In contrast, the dissolution of potassium nitrate (KNO3) is an endothermic process, meaning it absorbs heat from the surroundings, causing the solution temperature to drop. Sodium chloride (NaCl) has a very small positive enthalpy of solution, making its dissolution slightly endothermic or nearly athermal under standard conditions. Glucose dissolution is also endothermic as it requires energy to break the intermolecular hydrogen bonds in the solid crystal lattice. Therefore, only calcium oxide evolves heat upon dissolution.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 1: Chemical Reactions and Equations > Figure 1.3 > p. 6
2. [2] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures > Activity 8.1: Let us experiment > p. 118