Which energy change occurs during boiling?
Some heat energy of the substance is lost.
Heat energy of the substance remains the same
Most heat energy of the substance is lost.
Heat energy is absorbed by the substance.
The Correct Answer and Explanation is :
The correct answer is “Heat energy is absorbed by the substance.”
When a substance boils, it undergoes a phase change from liquid to gas. This process requires energy, specifically in the form of heat, to overcome the intermolecular forces holding the liquid molecules together.
Explanation:
Boiling occurs when a liquid reaches its boiling point, which is the temperature at which the vapor pressure of the liquid equals the atmospheric pressure. At this point, the liquid molecules gain enough kinetic energy to break free from the surface of the liquid and enter the gas phase. This process involves the absorption of heat energy, known as latent heat of vaporization.
- Absorption of Heat Energy:
During boiling, heat is continuously supplied to the liquid. This energy doesn’t increase the temperature of the liquid but is used to break the bonds between molecules. The temperature of the liquid stays constant during boiling, even as heat is absorbed. - Molecular Behavior:
In the liquid state, molecules are held together by intermolecular forces (such as hydrogen bonds, Van der Waals forces, or dipole-dipole interactions). As heat energy is absorbed, the molecules vibrate faster, increasing their kinetic energy. At the boiling point, enough energy is provided for the molecules to escape the liquid phase and become vapor. - Latent Heat:
The heat energy absorbed during boiling is called latent heat because it doesn’t result in a temperature change but is instead used for the phase transition. This heat energy is necessary to overcome the forces that hold the molecules in the liquid phase. - No Energy Loss:
The statement that energy is “lost” or “remains the same” during boiling is incorrect because boiling requires a continuous influx of heat energy to facilitate the transition to the gas phase.
In summary, boiling is an endothermic process where heat energy is absorbed by the liquid to enable the phase change from liquid to gas.