Question

The processes of melting and evaporation involve changes in the ___ of a substance.

Short answer

The processes of melting and evaporation involve changes in the internal energy of a substance.

Step-by-step solution

Understanding Melting

Melting is the process in which a substance changes from its solid phase to its liquid phase. This typically occurs when the substance is heated and reaches its melting point - the temperature at which the substance changes from a solid to a liquid.

Understanding Evaporation

Evaporation is the process in which a substance changes from its liquid phase to its gaseous phase. This occurs when the molecules in the liquid phase gain enough energy to overcome the intermolecular forces holding them together, allowing them to enter the gaseous phase. Evaporation can occur at any temperature, but it is more likely to happen at higher temperatures.

Identifying the Changes

In both melting and evaporation, a substance undergoes a change in its phase (solid to liquid or liquid to gas). This phase change is a result of the substance gaining or losing a specific amount of energy, often in the form of heat. The property that changes during these processes is the internal energy of the substance, which is related to the substance's temperature and the amount of heat transferred to or from the substance.

Completing the Sentence

Based on our understanding of melting and evaporation, we can now complete the sentence: The processes of melting and evaporation involve changes in the internal energy of a substance.

Key concepts

Melting

Melting is a fascinating phase change where a solid becomes a liquid. This transformation happens when a substance gains enough thermal energy to reach its melting point. At this point, the organized structure of the solid begins to lose its rigid arrangement, and the particles start moving more freely in a liquid state.
The act of melting doesn't just happen abruptly. You have probably noticed ice melting into water. It begins at the surface where it interacts with its environment, absorbing heat gradually. This absorbed energy is used to break the rigid bonds in the solid structure.

• Melting point: The specific temperature at which this process occurs.
• Energy absorption: Essential for breaking the rigid structured bonds.

Melting is significant in many natural and industrial processes, making it a fundamental concept in science.

Evaporation

Evaporation is the process by which liquid transforms into vapor without reaching boiling point. It's a slower process than boiling and can happen at any temperature. How does it work? It's all about energy and intermolecular forces.
To evaporate, liquid molecules need to gain enough energy to break free from the cohesive forces holding them together. This energy often comes from surrounding air or direct sunlight. When these molecules gain enough kinetic energy, they "vaporize" into the air.

• Occurs at any temperature: Unlike boiling, evaporation isn't limited to extreme heat.
• Energy gain: Necessary for molecules to escape the liquid phase.

Plant leaves, human skin, and large water bodies all showcase evaporation, a critical process for cooling and water cycle systems.

Internal Energy

Internal energy is the total energy stored within a substance. It plays a critical role in phase changes like melting and evaporation. This energy points to both the kinetic energy (movement of particles) and potential energy (stored in bonds between particles) within a substance.
When a substance undergoes a phase change, its internal energy either increases or decreases. For melting and evaporation, the internal energy increases as the substance absorbs heat energy.

• Kinetic energy: The motion and speed of molecules.
• Potential energy: Stored energy due to molecular arrangements.

The significance of internal energy is that it helps explain why energy changes don't always lead to temperature changes during phase transitions.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring particles (like atoms or molecules). These forces determine many physical properties and behaviors of substances. They are especially important when understanding phase changes like melting and evaporation.
During melting, intermolecular forces reduce as the solid structure loosens into a more fluid arrangement. For evaporation, molecules must fully overcome these forces to escape as gas particles.

• Types of forces: Include hydrogen bonding, dipole-dipole interactions, and Van der Waals forces.
• Impact on properties: Influence boiling points, melting points, and more.

These forces are the "glue" that holds substances together; understanding them is key to grasping why and how phase changes occur.