Question

A compound with an incongruent melting point decomposes on heating into (a) a liquid of the same composition as solid (b) a new solid phase and a solution with a composition different from that of the solid phase (c) a new solid phase and a solution with the same composition as that of the solid phase (d) a solution of fixed composition.

Short answer

The correct answer is (b) a new solid phase and a solution with a composition different from that of the solid phase.

Step-by-step solution

Understanding Incongruent Melting Point

Incongruent melting point refers to the temperature at which a compound starts changing from solid state to liquid state, but instead of the entire compound converting into a homogeneous liquid, a part of it changes into a new solid phase, and the remaining part forms a second liquid phase. Both phases formed have different composition from the original solid.

Identifying the Correct Option

Given this definition, the closest matching option is (b) a new solid phase and a solution with a composition different from that of the solid phase. This option accurately describes the process that occurs when a compound with an incongruent melting point is heated. The solid phase does not convert entirely into a homogeneous liquid, but forms a new solid phase and a solution.

Key concepts

Phase Transition

When we talk about phase transitions, we're referring to the changes that occur when a substance goes from one state of matter to another—like ice melting into water, or water evaporating into steam. These transitions typically occur at a specific temperature and pressure. However, not all transitions are straightforward. When a substance has an incongruent melting point, it doesn't just go from solid to liquid. Instead, it decomposes, resulting in a new solid forming alongside a liquid phase with a different composition than the original material. This is a fascinating exception to the more common congruent melting, where a pure substance transforms directly into a liquid with the same composition.

Understanding the nuances of phase transitions is crucial in diverse fields, from material science to meteorology, because it influences the physical properties and the stability of a substance under different environmental conditions.

Compound Decomposition

The term compound decomposition might conjure up images of something breaking down over time, like leaves decomposing in a forest. But in a chemical context, it means something quite specific—the breakdown of a compound into other substances when energy is applied, such as heat. During incongruent melting, a compound undergoes decomposition as it transitions from a solid phase. This generates a new solid phase and a liquid phase, each with its own distinct composition. This process can be critical in understanding how substances change under heat, which is not only interesting for scientists but also essential in industries dealing with materials processing, where controlling the phases of a material is often key to achieving desired properties.

In the case of incongruent melting, the original solid doesn't simply liquefy; it chemically alters, creating new compounds that may have radically different characteristics from the starting material. This phenomenon showcases the transformative power of heat in the realm of chemical substances.

Solid-Liquid Phase

In the realm of solid-liquid phases, the focus is on substances that exist partly in solid form and partly in liquid form simultaneously. This co-existence of phases plays a central role in the interpretation of melting points. While most substances usually convert directly from solid to liquid at their melting point, an incongruent melting point involves the coexistence of a solid and liquid phase but with a twist—the phases are not consistent in composition. This departure from the norm is fascinating as it challenges our understanding of how substances change phase. It illustrates that the journey from solid to liquid can be a complex process involving the creation of entirely new substances.

As such, the study of solid-liquid phases is especially relevant in material science and engineering, where understanding and controlling these phase relationships are pivotal. For example, in the creation of alloys or the synthetic growth of crystals, knowing how different compositions affect the solid-liquid equilibrium can be the difference between success and failure.

Chemical Thermodynamics

Diving into chemical thermodynamics, this subject gives us the tools to predict and explain energy changes within chemical systems. It intertwinely connects to the theory of phase transitions and compound decomposition. Chemical thermodynamics provides a framework for understanding how heat energy affects the stability and transformations of substances. As such, when a compound with an incongruent melting point decomposes, thermodynamics explains it as an energy redistribution process—some of it breaks bonds in the solid phase to form a new solid, while the rest creates a separate liquid phase.

The principles of thermodynamics are also instrumental in determining the conditions under which phase transitions occur. In the context of an incongruent melting point, it helps describe how heat can lead to decomposition and the formation of phases with new compositions. For students and professionals alike, a strong grasp of chemical thermodynamics is essential for interpreting these complex phase-related phenomena, which underscore much of the physical change observed in chemical systems.