Problem 50 - Chemistry The Molecular Science

Question

Give a nanoscale interpretation of the fact that at the melting point the density of solid mercury is greater than the density of liquid mercury, and at the boiling point the density of liquid mercury is greater than the density of gaseous mercury.

Step-by-Step Solution

Answer

Solid mercury is denser than liquid due to tightly packed atoms; gaseous mercury is less dense than liquid due to spreading of atoms.

Step 1: Understanding Solid to Liquid Transition

At the melting point, solid mercury transitions to liquid mercury. In this state, the atoms in solid mercury maintain a fixed, orderly arrangement, allowing them to be packed closely together. In contrast, when mercury melts, the atoms move more freely and spread apart, increasing the atomic spacing and decreasing the overall density.

Step 2: Understanding Liquid to Gas Transition

At the boiling point, liquid mercury becomes gaseous mercury. In the liquid state, while mercury atoms move more freely than in a solid, they remain relatively close due to intermolecular forces. When mercury boils, atom dispersion increases significantly as these forces weaken, leading to a larger volume occupied and, consequently, a lower density in the gaseous state.

Step 3: Conclusion on Density Changes

The density hierarchy is a result of the arrangement and movement of mercury atoms. In solids, atoms are closely packed, making solids denser than liquids. As liquids transition to gases, atoms disperse even further, decreasing density considerably in the gaseous state.

Key Concepts

Solid to Liquid Transition

When solid mercury melts into a liquid, a fascinating transformation occurs at the atomic level. In its solid form, mercury atoms are tightly packed in a structured lattice. This orderly arrangement allows the atoms to be closely bound, resulting in a higher density.
Once heat is applied and the melting point is reached, atoms gain energy and start moving more freely. This added freedom lets them rearrange into a less ordered structure. When atoms are no longer fixed in a rigid pattern, they spread apart, increasing the distance between them, which leads to a decrease in density.
It's important to understand that during this transition:

• The atomic motion increases significantly.
• The orderly structure of the solid breaks down.
• The volume of the mercury expands slightly as it becomes a liquid.

These changes explain why the density of mercury decreases as it transitions from a solid to a liquid.

Liquid to Gas Transition

During the transition from liquid to gas, mercury experiences even greater separation at the atomic level. In a liquid, mercury atoms still remain relatively close due to intermolecular forces, although they move freely compared to a solid.
At the boiling point, these intermolecular forces weaken significantly, allowing atoms to escape into the air as a gas. This results in a dramatic expansion as the atoms occupy a much larger volume than before.
Key points about this transformation include:

• A marked decrease in the strength of intermolecular attractions.
• Atoms moving independently in the gaseous state.
• A significant increase in the space between the atoms.

As these atoms disperse extensively, the density of mercury drops, making its gaseous state the least dense among the three states considered.

Atomic Spacing and Density

The concept of atomic spacing is crucial to understanding changes in density across different states of matter. Essentially, density is influenced by how closely atoms are packed within a given volume.
In a solid, atoms are tightly packed, which results in a higher density. When transitioning to a liquid, atoms begin to spread out, decreasing the density slightly. Finally, as the substance becomes gaseous, vast spaces develop between the atoms, leading to a further reduced density.
These changes in density are dictated by:

• The arrangement of atoms and their freedom of movement.
• The effect of temperature on atomic energy and spacing.
• The changes in intermolecular forces within different states.

Grasping the relation between atomic spacing and density is essential for understanding the properties of substances in different states.