Question

Explain why ice floats in water but solid benzene sinks in liquid benzene. Which behavior is more “normal”?

Short answer

Ice floats in water because its hexagonal lattice structure results in a lower density compared to liquid water. On the other hand, solid benzene sinks in liquid benzene as its molecules pack closely together in a more compact structure, making it denser than liquid benzene. The behavior of benzene, where the solid sinks in the liquid, is more "normal" among substances, as they typically become denser when transitioning from liquid to solid states.

Step-by-step solution

Define density

Density is defined as mass per unit volume. It is calculated using the formula: \(density = \frac{mass}{volume}\). The density of a substance depends on the arrangement of its particles and can change when it transitions between different states, such as solid, liquid, or gas.

Understand the density of ice and water

When water freezes to form ice, its molecules arrange themselves in a hexagonal lattice structure. This structure results in open spaces between the water molecules, which cause the volume of ice to be greater than that of an equal mass of liquid water. Since the mass of the water molecules does not change, the increase in volume leads to a decrease in density. Therefore, ice is less dense than liquid water, which causes it to float.

Understand the density of solid benzene and liquid benzene

When benzene transitions from a liquid to the solid-state, its molecules pack closely together, forming a more ordered and compact structure. This arrangement leads to a smaller volume for an equal mass of solid benzene compared to liquid benzene. With the mass remaining the same and the volume decreasing, the density of solid benzene is higher than that of liquid benzene. As a result, solid benzene sinks in liquid benzene.

Determine the "normal" behavior

Typically, substances become denser as they change from a liquid to a solid state. This is because molecules usually pack more closely together when they are in their solid form and move less than when they are in a liquid state. The behavior of benzene, where the solid sinks in the liquid, is more "normal" among substances.

Conclusion

Ice floats in water because its hexagonal lattice structure makes it less dense than liquid water. In contrast, solid benzene sinks in liquid benzene because its structure becomes more compact and denser as it solidifies. The behavior of solid benzene sinking in liquid benzene is more "normal" based on the behavior of substances typically becoming denser as they transition from liquid to solid states.

Key concepts

Density and State of Matter

In the realm of physical science, the concept of density serves as a pivotal metric for understanding how substances interact based on their mass and volume.

Density, at its core, is simply the ratio of mass to volume, captured by the formula: \(density = \frac{mass}{volume}\). The intriguing part about density is that it isn't static across the states of matter. For solids, molecules are tightly packed together, usually leading to higher densities. In liquids, the molecules have a bit more freedom to move, typically resulting in a moderate density. And in gases, where molecules wander freely, the density is usually the lowest.

These differences in molecular arrangement and hence density are what make certain items float and others sink when placed in a liquid, which leads to captivating phenomena like ice floating in water or solid benzene sinking in its liquid form.

Hexagonal Lattice Structure of Ice

Water is an exceptional substance, and its solid form, ice, boasts a fascinating hexagonal lattice structure which is a direct result of hydrogen bonding among water molecules.

When water freezes, each molecule forms a bond with four neighbors, creating a 3-dimensional network with a hexagonal pattern. This structure is rather spacious, housing generous pockets of empty space, which explains why ice is less dense than its liquid counterpart.

Significance of the Hexagonal Structure

Understanding this hexagonal lattice is crucial because it underpins the rare occurrence of a solid (ice) being able to float on its own liquid phase, and it's integral to theories explaining the unique properties of water.

Floating Ice in Liquid Water

Imagine placing an ice cube in a glass of water and watching it bob gently on the surface. This observation is not merely a common experience but also a manifestation of physical principles.

Ice floats in liquid water simply because it's less dense—the culmination of water molecules expanding as they freeze and arrange themselves into the hexagonal structure mentioned earlier. This peculiar behavior of ice contributes greatly to the Earth's ecological balance.

Ecological Importance

It allows frozen surfaces on bodies of water to insulate the liquid underneath, ensuring aquatic life can survive even when temperatures plummet. If ice did not float, many ecosystems might not sustain life as they do now.

Sinking Solid in Liquid State

Turning our attention to other substances, like benzene, we witness a reversal of water's idiosyncratic behavior. Solid benzene has a tendency to sink in its liquid phase.

This sinking is explained by tighter molecular packing in the solid form, increasing the density above that of the liquid and causing the solid to descend. Such behavior aligns with the typical behavior of matter, where a cooler temperature generally compresses molecules into a denser arrangement.

Understanding Density Shifts

For many substances, including benzene, the shift from a liquid to a solid involves a transition to a more ordered, denser state—which explains why finding ice floating atop water is quite the exceptional case in the study of matter.