Question

Solid mothballs work by filling a closet with a chemical vapor that is toxic to moths, and the balls shrink and disappear slowly with time. What is going on?

Short answer

Solid mothballs are made of either naphthalene or paradichlorobenzene, which can easily change from a solid to a gaseous state due to their high vapor pressures. This occurs through the process of sublimation, where a substance transitions directly from the solid phase to the gas phase without passing through an intermediate liquid phase. As the solid mothball sublimates, it fills the closet with a toxic chemical vapor that kills moths, protecting clothes from damage caused by moth larvae. Over time, as more molecules transition from the solid to the gas phase, the mothball shrinks and eventually disappears.

Step-by-step solution

Understand the Composition of Mothballs

Mothballs are typically made from a chemical compound called naphthalene or sometimes paradichlorobenzene. These compounds are solid at room temperature but have relatively high vapor pressures, meaning that they can easily transition from a solid to a gaseous state.

Explain the Concept of Sublimation

Sublimation is the process of a substance transitioning directly from the solid phase to the gas phase without passing through an intermediate liquid phase. This can occur when the vapor pressure of a solid exceeds the atmospheric pressure, causing the solid to turn into gas molecules. In the case of mothballs, the chemical compounds that make up the mothballs have high vapor pressures, allowing them to sublimate over time.

Describe How Mothballs Sublimate

As the naphthalene or paradichlorobenzene molecules in the mothballs gain energy from thermal fluctuations, they break free from the solid structure and enter the gas phase. Because the vapor pressure of these chemicals is relatively high, this process occurs at room temperature.

Explain the Effect of Sublimation on Moths

The chemical vapor released by mothballs is toxic to moths and other pests. As the solid mothball sublimates, it fills the closet with gaseous molecules of the repellent chemical. When moths come into contact with the vapor, they become sick and die, thereby protecting the clothes stored in the closet from damage caused by moth larvae feeding on the fabric.

Discuss the Shrinking and Disappearance of Mothballs

As sublimation occurs, the number of solid molecules in the mothball continuously decreases as they transition into the gaseous phase. This leads to a reduction in the size of the mothball, causing it to shrink. Eventually, the mothball will disappear entirely after all its molecules have sublimated into the gas phase.

Key concepts

Understanding Naphthalene

Naphthalene, a key component in some mothballs, is an intriguing substance from both a chemical and practical perspective. Structurally, it consists of two benzene rings fused together, making it recognizable as a polycyclic aromatic hydrocarbon. This white crystalline solid has a distinctive smell and, importantly for our discussion, a significant vapor pressure at room temperature.

Its use in mothballs capitalizes on these properties, especially its ability to transition from a solid directly to a gas, which is the basis for its pest-repelling capabilities. In households, this substance is valued for its ability to fight off moths without leaving a liquid residue, a quality due to its sublimation under normal conditions.

Vapor Pressure and Its Role

Vapor pressure is a critical concept in understanding how substances like naphthalene function as moth repellents. It's the measure of the propensity of a substance to evaporate, indicating how easily a compound can turn into vapor at a given temperature. Substances with higher vapor pressures at a particular temperature have a greater tendency to change into a gaseous state.

In the case of naphthalene, its relatively high vapor pressure means that even at room temperature, it can readily sublimate, releasing gas molecules into the air without ever becoming liquid. The result is a gradual decrease in the size of naphthalene-containing mothballs, as they consistently release repellent vapors that keep moths at bay.

Phase Transition: Solid to Gas

The transformation of a substance from one state of matter to another is known as a phase transition. Among the various types of phase transitions, sublimation is particularly fascinating. It describes the process of a substance moving directly from a solid state to a gaseous state without passing through the liquid phase in between.

In most environments, this occurs when the substance's vapor pressure exceeds the atmospheric pressure, prompting the solid to vaporize. Sublimation is precisely what happens in mothballs made of naphthalene. When placed in a closet, the mothballs slowly transition to gas, circulating a chemical vapor that proves toxic to moths, preventing damage to clothing and other textiles.