Question

Naphthalene balls disappear without leaving any solid. Give a reason.

Answer 1

Hint: Different materials undergo different types of phase transformation reactions at given conditions of temperature and pressure. The disappearance of Naphthalene balls without leaving solid behind is in itself a phase transformation that takes place at room temperature.

Complete answer:The Naphthalene balls also known as moth balls are a chemical combination of pesticide and deodorant. These white coloured solid balls can be identified from their distinctive odour. Clothes and many other materials are incapable of resisting moths and other pests that can cause significant damage to the material. Therefore these balls are used for protecting the materials while they are being stored.
Naphthalene is in itself an aromatic organic compound consisting of two benzene rings fused together. Like all aromatic compounds it is toxic and should not be ingested at any point.
At room temperatures the naphthalene balls disappear with time without leaving any solid. This phenomenon is due to a phase transformation called sublimation. Certain substances are capable of transforming into a gaseous phase directly from the solid phase without going through a liquid phase even at room temperatures. These substances are called sublime substances and Naphthalene is one such substance.
The heat present at room temperature is sufficient to bring about sublimation and the gaseous form of the substances gets mixed with air and the substance seem to disappear. This process happens throughout the bulk of the substance.
Hence Naphthalene balls disappear due to sublimation.

Note:
The enthalpy of sublimation i.e. the amount of heat required by a substance to undergo sublimation is actually the sum of enthalpy of two phase transformations namely fusion and evaporation (solid to liquid and then liquid to gas).