Because they goes sublimation easily i.e. the change state of naphthalene from solid to gas takes place easily.

**Explanation:**

Naphthalene balls, also known as mothballs, are solid white balls that have a strong odor. They are commonly used to repel insects, such as moths and cockroaches, and prevent them from damaging clothes and other stored items. Over time, naphthalene balls tend to disappear without leaving any solid residue behind. This phenomenon can be explained by a process called sublimation.

**What is sublimation?**
Sublimation is the process in which a solid directly converts into a gas without going through the liquid phase. Normally, when a solid is heated, it melts and turns into a liquid before vaporizing into a gas. However, some substances, like naphthalene, have a high vapor pressure and can undergo sublimation at room temperature.

**The process of sublimation in naphthalene balls:**
1. Naphthalene balls contain naphthalene as the active ingredient. Naphthalene has a high vapor pressure, which means it easily evaporates and forms a gas.
2. When exposed to air, the naphthalene molecules on the surface of the balls start to escape into the surrounding environment as gas molecules.
3. As more and more naphthalene molecules escape, the surface of the balls gradually diminishes, and the balls start to shrink in size.
4. The escaping gas molecules mix with the surrounding air, contributing to the characteristic strong odor of naphthalene.
5. Eventually, all the naphthalene molecules evaporate, and the balls disappear completely without leaving any solid residue.

**Factors affecting the rate of sublimation:**
Several factors can influence the rate at which naphthalene balls disappear through sublimation:
- Temperature: Higher temperatures accelerate the sublimation process, causing the balls to disappear more quickly.
- Air circulation: Good ventilation and air movement facilitate the escape of naphthalene gas molecules, increasing the rate of sublimation.
- Surface area: A larger surface area of naphthalene balls allows more molecules to escape simultaneously, leading to faster sublimation.

In conclusion, naphthalene balls disappear over time due to sublimation, a process where the solid naphthalene directly converts into a gas without leaving any solid residue. Factors such as temperature, air circulation, and surface area can affect the rate at which the sublimation occurs.