Introduction:
Water is a good conductor of electricity due to the presence of impurities. These impurities can be in the form of dissolved salts, minerals, or gases. When these impurities dissolve in water, they break down into charged particles called ions. These ions are responsible for increasing the conductivity of water.

Reasons why impurities dissolved in water increase its conductivity:

1. Presence of ions:
- When impurities dissolve in water, they release ions into the solution.
- These ions can be positively charged (cations) or negatively charged (anions).
- The presence of these ions allows the flow of electric current through the water.

2. Ion mobility:
- Ions in the water are free to move around due to their charge.
- The presence of dissolved impurities increases the concentration of ions in the water.
- This increased concentration of ions enhances the mobility of ions, allowing them to move more freely in the solution.
- The greater the mobility of ions, the higher the conductivity of water.

3. Increased collision frequency:
- As ions move in the water, they collide with other ions and water molecules.
- These collisions facilitate the transfer of charges, allowing the flow of electric current.
- The increased concentration of ions due to dissolved impurities leads to a higher collision frequency, promoting conductivity.

4. Ionization of water:
- Pure water can also conduct electricity to some extent due to the self-ionization of water molecules.
- In pure water, a small fraction of water molecules dissociate into H+ and OH- ions.
- These ions contribute to the conductivity of pure water.
- However, the conductivity of pure water is significantly lower compared to water containing dissolved impurities.

Conclusion:
Impurities dissolved in water increase its conductivity by releasing ions into the solution, enhancing ion mobility, increasing collision frequency, and facilitating the flow of electric current. These impurities provide a pathway for the conduction of electricity through water, making it a good conductor.