Dissolved salts and minerals in tap water.
Explanation
Electrical conductivity in liquids depends on the availability of free-moving charge carriers. In aqueous solutions, these charge carriers are ions (cations and anions) rather than free electrons, which are responsible for conduction in metals.
Analysis
- Distilled Water: Distilled water is chemically pure $H_2O$. Water molecules are covalently bonded and undergo negligible ionization (self-ionization into $H^+$ and $OH^-$ is extremely low). Due to the absence of free ions, it offers very high resistance to the flow of current and acts as an insulator.
- Tap Water: Tap water is not pure; it contains dissolved salts and minerals (such as chlorides, sulphates, and carbonates of calcium, magnesium, and sodium). These compounds are electrolytes that dissociate into positive and negative ions when dissolved.
- Mechanism of Conduction: When an electric potential is applied across tap water, these dissolved ions move towards oppositely charged electrodes, constituting an electric current. Therefore, the presence of dissolved salts and minerals is the primary reason for its conductivity.
Relevance of other options:
- Dissolved gases in tap water. Dissolved gases: While gases like $CO_2$ can dissolve to form weak acids (e.g., carbonic acid) and contribute slightly to ion concentration, the conductivity is predominantly driven by dissolved mineral salts.
- Acidic impurities in distilled water. Acidic impurities: If distilled water contained acidic impurities, it would conduct electricity.
- Suspended solid particles in tap water. Suspended solid particles: Suspended solids (insoluble matter) do not contribute to electrical conduction; only dissolved ionic species facilitate current flow.
Key Takeaway: Pure water is a poor conductor of electricity. The conductivity of water is directly determined by the concentration of dissolved electrolytes (salts and minerals) which provide the necessary ions for charge transport.