Introduction:
When electricity passes through a potato, a greenish blue color is formed on the positive terminal, not on the negative terminal. This phenomenon can be explained by understanding the process of electrolysis and the chemical reactions occurring within the potato.

Electrolysis in Potatoes:
1. Electrolysis: When an electric current is passed through a conductive solution, such as the moisture in a potato, electrolysis occurs. This process involves the movement of ions towards the opposite charged electrodes.

2. Positive Terminal: The positive terminal, also known as the anode, attracts negative ions (anions) present in the potato. These anions can include chloride ions (Cl-) or other negatively charged ions.

3. Oxidation Reaction: As the anions reach the positive terminal, they undergo an oxidation reaction. This reaction involves the loss of electrons from the anions, resulting in the production of positive ions and the release of electrons.

4. Electron Transfer: The released electrons then flow through the circuit, creating an electric current. This movement of electrons is what powers electrical devices connected to the circuit.

5. Negative Terminal: On the other hand, the negative terminal, or cathode, attracts positively charged ions (cations). These cations can include potassium ions (K+) or other positively charged ions.

6. Reduction Reaction: As the cations reach the negative terminal, they undergo a reduction reaction. This reaction involves the gain of electrons by the cations, which results in the formation of neutral atoms or molecules.

Reasons for the Greenish Blue Color:
1. Copper Ions: The greenish blue color observed on the positive terminal of the potato is likely due to the presence of copper ions in the potato. These ions may come from impurities in the metal used for the terminal or from other sources.

2. Copper Oxidation: When the copper ions come into contact with the positive terminal, they undergo an oxidation reaction. This leads to the formation of copper oxide or copper hydroxide, which gives the terminal a greenish blue color.

3. Lack of Copper Ions on Negative Terminal: The absence of copper ions on the negative terminal explains why the greenish blue color is not observed there. Since the negative terminal attracts positively charged ions (cations) and copper ions are negatively charged (anions), there is no interaction between the copper ions and the negative terminal.

Conclusion:
The greenish blue color formed on the positive terminal of a potato when electricity passes through it is due to the presence of copper ions and their oxidation reaction. The absence of copper ions on the negative terminal explains why the color is not observed there. Understanding the process of electrolysis and the chemical reactions occurring within the potato helps explain this phenomenon.