The most important characteristic of water to act as a good solvent is its polar nature.
Option 1: Purity of water does not directly affect its ability to act as a solvent. Water can still act as a solvent even if it contains impurities.
Option 2: The ionizing power of water refers to its ability to dissociate into ions. While this property is important for certain chemical reactions, it is not the main reason why water is a good solvent.
Option 3: This is the correct answer. Water is a polar molecule, meaning it has a partial positive charge on one end and a partial negative charge on the other. This polarity allows water to form hydrogen bonds with other polar substances, facilitating the dissolution process.
Option 4: Dissolving power of water is a result of its polar nature. It allows water to dissolve many different types of substances, making it an effective solvent.
In conclusion, the polar nature of water, option 3, is the most important characteristic that enables it to act as a good solvent.