Which substance is known as the universal solvent?

Water is known as the universal solvent due to its profound ability to dissolve a wide variety of substances. This remarkable property stems from its polar molecular structure, where the oxygen atom carries a slight negative charge while the hydrogen atoms have a slight positive charge. This polarity enables water molecules to surround and interact with different ions and polar molecules, effectively breaking them apart and leading to dissolution.

In biological and chemical processes, the role of water as a solvent is crucial. It facilitates numerous reactions within living organisms, helps transport nutrients and waste, and is integral to maintaining cellular functions. Other substances like oil and alcohol do not share this wide-ranging capacity to dissolve many compounds, particularly ionic substances. Saltwater contains dissolved salt, but it is not considered a separate universal solvent in its own right; rather, it is water that has salt as a solute. Hence, while other options may have solvent properties, water's unparalleled ability to dissolve many substances justifies its title as the universal solvent.