Water is known as the universal solvent because more solutes dissolve in it than any other substance. A solvent is a substance, usually a liquid, that dissolves other substances to form a solution.
According to About, water's ability to dissolve a variety of substances stems from the polarity of its hydrogen molecules. Due to its polar nature, water easily dissolves substances that are polar or ionic.
The water molecule carries electric charges that are slightly positive on the hydrogen side and slightly negative on the oxygen side. As such, ionic compounds easily dissociate in water into positive and negative ions.
The positive ion of the compound attracts the oxygen side of the water molecule while the negative ion attracts the hydrogen side. This unique structure of the water molecule results in the relatively unusual behavior of water, such as its increase in density at lower temperatures.
Despite water being dubbed the universal solvent, there are many substances that do not properly dissolve in water. Solubility decreases if the attraction between opposite-charged molecules in a compound is high. For instance, most hydroxides exhibit low solubility in water. In addition, many organic compounds do not dissolve in water, including waxes and fats.