Water is often called the “universal solvent” because it can dissolve more substances than any other known liquid. This powerful dissolving ability explains why water is so deeply intertwined with life on Earth, from the transport of nutrients within living organisms to the shaping of the planet’s geology. Understanding why this is the case requires a close look at the electrically charged structure of the water molecule itself.
The Unique Polarity of the Water Molecule
The water molecule, chemically represented as \(\text{H}_2\text{O}\), is constructed from one oxygen atom and two hydrogen atoms joined by covalent bonds. The molecule is not arranged in a straight line but has a bent or V-shape, with the two hydrogen atoms positioned at an angle of about 104.5 degrees from each other. This geometry is a direct result of the electron distribution around the central oxygen atom.
The unique shape and the nature of the atoms create a significant electrical imbalance, a property known as polarity. Oxygen is more electronegative than hydrogen, meaning the shared electrons spend more time orbiting the oxygen nucleus than the hydrogen nuclei.
This unequal sharing effectively gives the oxygen side of the molecule a slight negative charge, while the two hydrogen sides each acquire a slight positive charge. This separated charge distribution, or dipole, is the most important factor that allows water to act as an exceptional solvent.
The Mechanics of Dissolution: How Water Pulls Apart Solutes
Water’s polarity enables it to interact forcefully with and dissolve a wide range of substances, primarily those that are themselves charged or polar. When water encounters an ionic compound, such as table salt (sodium chloride, NaCl), the charged ends of the water molecules are strongly attracted to the oppositely charged ions in the salt crystal lattice.
The slight negative charge on the oxygen atom attracts the positive sodium ions (\(\text{Na}^{+}\)), while the slight positive charge on the hydrogen atoms attracts the negative chloride ions (\(\text{Cl}^{-}\)). As water molecules collide with the crystal, these attractions are powerful enough to overcome the strong ionic bonds holding the salt crystal together. The water molecules effectively “tug” the ions away from the solid structure.
Once separated, the individual ions are completely surrounded by a shell of water molecules, known as a hydration shell. These shells stabilize the ions and prevent them from reattaching to each other and reforming the crystal, keeping the solute dissolved and dispersed evenly throughout the water. This process is called dissociation.
Water also dissolves many other polar substances, such as sugar or alcohol, which are made of polar covalent molecules. These molecules do not break apart but possess regions of partial positive and negative charge. Water molecules use their own polarity to form hydrogen bonds with these charged regions, pulling the individual solute molecules away from each other to mix seamlessly with the solvent.
Why Water Isn’t Truly “Universal”
Despite its name, water is not literally a universal solvent, as it does not dissolve every single substance. The label is maintained because water dissolves more types of substances and in greater quantity than any other liquid. The limitation of water’s dissolving power is best explained by the chemical principle “like dissolves like”.
Water, being a highly polar solvent, is excellent at dissolving other polar molecules and charged ions. However, it struggles to interact with non-polar substances, such as oils, fats, and waxes. Non-polar molecules have an even distribution of electrical charge and lack the positive and negative poles that water requires to form attractions.
When a non-polar substance like oil is mixed with water, the water molecules are more strongly attracted to each other than they are to the oil molecules. The water molecules effectively push the non-polar molecules aside, causing them to clump together and form a separate layer or droplets.