Water is frequently called the “universal solvent” because it can dissolve more substances than any other known liquid on Earth. This unique capability reflects its profound chemical properties, which allow it to interact with a vast array of compounds. This solvent power is fundamental to life, providing the medium for countless chemical reactions within cells and enabling the transport of nutrients and waste throughout biological systems. The reason for this dissolving ability lies within the specific structure of the water molecule itself.
The Foundation: Water’s Molecular Polarity
A single water molecule is composed of two hydrogen atoms and one oxygen atom (H₂O). The molecule has a bent shape, with the hydrogen atoms attached at an angle to the central oxygen atom. This geometry, combined with a property called electronegativity, is the source of water’s solvent power.
Electronegativity describes an atom’s ability to pull electrons toward itself within a chemical bond. In water, the oxygen atom is significantly more electronegative than the hydrogen atoms. This means the shared electrons spend more time near the oxygen atom, giving that side of the molecule a slight negative electrical charge.
Conversely, the hydrogen atoms are left with a slight positive charge because the shared electrons are pulled away from them. This unequal distribution of charge creates a molecular dipole, essentially making the water molecule a tiny magnet with distinct positive and negative ends. This separation of charge is defined as polarity, and it is the characteristic that allows water to act as an extraordinary solvent.
How Water Dissolves Ionic Compounds
Water’s polarity makes it particularly effective at dissolving ionic compounds, such as table salt (sodium chloride or NaCl). These compounds are held together by the strong electrostatic attraction between positively and negatively charged ions. When salt is dropped into water, the charged ends of the water molecules are drawn to the ions in the salt crystal.
The slightly negative oxygen end of the water molecule is attracted to the positive sodium ions (Na+), while the slightly positive hydrogen ends are attracted to the negative chloride ions (Cl-). These ion-dipole attractions are strong enough to overcome the internal forces holding the salt crystal together. Water molecules then swarm the individual ions, pulling them apart in a process called dissociation.
Once separated, the ions become surrounded by a sphere of water molecules, known as a hydration shell. This shell shields the ion from reattaching to other ions, allowing it to remain dissolved and evenly dispersed throughout the solution.
How Water Dissolves Polar Covalent Compounds
Water also readily dissolves other substances that are polar, even if they are not composed of full ions. This includes many organic molecules like sugars and alcohols, which are held together by covalent bonds but still possess regions of partial positive and negative charge. The mechanism for dissolving these compounds involves a specific type of attraction called hydrogen bonding.
The partially positive hydrogen atoms of a water molecule are attracted to the partially negative regions on another polar molecule, such as the oxygen in an alcohol molecule. This attraction allows water molecules to form temporary bonds with the solute molecules. These continuous hydrogen bonds between the water and the solute enable the two substances to intermix completely and form a solution.
Why Water Isn’t Truly Universal
While water is an unmatched solvent, it is not truly “universal” because it cannot dissolve every substance, particularly those that are non-polar. Non-polar molecules, such as oils, fats, and waxes, have an equal distribution of electrons and lack the partial positive and negative charges that water possesses. These substances do not attract water molecules with enough force to break the hydrogen bonds that hold the water molecules together.
When non-polar molecules are introduced, the water molecules cluster together and effectively push the non-polar substance out, causing it to separate. This is why oil and water do not mix. This behavior highlights the principle that polar solvents tend to dissolve polar solutes, and non-polar solvents dissolve non-polar solutes.