Water: The Universal Solvent

Core Idea

Water is sometimes called the "universal solvent" because it dissolves more substances than any other common liquid. This ability comes from water's molecular structure — the oxygen end of a water molecule has a slight negative charge and the hydrogen end has a slight positive charge, making water a polar molecule. This polarity allows water to attract and pull apart many ionic compounds and other polar substances. Water cannot dissolve everything, though — nonpolar substances like oil do not mix well with water.

How It's Best Learned

Dissolve different substances in water (salt, sugar, baking soda, cooking oil, sand) and observe which dissolve easily and which do not. Then connect the results to the idea of polar and nonpolar molecules — "like dissolves like."

Common Misconceptions

• Water is not truly "universal" — it cannot dissolve everything. Nonpolar substances like fats and oils do not dissolve in water.
• Dissolving is not the same as melting — when salt dissolves in water, the salt particles separate and spread out among water molecules, but nothing is being heated to a liquid state.
• Clear solutions are not the same as pure water — a solution of salt in water looks clear but contains dissolved particles.
• Water dissolves ionic compounds by pulling ions apart, not by breaking the chemical bonds within the ions themselves.

Explainer

You already know how dissolving works — a solute breaks apart and spreads evenly throughout a solvent to form a solution. Now it is time to understand why water is so extraordinarily good at dissolving things that it has earned the nickname "the universal solvent."

The secret lies in water's molecular structure. A water molecule (H2O) has one oxygen atom bonded to two hydrogen atoms, but the molecule is not shaped in a straight line — it is bent. Because oxygen attracts electrons more strongly than hydrogen does, the shared electrons in the bonds spend more time near the oxygen. This gives the oxygen end of the molecule a slight negative charge and each hydrogen end a slight positive charge. A molecule with uneven charge distribution like this is called a polar molecule.

This polarity is what makes water such a powerful solvent. When you drop a crystal of table salt (NaCl) into water, the polar water molecules surround the crystal. The slightly negative oxygen ends of water molecules are attracted to the positive sodium ions (Na+), and the slightly positive hydrogen ends are attracted to the negative chloride ions (Cl-). Water molecules tug on these ions, pulling them away from the crystal lattice one by one and surrounding each ion in a shell of water molecules. This process — called hydration — continues until the entire crystal has dissolved and the ions are dispersed throughout the water.

Water dissolves many other polar and ionic substances by the same principle: its polar molecules attract charged or partially charged particles in the solute. Sugar dissolves readily in water because sugar molecules have many polar regions that interact with water molecules. Acids, bases, and many biological molecules dissolve in water for similar reasons. This is why water is the basis of almost all biological chemistry — your blood, your cells, and every organism on Earth runs on aqueous (water-based) solutions.

However, the nickname "universal solvent" is not entirely accurate. Water is excellent at dissolving ionic and polar substances, but it struggles with nonpolar substances — those whose molecules have evenly distributed charges. Cooking oil, wax, and grease are nonpolar. When you pour oil into water, the oil sits on top in a separate layer because water molecules are far more attracted to each other than to the nonpolar oil molecules. This is the basis of the principle "like dissolves like": polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes. Understanding this principle helps you predict what will dissolve in what — and why your greasy hands need soap (which has both polar and nonpolar parts) rather than just water.