Why Is Water the Universal Solvent?

Why water dissolves so many different chemicals

Water is valued as a solvent in part because so many types of molecules will dissolve in it.
Water is valued as a solvent in part because so many types of molecules will dissolve in it. Trish Gant / Getty Images

Water is known as the universal solvent. Here is an explanation of why water is called the universal solvent and what properties make it good at dissolving other substances.

Chemistry Makes Water A Great Solvent

Water is called the universal solvent because more substances dissolve in water than in any other chemical. This has to do with the polarity of each water molecule. The hydrogen side of each water (H2O) molecule carries a slight positive electric charge, while the oxygen side carries a slight negative electric charge.

This helps water dissociate ionic compounds into their positive and negative ions. The positive part of an ionic compound is attracted to the oxygen side of water while the negative portion of the compound is attracted to the hydrogen side of the water.

Why Salt Dissolves in Water

For example, consider what happens when salt dissolves in water. Salt is sodium chloride, NaCl. The sodium portion of the compounds carries a positive charge, while the chlorine part carries a negative charge. The two ions are connected by an ionic bond. The hydrogen and oxygen in the water, on the other hand, are connected by covalent bonds. Hydrogen and oxygen atoms from different water molecules are also connected via hydrogen bonds. When salt is mixed with water, the water molecules orient so that the negative charge oxygen anions faces the sodium ion, while the positive-charged hydrogen cations face the chloride ion.

Although ionic bonds are strong, the net effect of the polarity of all the water molecules is enough to pull the sodium and chlorine atoms apart. Once the salt is pulled apart, its ions become evenly distributed, forming a homogeneous solution.

If a lot of salt is mixed with water, it won't all dissolve.

In this situation, dissolution proceeds until there are too many sodium and chlorine ions in the mixture for water to win the tug-of-water with undissolved salt. Basically, the ions get in the way and prevent the water molecules from completely surrounding the sodium chloride compound. Raising the temperature increases the kinetic energy of the particles, increasing the amount of salt that can be dissolved in the water.

Water Doesn't Dissolve Everything

Despite its name as the "universal solvent" there are many compounds water won't dissolve or won't dissolve well. If the attraction is high between the opposite charged ions in a compound, then the solubility will be low. For example, most of the hydroxide exhibit low solubility in water. Also, nonpolar molecules don't dissolve very well in water, including many organic compounds, such as fats and waxes.

In summary, water is called the universal solvent because it dissolves the most substances, not because it actually dissolves every single compound.