Why Is Water Called the Universal Solvent?


Water is the universal solvent because it is a polar molecule.
Water is the universal solvent because it is a polar molecule. It’s not actually universal, though, because it doesn’t dissolve nonpolar molecules very well.

You may hear water called the universal solvent. Here is a look at whether this is true and why water is so good at dissolving other compounds.

Polarity Makes Water a Great Solvent

Water dissolves more compounds than any other solvent. The biggest reason water is a great solvent is because it’s a polar molecule. What this means is that water is a neutral atom, but one part has a partial positive charge and another part has a partial negative charge. A water molecule has a bent shape with a 104.5 degree angle between the two hydrogen atoms, causing the polarity. The two hydrogen atoms of each water molecule have a partial positive charge, while the oxygen atom has a partial negative charge. Polar molecules readily dissolve in water because the positive part of the polar molecule is attracted to the oxygen atom, while the negative part is attracted to the hydrogen atom.

In addition to polarity, water is a great solvent because it is amphoteric. What this means is that water can act as both an acid and a base. Amphoterism makes water a better solvent than most other polar molecules.

Example: Dissolving Salt in Water

For example, consider how ordinary table salt (NaCl) dissolves in water. Salt is an ionic compound that dissolves into a sodium ion (Na+) and a chlorine ion (Cl). When you stir salt into water, the water molecules orient so that the negatively-charged oxygen atoms face the positively-charged sodium atoms in salt, while the positively-charged hydrogen atoms face the negatively-charged chlorine atoms. The ionic bond is a strong chemical bond, but the action of all the water molecules is sufficient to pull the sodium and chlorine atoms apart. Once separated, the ions evenly distribute and form a chemical solution.

This brings up an important point about solvents. Their activity depends on temperature. If you add salt to ice water, very little dissolves. If you add salt to boiling water, much more salt dissolves. Raising the temperature typically increases the effectiveness of a solvent because it increases the kinetic energy of particles. More kinetic energy results in more interaction between particles, so dissolving occurs more quickly.

Why Water Isn’t Really a Universal Solvent

Water dissolves polar molecules, including salts, sugars, many gases, proteins, simple alcohols, and DNA. But, it isn’t a universal solvent because it can’t dissolve hydrophobic or nonpolar molecules, such as fats, oils, some hydroxides, and most metal oxides, silicates, and sulfides.

There is no true universal solvent. The alchemists sought such a compound, which they called alkahest. One problem with a universal solvent would be its ability to dissolve any container. The alchemists got around this issue by stating alkahest could only dissolve compounds and not elements. Of course, such a substance does not exist, but the alchemists did find useful solvents, such as caustic potash in alcohol.

References

  • Ball, Philip (2001). Life’s Matrix : A Biography of Water (1st ed.). Farrar, Straus, and Giroux. ISBN 9780520230088.
  • Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 978-0-13-250882-7.
  • Franks, Felix (2007). Water : A Matrix of Life (2nd ed.). Royal Society of Chemistry. ISBN 9781847552341.

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