Because water is a polar molecule, it makes an excellent solvent for polar and

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Learning Objectives

• Explain why some molecules do not dissolve in water.

Water, which not only dissolves many compounds but also dissolves more substances than any other liquid, is considered the universal solvent. A polar molecule with partially-positive and negative charges, it readily dissolves ions and polar molecules. Water is therefore referred to as a solvent: a substance capable of dissolving other polar molecules and ionic compounds. The charges associated with these molecules form hydrogen bonds with water, surrounding the particle with water molecules. This is referred to as a sphere of hydration, or a hydration shell, and serves to keep the particles separated or dispersed in the water.

When ionic compounds are added to water, individual ions interact with the polar regions of the water molecules during the dissociation process, disrupting their ionic bonds. Dissociation occurs when atoms or groups of atoms break off from molecules and form ions. Consider table salt (NaCl, or sodium chloride): when NaCl crystals are added to water, the molecules of NaCl dissociate into Na+ and Cl–ions, and spheres of hydration form around the ions. The positively-charged sodium ion is surrounded by the partially-negative charge of the water molecule’s oxygen; the negatively-charged chloride ion is surrounded by the partially-positive charge of the hydrogen in the water molecule.

Figure \(\PageIndex{1}\): Dissociation of NaCl in water: When table salt (NaCl) is mixed in water, spheres of hydration form around the ions.

Since many biomolecules are either polar or charged, water readily dissolves these hydrophilic compounds. Water is a poor solvent, however, for hydrophobic molecules such as lipids. Nonpolar molecules experience hydrophobic interactions in water: the water changes its hydrogen bonding patterns around the hydrophobic molecules to produce a cage-like structure called a clathrate. This change in the hydrogen-bonding pattern of the water solvent causes the system’s overall entropy to greatly decrease, as the molecules become more ordered than in liquid water. Thermodynamically, such a large decrease in entropy is not spontaneous, and the hydrophobic molecule will not dissolve.

• Water dissociates salts by separating the cations and anions and forming new interactions between the water and ions.
• Water dissolves many biomolecules, because they are polar and therefore hydrophilic.

• dissociation: The process by which a compound or complex body breaks up into simpler constituents such as atoms or ions, usually reversibly.
• hydration shell: The term given to a solvation shell (a structure composed of a chemical that acts as a solvent and surrounds a solute species) with a water solvent; also referred to as a hydration sphere.

Water is called the "universal solvent" because it dissolves more substances than any other liquid -- why is that the case.

A water molecule is formed when two atoms of hydrogen bond covalently with an atom of oxygen. In a covalent bond electrons are shared between atoms. In water the sharing is not equal. The oxygen atom attracts the electrons more strongly than the hydrogen. This gives water an asymmetrical distribution of charge. Molecules that have ends with partial negative and positive charges are known as polar molecules. It is this polar property that allows water to separate polar solute molecules and explains why water can dissolve so many substances. 

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Water is a good solvent due to its polarity.

The solvent properties of water are vital in biology, because many biochemical reactions take place only within aqueous solutions 

When an ionic or polar compound enters water, it is surrounded by water molecules. The relatively small size of water molecules typically allows many water molecules to surround one molecule of solute. The partially negative dipoles of the water are attracted to positively charged components of the solute, and vice versa for the positive dipoles.

An example of an ionic solute is table salt. The dipole forces of water  can disrupt the attractive forces that hold the sodium and chloride in the salt molecule together and, thus, dissolve it.

Liquid water has a partially ordered structure in which hydrogen bonds  are constantly being formed and breaking up. 

The strong hydrogen bonds also give water a high cohesiveness and, consequently, surface tension. This is evident when small quantities of water are put onto a nonsoluble surface and the water stays together as drops.

What is the distance in angstroms for the hydrogen bond shown above?

How to measure distance

Label Atoms first

Style -- > Labels -- > with atom Name

Then measure distance

Right mouse button -->Measurements -->

Double click on hydrogen (H31) then drag to oxygen (O28) -- double click again

How to measure angles

Double click on first atom, once on middle atom, twice on atom three.

Check your measurements below

1) What is the distance of the hydrogen bond show?

Please enter your answer in the space provided:

2) What is the measure of the HOH angle in a single molecule of water?

3)What is distance between the oxygen atoms in a water dimer?

Test your Understanding:

Water is a polar molecule and also acts as a polar solvent. When a chemical species is said to be "polar," this means that the positive and negative electrical charges are unevenly distributed. The positive charge comes from the atomic nucleus, while the electrons supply the negative charge. It's the movement of electrons that determines polarity. Here's how it works for water.

• Water is polar because it has a bent geometry that places the positively-charged hydrogen atoms on one side of the molecule and the negatively-charged oxygen atom on the other side of the molecule.
• The net effect is a partial dipole, where the hydrogens have a partial positive charge and the oxygen atom has a partial negative charge.
• The reason water is bent is because the oxygen atom still has two lone pairs of electrons after it bonds with hydrogen. These electrons repel each other, bending the O-H bond away from the linear angle.

Water (H2O) is polar because of the bent shape of the molecule. The shape means most of the negative charge from the oxygen on side of the molecule and the positive charge of the hydrogen atoms is on the other side of the molecule. This is an example of polar covalent chemical bonding. When solutes are added to water, they may be affected by the charge distribution.

The reason the shape of the molecule isn't linear and nonpolar (e.g., like CO2) is because of the difference in electronegativity between hydrogen and oxygen. The electronegativity value of hydrogen is 2.1, while the electronegativity of oxygen is 3.5. The smaller the difference between electronegativity values, the more likely atoms will form a covalent bond. A large difference between electronegativity values is seen with ionic bonds. Hydrogen and oxygen are both acting as nonmetals under ordinary conditions, but oxygen is quite a bit more electronegative than hydrogen, so the two atoms form a covalent chemical bond, but it's polar.

The highly electronegative oxygen atom attracts electrons or negative charge to it, making the region around the oxygen more negative than the areas around the two hydrogen atoms. The electrically positive portions of the molecule (the hydrogen atoms) are flexed away from the two filled orbitals of the oxygen. Basically, both hydrogen atoms are attracted to the same side of the oxygen atom, but they are as far apart from each other as they can be because the hydrogen atoms both carry a positive charge. The bent conformation is a balance between attraction and repulsion.

Remember that even though the covalent bond between each hydrogen and oxygen in water is polar, a water molecule is an electrically neutral molecule overall. Each water molecule has 10 protons and 10 electrons, for a net charge of 0.

The shape of each water molecule influences the way it interacts with other water molecules and with other substances. Water acts as a polar solvent because it can be attracted to either the positive or negative electrical charge on a solute. The slight negative charge near the oxygen atom attracts nearby hydrogen atoms from water or positive-charged regions of other molecules. The slightly positive hydrogen side of each water molecule attracts other oxygen atoms and negatively-charged regions of other molecules. The hydrogen bond between the hydrogen of one water molecule and oxygen of another holds water together and gives it interesting properties, yet hydrogen bonds are not as strong as covalent bonds. While the water molecules are attracted to each other via hydrogen bonding, about 20% of them are free at any given time to interact with other chemical species. This interaction is called hydration or dissolving.

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• Clough, Shepard A.; Beers, Yardley; Klein, Gerald P.; Rothman, Laurence S. (1973). "Dipole moment of water from Stark measurements of H2O, HDO, and D2O". The Journal of Chemical Physics. 59 (5): 2254–2259. doi:10.1063/1.1680328
• Gubskaya, Anna V.; Kusalik, Peter G. (2002). "The total molecular dipole moment for liquid water". The Journal of Chemical Physics. 117 (11): 5290–5302. doi:10.1063/1.1501122.
• Pauling, L. (1960). The Nature of the Chemical Bond (3rd ed.). Oxford University Press. ISBN 0801403332.