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Water: Properties, Polarity and Its Role as a Universal Solvent

The Water Molecule

Water is a very simple molecule: it is composed of two hydrogen atoms combined with one oxygen atom. Yet it is an extraordinary molecule with several exceptional properties that make it the most important solvent on Earth, and essential for life. Water is involved in many biochemical reactions, including photosynthesis and cellular respiration, and serves as a medium for transporting substances within and between cells.

Despite its apparent simplicity, water remains an important subject of curiosity in the scientific community.

Diagram illustrating the polarity of a water molecule, showing the oxygen atom with a partial negative charge (δ-) and two hydrogen atoms with partial positive charges (δ+), along with an arrow indicating the dipole moment

Figure 1. Polarity of a Water MoleculeThe oxygen atom has a partial negative charge (δ-), while the hydrogen atoms each have a partial positive charge (δ+). The dipole moment is represented by the arrow going from the positive to negative side.

Polarity of water

A water molecule has a bent shape with a 104.5-degree angle between the two hydrogen atoms, causing its polarity. While the water molecule has no charge, its oxygen atom is more electronegative than the hydrogen atoms (i.e., it is more attractive to electrons). As a result, the side of the molecule with the oxygen atom has a slight negative charge, and each of the hydrogen atoms has a slight positive charge. A molecule with such a charge difference is called a dipole (Figure 1).

Diagram showing the concept of hydrogen bonding between three water molecules. Hydrogen bonds are represented by dotted lines between hydrogen and oxygen atoms

Figure 2. Hydrogen bonding of water molecules.

Hydrogen bonding and water

This dipole moment of water molecules leads them to be attracted to each other (the slightly positive hydrogen side is attracted to the slightly negative oxygen side) and to other polar molecules. This attraction is called hydrogen bonding (Figure 2).

This hydrogen bonding is responsible for many of water's unique properties, including the fact that ice has a lower density than liquid water, the high surface tension and viscosity of water and, to some extent, the ability of water to dissolve many substances. The high melting and boiling point temperatures of water are due to the energy required to break hydrogen bonds.

Water is an Excellent Solvent

Water can dissolve more substances than any other liquid. It has a high dielectric constant, which means it can reduce the electrostatic forces between charged particles. This makes it easier for ionic compounds to dissociate in water. It is an excellent solvent for salts because these are made of negatively and positively charged ions that will be surrounded by the dipolar water molecules (Figure 3).

Illustration showing the interaction of water molecules with negatively charged chloride ions (Cl-) and positively charged sodium ions (Na+), highlighting how water molecules surround each ion.

Figure 3. Salt (sodium chloride) dissolved by water molecules.

Polar molecules easily 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, water can act as both an acid and a base (i.e., it is amphoteric), which allows it to interact with a wide range of substances. It is also a good solvent for neutral organic molecules and establishes hydrogen bonding with many molecules involved in the life processes such as sugars, proteins and nucleic acids.

However, water is not actually a universal solvent as it cannot dissolve hydrophobic or nonpolar molecules.

Physico-Chemical Properties of Water

Water has atypical physico-chemical properties, such as high values of viscosity, surface tension and heat of vaporization. This is largely due to the hydrogen bonds that connect its molecules. Unlike most substances, water is less dense as a solid (ice) than as a liquid.

Some key parameters of high purity (ultrapure) water are listed in Table 1.

The water molecule has another interesting characteristic: its ability to dissociate itself:

2 H₂O ⇌ H₃O⁺ + OH⁻

This ability to behave as both an acid (a proton donor) and a base (a proton acceptor) is characteristic of amphoteric substances. The most common method for specifying the acidity of a solution is its pH, which is defined in terms of the hydrogen ion concentration:

pH = −log [H⁺]

Questions are often raised about the value of pH for ultrapure water and attempts to measure the pH of ultrapure water in laboratories sometimes deliver puzzling results. Refer to this article for more information about the pH of ultrapure water.