Question

Water has a high boiling point given its relatively low molar mass. Why?

Short answer

Water has a high boiling point despite its low molar mass due to strong hydrogen bonding between the water molecules, which requires significant energy to overcome during boiling.

Step-by-step solution

Understanding Hydrogen Bonding

Water has a high boiling point due to the presence of hydrogen bonds between water molecules. Hydrogen bonds are strong dipole-dipole interactions that occur when a hydrogen atom is covalently bonded to a highly electronegative atom, like oxygen, and is attracted to another electronegative atom in a nearby molecule.

Analyzing Water Structure

Each water molecule can form up to four hydrogen bonds with surrounding water molecules due to its bent shape and the two lone pairs of electrons on the oxygen atom. This extensive hydrogen bonding network is responsible for the high boiling point of water.

Comparing with Other Substances

When comparing water to other similar molecular mass compounds, water has a significantly higher boiling point. This can be attributed to the strong hydrogen bonding which requires more energy to break during the phase transition from liquid to gas.

Key concepts

High Boiling Points of Water

The intriguingly high boiling point of water is a consequence of the extensive hydrogen bonding network among its molecules. Despite its relatively low molecular weight, water boils at 100°C, which is significantly higher than other molecules of comparable size. This anomalous characteristic is due to the fact that each water molecule can participate in up to four hydrogen bonds - two through its hydrogen atoms and two through the lone pairs of electrons on the oxygen atom.
These bonds create a robust interconnected structure, imparting water with a high degree of cohesion. As a result, a substantial amount of energy in the form of heat is required to disrupt these interactions and transition water from its liquid state to its gaseous state. In essence, the heat provided during boiling primarily goes into breaking the hydrogen bonds rather than increasing the kinetic energy of the water molecules immediately.

Hydrogen Bonding: A Key Player

Hydrogen bonds, while weaker than covalent bonds, are significantly stronger than other types of dipole-dipole interactions and van der Waals forces. These specific interactions contribute not only to the high boiling point but also to the high melting point and the exceptional surface tension of water, affecting a wide range of physical properties.

Dipole-Dipole Interactions

Dipole-dipole interactions are forces of attraction that exist between polar molecules, which have regions of positive and negative charge due to an uneven distribution of electrons. Water, with its polar 'V'-shaped molecular structure, has a partial negative charge near the oxygen atom and a partial positive charge near the hydrogen atoms, making it an exemplary polar molecule.
These polar regions are attracted to the opposite charges of neighboring molecules, forming the basis for dipole-dipole interactions. However, in the case of water, these interactions are specialized and known as hydrogen bonds - a particularly strong type of dipole-dipole interaction.

Hydrogen Bonds: The Strongest Dipole-Dipole Interaction

Hydrogen bonding occurs when hydrogen is covalently attached to a highly electronegative atom (like oxygen, nitrogen, or fluorine) and is in close proximity to the electronegative atom of another molecule. This unique bond is a major factor contributing to the physical properties of water and many biological phenomena, such as the structure of DNA and the properties of proteins.

Phase Transition

Phase transition refers to the transformation of matter from one state of aggregation to another, such as from a liquid to a gas. In water, this phase transition requires breaking the hydrogen bonds that hold the water molecules together in the liquid state.
The energy needed for this process is known as the latent heat of vaporization. Since water's hydrogen bonds are particularly strong, the amount of energy needed is considerably higher than for other liquids of similar molecular mass. When water is heated to its boiling point, the energy input disrupts these hydrogen bonds, allowing molecules to escape into the air as steam—a gaseous form of water.
Despite the need for significant energy to overcome hydrogen bonding, once water reaches the gaseous state, its molecules are far apart, and the influence of these bonds is minimal, leading to water vapor behaving more like an ideal gas until it condenses and once again becomes influenced by hydrogen bonding.