Question

Is the boiling point of water unusually high or low?

Short answer

The boiling point of water is unusually high among small molecules due to hydrogen bonding, but low compared to substances with stronger intermolecular forces.

Step-by-step solution

Understanding the Context of Boiling Points

The boiling point of water is the temperature at which it changes from a liquid to a gas. Typically, water boils at 100°C (212°F) at 1 atmosphere of pressure, which is standard atmospheric pressure at sea level. This temperature is often used as a reference point for boiling points in other contexts.

Comparing to Other Common Liquids

To determine if the boiling point of water is unusually high or low, we compare it to the boiling points of other common liquids. For example, ethanol boils at 78.37°C, and acetone boils at 56°C. These boiling points are lower than that of water, but many other liquids (especially those with larger molecular weights and strong inter-molecular forces) have much higher boiling points than water.

Considering Factors Affecting Boiling Points

Boiling points are influenced by factors like molecular weight and intermolecular forces. Water has hydrogen bonds, a strong type of intermolecular attraction, which contributes to its relatively high boiling point compared to other small molecules like methane, which boils at -164°C.

Evaluating the Unusual Nature Based on Context

Whether water's boiling point is considered unusually high or low can depend on context. In comparison to substances with stronger intermolecular forces or higher molecular weights, water's boiling point is low. However, among small molecules with weaker intermolecular forces, water's boiling point is relatively high.

Key concepts

Intermolecular Forces

Intermolecular forces play a significant role in determining the boiling point of a substance. These are forces that hold molecules together within a liquid phase.
In water, the dominant intermolecular force is hydrogen bonding.
Hydrogen bonds are relatively strong types of dipole-dipole attractions that occur when hydrogen is bonded to electronegative elements like oxygen. These forces cause water molecules to be highly attracted to each other, which results in requiring more heat energy to break these bonds and turn water into vapor.

• Hydrogen bonding is stronger than Van der Waals forces present in many other liquids like methane.
• Despite its small size, water exhibits strong cohesion due to these hydrogen bonds. This contributes to water's high surface tension and heat capacity as well.

Intermolecular forces are extremely important when considering boiling points. The stronger these forces are, the higher the energy (heat) required, leading to higher boiling points.

Molecular Weight

Molecular weight also influences the boiling point of a substance, but not as straightforwardly as intermolecular forces do. Molecular weight is the sum of the masses of all the atoms in a molecule.
Generally, molecules with higher molecular weights tend to have higher boiling points because they have a greater number of atoms which can contribute to larger surface areas for intermolecular attractions. However, in water's case, its relatively low molecular weight does not prevent it from having a decent boiling point for its size.
The reason water boils at 100°C instead of a lower temperature is due to the combination of factors: strong hydrogen bonding coupled with a moderate molecular weight.

• For example, larger molecules such as hexane have a boiling point higher than that of water because of larger molecular size and correspondingly greater London dispersion forces.
• But when considering other small molecules like ammonia, water's boiling point is higher due to stronger intermolecular attractions.

Understanding molecular weight alongside intermolecular forces offers insight into why specific substances boil at given temperatures, reinforcing the complex interplay of these factors.

Comparison of Boiling Points

When comparing boiling points, it's crucial to consider both intermolecular forces and molecular weight.
For instance, if you compare water to ethanol, which has a boiling point of 78.37°C, water's higher boiling point can be attributed to stronger hydrogen bonds despite not having much larger molecular weight.

• In contrast, heavy molecules with extensive Van der Waals forces tend to have high boiling points.
• Substances like methanol and methane have significantly lower boiling points than water due to weaker intermolecular interactions.

The unique combination of modest molecular weight and strong hydrogen bonding positions water within a distinctive boiling point range. Water’s boiling point is unusual because, when compared to similarly-sized molecules that lack strong hydrogen bonds, it is quite high, but when juxtaposed with larger molecules or those exhibiting stronger bonds like ionic compounds, it appears relatively low.
This dual perspective teaches us that boiling points cannot be attributed to a single factor; they arise from a myriad of interconnected molecular characteristics.