Why Hydrogen Bonds Raise the Boiling Point of Water
- Increase in Intermolecular Forces: The boiling point of a substance is closely related to the intermolecular forces present. Hydrogen bonds are a type of relatively strong intermolecular force, and water molecules are connected by numerous hydrogen bonds. During the transition of water from liquid to gas, not only must van der Waals forces be overcome, but additional energy is required to break these hydrogen bonds. Therefore, higher temperatures are needed to provide sufficient energy, which raises the boiling point of water.
- Formation of a Stable Hydrogen Bond Network: The oxygen atom in each water molecule can form hydrogen bonds with the hydrogen atoms of two other water molecules. Meanwhile, the two hydrogen atoms in each water molecule can also form hydrogen bonds with the oxygen atoms of other water molecules. On average, each water molecule forms two hydrogen bonds. A large and stable three-dimensional network of hydrogen bonds forms as many water molecules are interconnected. To turn water from liquid to gas, sufficient energy is required to break this extensive hydrogen bond network, allowing the water molecules to escape each others bonds and enter the gas phase. This results in a significantly higher boiling point for water.
- The Need to Break Numerous Hydrogen Bonds During Vaporization: Even near the boiling point, a significant number of hydrogen bonds remain in liquid water. During vaporization, continuous energy must be supplied to break these hydrogen bonds, allowing water molecules to overcome the constraints of hydrogen bonding and enter the gas phase. This further contributes to maintaining water’s boiling point at a higher temperature.
Methods to Lower the Boiling Point of Water
- Lowering Atmospheric Pressure: According to physical chemistry principles, the boiling point of a liquid is directly proportional to the external atmospheric pressure. Therefore, reducing external pressure will also lower the boiling point of water. For example, at high altitudes where the pressure is lower, water boils at a temperature lower than 100°C. In laboratory settings, a vacuum pump can be used to extract air from a sealed container, artificially creating a low-pressure environment to lower the boiling point of water.
- Adding Substances that Form Azeotropes: Adding certain substances that can form azeotropic mixtures with water can lower its boiling point. For instance, adding alcohols like ethanol, isobutanol, isopropanol, or tert-butanol can form azeotropes with water. A mixture of 90% water and isobutanol, isopropanol, or tert-butanol has a boiling point reduced to around 85-87°C.
- Other Methods: Other substances can also be added to alter the properties of water and reduce its boiling point. For example, adding small amounts of vinegar can slightly lower the boiling point of water, although the effect may not be as pronounced as the previous methods.
