Hydrogen Bonds — Reading Comprehension

Hydrogen bonds play a vital role in shaping the world around us. For example, water’s unusually high boiling point, its ability to form droplets, and the way ice floats are all explained by hydrogen bonds. Scientists have discovered that these strong attractions between molecules are essential to many chemical and biological processes. Unlike covalent bonds, which involve the sharing of electrons within a molecule, hydrogen bonds are not true chemical bonds, but rather powerful forces that act between molecules.

How Hydrogen Bonds Form
Hydrogen bonds occur when a hydrogen atom attached to a highly electronegative atom—such as oxygen, nitrogen, or fluorine—attracts another electronegative atom in a different molecule. In a water molecule (H₂O), the oxygen atom pulls electrons closer, giving it a slight negative charge, while the hydrogen atoms become slightly positive. This charge difference creates an intermolecular force, known as a hydrogen bond, between the hydrogen of one molecule and the oxygen of another. These bonds are much weaker than covalent bonds, but stronger than most other intermolecular forces. In liquid water, each molecule can form up to four hydrogen bonds at a time, creating a dynamic, interconnected network. This network is responsible for water’s high surface tension, its ability to dissolve many substances, and its high boiling point (100°C), which is much higher than similar-sized molecules.

Hydrogen Bonds in Water and DNA
The effects of hydrogen bonding are seen in both living and nonliving systems. In water, hydrogen bonds cause molecules to stick together, supporting insects that walk on water and causing water to bead on surfaces. When water freezes, the hydrogen bonds arrange molecules in an open, hexagonal pattern. This makes ice less dense than liquid water, so it floats—an unusual property that helps aquatic life survive cold winters. In living systems, hydrogen bonds hold together the two strands of the DNA double helix. Each base pair in DNA is connected by specific hydrogen bonds: adenine pairs with thymine using two hydrogen bonds, while cytosine pairs with guanine using three. These bonds are strong enough to keep the DNA strands together but weak enough to allow them to separate during cell division and replication.

Broader Importance and Misconceptions
Hydrogen bonds are not the same as covalent bonds. While covalent bonds involve the sharing of electrons between atoms in a single molecule, hydrogen bonds are attractions between different molecules or parts of large molecules. This distinction is important for understanding chemical reactions, physical properties, and biological functions. Hydrogen bonds also play a role in the structure of proteins, the behavior of ammonia and hydrogen fluoride, and in many industrial and technological applications. Understanding hydrogen bonds helps scientists develop new materials, medicines, and environmental solutions.

Hydrogen bonds are a key example of how invisible forces at the molecular level shape the properties of substances and the processes of life. They connect to broader scientific ideas about matter, energy, and interactions between particles.

Interesting Fact: 
Without hydrogen bonds, all water on Earth would be a gas at room temperature, making life as we know it impossible!

What is a hydrogen bond?

Which property of water is explained by hydrogen bonds?

What happens to water molecules when ice forms?

In DNA, what do hydrogen bonds do?

What does 'electronegative' mean in the context of hydrogen bonds?

Which of the following best describes a covalent bond?

Hydrogen bonds are responsible for which of the following effects? (Choose all that apply)

If hydrogen bonds did not exist, what would happen to water at room temperature?

True or False: Hydrogen bonds are stronger than covalent bonds.

True or False: Hydrogen bonds only occur in water.