Do you ever wonder why water sticks to your hair after showering? Or how water climbs up the root system of plants? Or why do summer and winter temperatures seem to be less harsh in coastal areas?
Water is one of the most abundant and important substances on Earth. Its many unique properties allow it to sustain life from the cellular level to the ecosystem. Many of water's unique qualities are due to the polarity of its molecules, notably their capacity to form hydrogen bonds with one another and with other molecules.
Here, we will define hydrogen bonding in water, elaborate on its mechanisms, and discuss the different properties of water imparted by hydrogen bonding.
What is hydrogen bonding?
A hydrogen (H) bond is a bond that forms between a partially positively charged hydrogen atom and an electronegative atom, typically fluorine (F), nitrogen (N), or oxygen (O).
Examples of where hydrogen bonds can be found include water molecules, amino acids in protein molecules, and the nucleobases that form nucleotides in the two strands of DNA.
How do hydrogen bonds form?
When atoms share valence electrons, a covalent bond is formed. Covalent bonds are either polar or non-polar depending on the electronegativity of the atoms (the ability of an atom to attract electrons when in a bond).
Due to the unequal sharing of electrons, a polar molecule has a partially positive region on one side and a partially negative region on the other. Because of this polarity, a hydrogen atom with a polar covalent bond to an electronegative atom (for example, nitrogen, fluorine, and oxygen) is attracted to electronegative ions or negatively charged atoms of other molecules.
This attraction leads to the formation of a hydrogen bond.
Hydrogen bonds are not 'real' bonds in the same way that covalent, ionic, and metallic bonds are. Covalent, ionic, and metallic bonds are intramolecular electrostatic attractions, meaning they hold atoms together within a molecule. On the other hand, hydrogen bonds are intermolecular forces meaning they occur between molecules. Although hydrogen bond attractions are weaker than real ionic or covalent interactions, they are powerful enough to create essential properties, which we will discuss later.
Hydrogen bonding in water: biology
Water consists of two hydrogen atoms attached via covalent bonds to one oxygen atom (H-O-H). Water is a polar molecule because its hydrogen and oxygen atoms share electrons unequally due to differences in electronegativity.
Each hydrogen atom contains a nucleus made up of a single positively-charged proton with one negatively-charged electron orbiting the nucleus. On the other hand, each oxygen atom contains a nucleus made up of eight positively charged protons and eight uncharged neutrons, with eight negatively charged electrons orbiting the nucleus.
The oxygen atom has a higher electronegativity than the hydrogen atom, so electrons are attracted to oxygen and repelled by hydrogen. When the water molecule is formed, the ten electrons pair up into five orbitals distributed as follows:
One pair is linked to the oxygen atom.
Two pairs are linked to the oxygen atom as outer electrons.
Two pairs form the two O-H covalent bonds.
When the water molecule is formed, two lone pairs are left. The two lone pairs associate themselves with the oxygen atom. As a result, oxygen atoms have a partial negative (δ-) charge, while hydrogen atoms have a partial positive (δ+) charge.
This means the water molecule has no net charge, but the hydrogen and oxygen atoms have partial charges.
Because the hydrogen atoms in a water molecule are partially positively charged, they are attracted to partially negative oxygen atoms in nearby water molecules, allowing hydrogen bonds to form between nearby water molecules or other molecules with a negative charge. Hydrogen bonding occurs constantly between water molecules. While individual hydrogen bonds tend to be weak, they create a considerable impact when they form in large numbers, which is usually the case for water and organic polymers.
What is the number of hydrogen bonds that can form in water molecules?
Water molecules contain two lone pairs and two hydrogen atoms, all of which are connected to the strongly electronegative oxygen atom. This means that up to four bonds (two where it is the receiving end of the h-bond, and two where it is the giver in the h-bond) can be formed by each water molecule.
However, because hydrogen bonds are weaker than covalent bonds, they form, break, and reconstruct easily in liquid water. As a result, the precise number of hydrogen bonds created per molecule varies.
What are the effects and consequences of hydrogen bonding in water?
Hydrogen bonding in water imparts several properties that are important in sustaining life. In the following section, we will talk about some of these properties.
Solvent property
Water molecules are excellent solvents. Polar molecules are hydrophilic ("water-loving") substances.
Hydrophilic molecules interact with and dissolve easily in water.
This is because the negative ion of the solute will attract the positively charged region of the water molecule and vice versa, causing the ions to dissolve.
Sodium chloride (NaCl), also known as table salt, is an example of a polar molecule. It dissolves easily in water because the partially negative oxygen atom of the water molecule is attracted to the partially positive Na+ ions. On the other hand, the partially positive hydrogen atoms are attracted to the partially negative Cl- ions. This causes the NaCl molecule to dissolve in water.
Moderation of temperature
The hydrogen bonds in water molecules react to changes in temperature, giving water its unique characteristics in its solid, liquid, and gas states.
In its liquid state, water molecules constantly move past each other as the hydrogen bonds continuously break and recombine.
In its gas state, water molecules have higher kinetic energy, causing hydrogen bonds to break.
In its solid state, water molecules expand because the hydrogen bonds push the water molecules apart. At the same time, the hydrogen bonds hold the water molecules together, forming a crystalline structure. This gives ice (solid water) a lower density compared to liquid water.
Hydrogen bonding in water molecules gives it a high specific heat capacity.
Specific heat refers to the amount of heat that must be taken in or lost by one gram of substance for its temperature to change by one degree Celsius.
The high specific heat capacity of water means it takes a lot of energy to cause changes in temperature. The high specific heat capacity of water allows it to maintain a stable temperature, vital in sustaining life on Earth.
Similarly, hydrogen bonding gives water high heat of vaporization,
The heat of vaporization is the amount of energy it takes for a liquid substance to become gaseous.
In fact, it takes 586 cal of heat energy to change one gram of water to gas. This is because hydrogen bonds need to be broken for liquid water to enter its gas state. Once it reaches its boiling point (100° C or 212° F), the hydrogen bonds in water break, causing water to evaporate.
Cohesion
Hydrogen bonding causes water molecules to stay close to each other which makes water a highly cohesive substance.
It is what makes water "sticky".
Cohesion refers to the attraction of similar molecules--in this case, water--holding the substance together.
Water clumps together to form "drops" because of its cohesive property. Cohesion results in another property of water: surface tension.
Surface tension
Surface tension is the property that allows a substance to resist tension and prevent rupture.
The surface tension created by hydrogen bonds in water is similar to people forming a human chain to prevent others from breaking through their joined hands.
Both the cohesion of water to itself and the strong adhesion of water to the surface that it is touching cause water molecules close to the surface to move down and to the side.
On the other hand, the air pulling up exerts a little force on the water's surface. As a result, a net force of attraction is produced between water molecules at the surface, resulting in a highly flat, thin sheet of molecules. Water molecules on the surface adhere to one another, preventing items lying on the surface from sinking.
Surface tension is why a paper clip that you carefully place on the water's surface can float. While this is the case, a heavy object, or one that you didn't place carefully on the water's surface, can break the surface tension, causing it to sink.
Adhesion
Adhesion refers to the attraction between different molecules.
Water is highly adhesive; it adheres to a wide range of various things. Water attaches to other things for the same reason it sticks to itself — it is polar; thus, it is attracted to charged substances. Water attaches to various surfaces, including plants, utensils, and even your hair when it is wet after showering.
In each of these scenarios, adhesion is the reason why water adheres to or wets something.
Capillarity
Capillarity (or capillary action) is the tendency of water to climb up a surface against the force of gravity due to its adhesive property.
This tendency is due to the water molecules being more attracted to such surfaces than other water molecules.
If you have dipped a paper towel in water before, you might have noticed that the water would "climb up" the paper towel against the force of gravity; this happens thanks to capillarity. Similarly, we can observe capillarity in fabric, soils, and other surfaces where there are small spaces through which liquids can move.
What is the importance of hydrogen bonding in water in biology?
In the previous section, we discussed the properties of water. How are these enabling biochemical and physical processes that are essential in sustaining life on Earth? Let's discuss some specific examples.
Water being an excellent solvent means it can dissolve a wide range of compounds. Since most crucial biochemical processes occur in a watery environment inside cells, this property of water is critical in allowing these processes to occur. Water's high specific heat capacity enables large bodies of water to regulate temperature.
For example, coastal areas get less harsh summer and winter temperatures than big land masses' because land masses lose heat more quickly than water.
Similarly, water's high heat of vaporization means that in the process of changing from liquid to gas state, a lot of energy is consumed, causing the surrounding environment to cool down.
For example, sweating in many living organisms (including humans) is a mechanism that maintains a homeostasis of body temperature by cooling down the body.
The cohesion, adhesion, and capillarity are important properties of water that enable the uptake of water in plants. Water can climb up the roots thanks to capillarity. It can also move through the xylem to bring water up to the branches and leaves.
Hydrogen Bonding in Water - Key takeaways
- A hydrogen bond is a bond that forms between a partially positively charged hydrogen atom and an electronegative atom.
- Water is a polar molecule: its oxygen atoms have a partial negative (δ-) charge, while its hydrogen atoms have a partial positive (δ+) charge.
- These partial charges allow hydrogen bonds to form between a water molecule and nearby water molecules or other molecules with a negative charge.
- Due to hydrogen bonding, water molecules have properties that are important in sustaining life.
- These properties include solvent capability, moderation of temperature, cohesion, surface tension, adhesion, and capillarity.
References
- Zedalis, Julianne, et al. Advanced Placement Biology for AP Courses Textbook. Texas Education Agency.
- Reece, Jane B., et al. Campbell Biology. Eleventh ed., Pearson Higher Education, 2016.
- University of Hawai‘i at Mānoa, Exploring Our Fluid Earth. Hydrogen Bonds Make Water Sticky.
- “15.1: Structure Of Water.” Chemistry LibreTexts, 27 June 2016.
- Belford, Robert. “11.5: Hydrogen Bonds.” Chemistry LibreTexts, 3 Jan. 2016.
- Water Science School. “Adhesion and Cohesion of Water.” U.S. Geological Survey, 22 Oct. 2019.
- Water Science School. “Capillary Action and Water.” U.S. Geological Survey, 22 Oct. 2019.
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