Properties of Water
Water looks simple, but its molecular structure gives it a remarkable collection of properties. Those properties help organisms regulate temperature, transport materials, maintain cells, and carry out chemical reactions.
The Big Idea
Many of water's unusual properties can be traced back to one important fact: water molecules are polar and form hydrogen bonds with one another.
A water molecule contains two hydrogen atoms bonded to one oxygen atom. Oxygen attracts the shared electrons more strongly than hydrogen does. As a result, the oxygen end of the molecule has a slight negative charge, while the hydrogen ends have slight positive charges.
This uneven distribution of charge is called polarity. Because opposite partial charges attract, neighboring water molecules are attracted to one another. These attractions are called hydrogen bonds.
Polarity
Polarity means that charge is distributed unevenly within a molecule, giving different regions of the molecule slightly different charges.
Water is polar because oxygen attracts the shared electrons more strongly than hydrogen. This gives the oxygen side of water a partial negative charge and the hydrogen sides partial positive charges.
Water's polarity is extremely important because it allows water molecules to interact with other polar molecules and ions.
When salt dissolves in water, the charged particles in the salt are attracted to the partial charges on water molecules. Water molecules surround the ions and help separate them.
Why can water interact strongly with ions such as Na⁺ and Cl⁻?
Because water is polar and has partial positive and negative regions that attract charged particles.
Hydrogen Bonding
A hydrogen bond is a weak attraction between a partially positive hydrogen atom and an electronegative atom such as oxygen in a nearby molecule.
Hydrogen bonds are not the same thing as the covalent bonds holding the atoms together inside a water molecule.
The O–H bonds within a water molecule are polar covalent bonds. Hydrogen bonds are attractions between separate molecules.
one molecule
another molecule
Which is stronger within a single water molecule: the O–H covalent bond or a hydrogen bond between water molecules?
The O–H covalent bond.
Cohesion
Cohesion is the attraction between molecules of the same substance.
Water molecules are attracted to other water molecules because of hydrogen bonding. This attraction allows water to stay together as a continuous substance.
When water pours from a container, individual molecules do not immediately separate from one another. Their attraction helps maintain a continuous stream.
Cohesion = water sticking to water.
Adhesion
Adhesion is the attraction between molecules of different substances.
Because water is polar, water molecules can be attracted to other polar or charged surfaces. This is why water can cling to glass, plant tissues, paper fibers, and other materials.
After a rainstorm, water droplets can remain attached to a spider web or blade of grass. The water is being attracted to the surface.
Cohesion: water → water
Adhesion: water → another surface
Surface Tension
Surface tension is the resistance of a liquid surface to being broken or stretched, caused by attractions between molecules at the surface.
Water molecules at the surface experience strong attractions to neighboring water molecules. This creates a surface that behaves somewhat like a thin elastic film.
A carefully placed needle can remain on the surface of water. Some insects can also move across the surface without sinking.
Capillary Action
Capillary action is the movement of a liquid through a narrow space caused by the combined effects of adhesion and cohesion.
Water can move upward through narrow tubes even when gravity would normally pull it downward. Adhesion pulls water toward the walls of the tube, while cohesion helps additional water molecules follow.
Plants rely on capillary action as water moves through tiny tubes in their tissues. Water can travel from the roots toward the leaves.
Paper towels provide another familiar example. Water adheres to the fibers while cohesion helps pull additional water molecules along.
High Specific Heat
Specific heat is the amount of heat required to raise the temperature of a substance by a given amount. Water has a relatively high specific heat.
Because water molecules are strongly attracted to one another, added energy can be absorbed without producing an immediate large increase in temperature.
This means water can absorb substantial amounts of heat while changing temperature relatively slowly.
Large bodies of water absorb and store heat from the Sun. As a result, coastal regions often experience less extreme temperature changes than comparable inland regions.
Water's high specific heat helps organisms maintain relatively stable internal temperatures even when the surrounding environment changes.
High Heat of Vaporization
Heat of vaporization is the energy required to change a liquid into a gas. Water has a relatively high heat of vaporization because considerable energy is needed to overcome attractions between water molecules.
When the highest-energy water molecules escape from the liquid during evaporation, the remaining liquid loses energy.
That is why evaporation can produce a cooling effect.
Sweat on your skin does not cool you simply because the water is "cold." As water evaporates, it requires energy to escape the liquid. That energy comes from the surroundings, including your skin.
Why does evaporation of sweat cool the body?
Evaporation requires energy, and that energy is taken from the surroundings, lowering the temperature of the skin.
Solvent Capabilities
Water is an excellent solvent because its polarity allows it to interact with and separate many ions and polar molecules.
When a substance dissolves, its particles become distributed throughout the solvent. Water can dissolve many substances that are important to living organisms.
Salt dissolves in water. Sugar dissolves in water. Many nutrients and other substances can also be transported in water-based solutions inside organisms.
This makes water especially useful for biological reactions and transport.
A substance that dissolves another substance is called the solvent. The substance being dissolved is the solute.
Expansion Upon Freezing
Unlike most substances, water expands when it freezes, making solid water less dense than liquid water.
As water freezes, hydrogen bonds help arrange water molecules into a more organized structure. The molecules occupy more space than they do in liquid water.
Because ice is less dense than liquid water, ice floats. This is extremely important in aquatic ecosystems.
When a pond freezes, ice forms at the surface. The ice can provide insulation, helping keep the liquid water underneath from freezing completely and allowing aquatic organisms to survive.
Expansion upon freezing also explains why freezing water can damage pipes, crack rocks, and create other physical changes in the environment.
Putting It All Together
The properties of water are not ten completely separate facts. They are connected.
Water's polarity leads to hydrogen bonding, and hydrogen bonding helps produce many of water's unique properties.
For example, hydrogen bonding contributes to cohesion. Cohesion contributes to surface tension. Adhesion and cohesion work together to produce capillary action. Hydrogen bonding also helps explain why significant energy is required to change water's temperature or change liquid water into a gas.
The same molecular characteristics that make water useful for transporting substances also help organisms maintain stable temperatures and survive in aquatic environments.
Common Misconceptions
Watch Out for These
Correction: Cohesion is attraction between molecules of the same substance. Adhesion is attraction between different substances.
Correction: The O–H bonds inside water are polar covalent bonds. Hydrogen bonds are attractions between separate molecules.
Correction: Ice is actually less dense than liquid water because its hydrogen-bonded structure holds molecules farther apart.
Correction: High specific heat means water resists changes in temperature. It takes a relatively large amount of energy to raise its temperature.
Correction: Evaporation requires energy. That energy comes from the surroundings, helping cool the skin.
Correction: Capillary action involves both adhesion to the surface and cohesion between water molecules.
Correction: Water is an excellent solvent for many ionic and polar substances, but it does not dissolve every substance equally well.
Check Your Understanding
Try these questions without looking back through the article. Then open each answer to check your thinking.
1. What makes water a polar molecule?
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2. What is the difference between cohesion and adhesion?
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3. Why can some insects walk across the surface of water?
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4. Why can water move upward through a plant?
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5. Why do coastal areas tend to have more moderate temperatures than inland areas?
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6. Why does sweating help cool the body?
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7. Why is water such an important solvent in living organisms?
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8. Why does ice float on liquid water?
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9. A paper towel absorbs a spill and the water moves through its fibers. Which property or combination of properties best explains this?
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10. Explain how polarity and hydrogen bonding are connected to several of water's other properties.
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Key Takeaways
Water is polar because its electrons are distributed unevenly.
Polar water molecules form hydrogen bonds with one another.
Hydrogen bonding contributes to cohesion, surface tension, specific heat, and heat of vaporization.
Adhesion + cohesion contribute to capillary action.
Water's polarity makes it an excellent solvent for many ionic and polar substances.
Water's unusual behavior when freezing makes ice less dense than liquid water.
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