Hydrogen bonding

What is the difference between a covalent bond and a hydrogen bond in water molecules?

• Covalent bonds are strong bonds that occur within a water molecule, where oxygen and hydrogen atoms share electrons.

• Hydrogen bonds are weak attractions that occur between water molecules, where the slightly positive hydrogen atom of one molecule is attracted to the slightly negative oxygen atom of another molecule.

What is the difference between cohesion and adhesion in water?

• Cohesion refers to water molecules sticking to each other, primarily due to hydrogen bonds. This is why water forms droplets and has surface tension.

• Adhesion refers to water molecules sticking to other materials or surfaces. This is why water climbs up plant stems (capillary action) or spreads on a glass surface.

What property of water is responsible for surface tension, and what causes it?

Surface tension is caused by the cohesion of water molecules due to hydrogen bonds. At the surface, water molecules experience a stronger pull toward each other, creating a “skin” that makes water resist breaking apart easily.

Why does ice float on water, even though most solids are denser than their liquid forms?

When water freezes, hydrogen bonds arrange the molecules into a hexagonal lattice structure. This arrangement forces the molecules farther apart, making ice less dense than liquid water. That’s why ice floats.

Why does water have a higher boiling point than other small molecules like methane (CH₄)?

Water’s high boiling point is due to hydrogen bonds between the molecules. These bonds require significant energy to break, so it takes more heat for water molecules to escape into a gas phase compared to non-polar molecules like methane.

How do hydrogen bonds contribute to water’s high specific heat capacity?

Hydrogen bonds between water molecules absorb heat energy without letting the molecules move too much right away. This means that much of the heat goes into loosening or breaking hydrogen bonds, which delays a change in temperature. This is why water can absorb a lot of heat without heating up quickly.

What would happen if water couldn’t form hydrogen bonds?

Without hydrogen bonds, water wouldn’t be a liquid at normal Earth temperatures—it would behave more like a gas. Cells would lose their ability to hold water in cytoplasm, water wouldn’t form droplets, rain wouldn’t fall, and water wouldn’t be a universal solvent, which would be catastrophic for life on Earth.

Why does ammonia (NH₃) have a lower boiling point than water, even though both form hydrogen bonds?

While ammonia does form hydrogen bonds, they are weaker than those in water. This is because nitrogen (in NH₃) is less electronegative than oxygen (in H₂O), making the partial charges in ammonia smaller and the hydrogen bonds weaker. Therefore, less energy is required to break these bonds in ammonia.

What would happen to water's surface tension if its hydrogen bonds were stronger?

If water had stronger hydrogen bonds, its surface tension would increase, making water molecules stick together even more tightly at the surface. This would result in a stronger "film" at the surface of the water, resisting deformation or breaking.

How does hydrogen bonding affect the density of ice compared to liquid water?

Hydrogen bonds form a crystalline structure in ice, which forces water molecules into a fixed pattern with more space between them. This spread-out structure makes ice less dense than liquid water, allowing it to float.

How do hydrogen bonds influence how water moves and behaves in everyday situations?

Hydrogen bonds cause water molecules to stick to each other (cohesion), which is why water flows in a continuous stream and forms droplets. This also affects how water moves through narrow spaces, such as in plant capillaries, because of the combination of cohesion and adhesion.