1.2 Water & Hydrogen Bonding

Describe the structure of a water molecule.

A water molecule consists of two hydrogen atoms covalently bonded to one oxygen atom; the oxygen atom has a greater attraction for the electrons, making the molecule dipolar.

What is meant by a dipolar molecule?

A molecule with a slightly negative charge on one end and a slightly positive charge on the other.

Describe hydrogen bonding between water molecules.

A hydrogen bond forms between the slightly negative oxygen atom of one water molecule and the slightly positive hydrogen atom of another.

Why do hydrogen bonds form in water?

Because oxygen has a greater attraction for electrons, making it slightly negative and hydrogen slightly positive.

State two characteristics of hydrogen bonds.

• Each hydrogen bond is very weak (about one tenth the strength of a covalent bond). * Hydrogen bonds break and reform continuously.

How many hydrogen bonds can one water molecule form?

Up to four hydrogen bonds with other water molecules.

What is cohesion?

Hydrogen bonds hold water molecules together.

Why are water molecules described as 'sticky'?

Because hydrogen bonds cause cohesion between molecules.

Why is water a good solvent?

Polar molecules and ions are attracted to the weak charges on water molecules and dissolve readily.

What types of substances dissolve easily in water?

Charged substances and polar molecules.

What is high specific heat capacity?

4.2 J are necessary to increase the temperature of 1 g of water by 1°C.

Why does water have a high specific heat capacity?

Thermal energy is used to break hydrogen bonds between water molecules.

What is high latent heat of vaporisation?

Much thermal energy is needed to change water into water vapour.

Why is evaporation of water effective for cooling?

A lot of thermal energy is needed to evaporate small quantities of water.

What is high latent heat of fusion?

Much thermal energy is needed to change ice to water.

State one role of water as a solvent in organisms.

Solvent within cells or in transport media such as blood plasma.

How is water involved in photosynthesis?

It is a raw material and provides hydrogen ions and electrons.

What is the role of water in hydrolysis?

It is used in hydrolysis reactions such as digestion.

How does water act as a coolant?

Loss of water (e.g. sweating, transpiration) removes heat energy.

How does water provide support in plants?

It maintains turgidity in plant cells.

How does water act as a hydrostatic skeleton?

It provides support in animals such as worms and sea anemones.

Why does ice float on water?

Ice is less dense than water.

Why is this important for aquatic organisms?

Ice acts as insulation, preventing water below from freezing.

How does water provide buoyancy?

It supports aquatic organisms so they do not need strong skeletons.

Why does water not evaporate quickly from habitats?

High latent heat of vaporisation means much energy is needed.

Why is water described as reactive?

It splits into hydrogen ions (H⁺) and hydroxyl ions (OH⁻).

How is hydrogen bonding responsible for properties of water?

Hydrogen bonds require energy to break, which gives water properties such as high specific heat capacity, cohesion, and high latent heat.

What percentage of animals and plants is water?

• Animals: ~70% * Plants: ~90%

What is a solvent?

A liquid in which a solute dissolves.

What is a solute?

A substance that dissolves in a solvent.

What is a soluble substance?

A substance that can dissolve in a solvent.

What is an insoluble substance?

A substance that cannot dissolve in a solvent.

What is a dipolar molecule?

A molecule with a slightly positive and slightly negative end.

What is cohesion?

Hydrogen bonds hold water molecules together.

Why do hydrogen bonds form between water molecules?

Because oxygen is more electronegative than hydrogen, making oxygen slightly negative (δ⁻) and hydrogen slightly positive (δ⁺), causing attraction between molecules.

Name biological molecules that dissolve in water (any seven).

• Glucose

• Amino acids

• Salts (NaCl)

• Nucleotides

• Many hormones

• Enzymes

• Urea

Name biological molecules that do not dissolve in water (any seven).

• Lipids

• Triglycerides

• Phospholipids

• Cholesterol

• Cellulose

• Starch (insoluble form in plants)

• Glycogen (insoluble form)

Explain the importance of water as a component of cells.

Water acts as a solvent, allowing metabolic reactions to occur and substances to dissolve and move within cells.

Explain the importance of water as a transport medium

Water dissolves substances such as nutrients and gases and transports them in blood plasma, lymph, xylem, and phloem.

Explain the importance of water as a coolant.

Water removes heat energy during evaporation (sweating and transpiration), cooling organisms.

State advantages of water as an environment for organisms.

• Stable temperature (high specific heat capacity)

• Buoyancy reduces need for strong skeletons

• Solvent for nutrients and gases

• Ice floats, insulating aquatic life

State disadvantages of water as an environment for organisms.

• Low oxygen concentration compared to air

• Light penetration decreases with depth

• Movement can be difficult in dense water

• Temperature changes can still occur in shallow water

Why is liquid water important for life on Earth?

Because it acts as a universal solvent, supports metabolic reactions, regulates temperature, and allows transport of substances in living organisms.

What happened to water on Venus?

Venus lost its liquid water due to extreme greenhouse heating, causing water to evaporate and escape into space.