2.5.1 Properties of Water

The properties of water that make it the medium of life are:

• High polarity
• Hydrogen bonding
• Cohesion and adhesion
• High specific heat capacity
• High heat of vaporization
• Hydrophobic exclusion
• Ionization
• Lower density of ice

1. High Polarity

Definition: A polar covalent bond is a chemical bond in which shared electrons are pulled closer to the more electronegative atom, making it partially negative and the other atom partially positive.

In simple words: Water is a polar molecule because it has a slightly negative end and a slightly positive end.

Key Points:

• The bonds which are formed by the mutual sharing of electrons between two atoms are called covalent bonds.
• Normally the sharing of electrons between two atoms is fairly equal and the covalent bond is nonpolar.
• In the case of water, the sharing of electrons between oxygen and hydrogen is not completely equal, so the covalent bond is polar.
• In H₂O:
• The O atom has a slight negative charge.
• Each H atom has a slight positive charge.
• Even though H₂O as a whole is neutral.
• Because of its polar covalent bonds, water is a polar molecule — it has a slightly negative pole and two slightly positive ones.

Importance of Polarity:

• The polarity of water molecules makes it an excellent or universal solvent for polar substances.
• Ionic compounds or electrolytes can be easily dissolved in water.
• Non-polar substances having charged groups in their molecules can also be dissolved in water.
• Such compounds, when dissolved in water, dissociate into positive and negative ions and are in a more favourable state to react with other molecules and ions.
• This is the reason why all chemical reactions in living beings occur in aqueous medium.

2. Hydrogen Bonding

Definition: The polarity of water molecules makes them interact with each other. The charged regions on each molecule are attracted to oppositely charged regions on neighbouring molecules, forming weak bonds.

In simple words: Hydrogen bonds are weak attractions between the positive and negative ends of water molecules.

Key Points:

• Since the positively charged region in this special type of bond is always an H atom, the bond is called a hydrogen bond.
• This bond is often represented by a dotted line because a hydrogen bond is easily broken.
• Because of hydrogen bonding, water is a liquid at temperatures suitable for life.
• The high cohesion and adhesion force of water is due to the presence of hydrogen bonds in water, which in turn makes water a transport medium.

3. Cohesion and Adhesion

Cohesion: Cohesion is the attraction among the water molecules which enables the water molecules to stick together.

Adhesion: Water molecules also have attraction to polar surfaces. This attraction is called adhesion.

• Cohesion = water molecules stick to each other.
• Adhesion = water molecules stick to other surfaces.

Key Points:

• Water flows freely due to cohesion.
• Both cohesion and adhesion are due to hydrogen bonds among water molecules.
• These properties of water enable it to circulate in living bodies and to act as a transport medium.

Examples:

• Cohesion: Water molecules forming droplets.
• Adhesion: Water sticking to the sides of a glass tube (capillary action).

4. High Specific Heat Capacity

Definition: Heat capacity can be defined as the amount of heat required for minimum increase in temperature of a substance.

Specific Heat Capacity of Water: The specific heat capacity of water can be represented as the number of calories required to raise the temperature of 1g of water up to 1°C, i.e., 1 Calorie (4.18 Joules).

In simple words: Water can absorb a lot of heat without its temperature rising much.

Key Points:

• Water has relatively a very high heat capacity than any other substance due to its hydrogen bonding.
• Much of the heat absorbed by water is utilized in the breakdown of hydrogen bonding.
• Therefore, it does not manifest itself to raise the temperature of water.
• Hence, a very large amount of heat can increase very little in temperature in water.
• Due to its high heat capacity, water works as a temperature stabilizer or regulator for organisms in hot environments.
• It protects living material against sudden thermal changes.

5. High Heat of Vaporization

Definition: Heat of vaporization is the amount of heat required to convert a unit mass of a liquid into gaseous form.

Heat of Vaporization of Water: Water has a high heat of vaporization, i.e., 574 calories per gram.

In simple words: Water requires a lot of heat to turn into vapor.

Key Points:

• The high heat of vaporization means that a large amount of heat can be lost with minimal loss of water from the body.
• It is the high heat of vaporization of water that gives animals an efficient way to release excess body heat in a hot environment.
• When an animal sweats, body heat is used to vaporize the sweat, thus cooling the animal.
• Due to this property of water, evaporation of only 2 ml out of one litre of water lowers the temperature of the remaining 998 ml water by 1°C.

6. Hydrophobic Exclusion

Definition: Hydrophobic exclusion can be defined as the reduction of the contact area between water and hydrophobic substances which are placed in water.

In simple words: Hydrophobic exclusion is the tendency of nonpolar substances to cluster together in water to minimize contact with water.

Example: If you place a few drops of oil on the surface of a water solution, the oil drops will tend to join into a single drop.

Biological Importance:

• Hydrophobic exclusion plays key roles in maintaining the integrity of lipid bilayer membranes.
• The hydrophobic tails of phospholipids face each other, away from water, while the hydrophilic heads face water.

7. Ionization

Definition: The dissociation of a molecule into ions is called ionization.

Ionization of Water: When a water molecule ionizes, it releases an equal number of positive hydrogen (H⁺) and negative hydroxyl (OH⁻) ions.

H_2O \rightleftharpoons H^+ + OH^-

Key Points:

• This reaction is reversible, but equilibrium is maintained at 25°C.
• The H⁺ and OH⁻ ions affect and take part in many of the reactions that occur in cells.
• It helps to maintain or change the pH of the medium.

8. Lower Density of Ice

Key Points:

• Ice floats on water because ice is less dense than water.
• The reason is that ice has a giant structure and shows the maximum number of hydrogen bonding among water molecules.
• Hence, they are arranged like a lattice.
• In freezing weather, ice forms on the surface of ponds and lakes, forming an insulating layer above the water below.
• This provides a living environment for some organisms until the ice melts.
• Organisms can also live under the ice.

Intermediate Level

Properties of Water and Their Importance

Why Water is Called the Universal Solvent

• Water is a polar molecule.
• It can dissolve ionic compounds (electrolytes) by surrounding the ions.
• It can dissolve polar molecules (e.g., sugars) by forming hydrogen bonds.
• It cannot dissolve nonpolar molecules (e.g., oils).
• Because it dissolves so many substances, water is called the universal solvent.

Why Water is Important for Temperature Regulation

• Water has a high specific heat capacity.
• It absorbs a lot of heat before its temperature rises.
• It releases a lot of heat before its temperature falls.
• This helps organisms maintain a stable internal temperature.
• Water also has a high heat of vaporization.
• Sweating removes excess body heat through evaporation.

Why Ice Floats

• Ice is less dense than water.
• Water molecules in ice are arranged in a lattice with maximum hydrogen bonding.
• This lattice has more space between molecules, making ice less dense.
• Ice floats on water and forms an insulating layer.
• This allows organisms to survive in water below the ice.

Important Points

• 1. Water is one of the main constituents on earth.
• 2. Origin, sustainability, and continuity of life depend on water.
• 3. More than two-thirds of the earth is covered by water.
• 4. Distribution of life on earth is directly proportional to the availability of water.
• 5. Approximately 70 percent of any organism is formed of water.
• 6. The lowest water content is 20% in seeds and bones.
• 7. The highest water content is 85–90% in brain cells.
• 8. Jellyfish has an exceptionally large amount of water — 99% — making the body of jellyfish transparent.
• 9. Properties of water that make it the medium of life:
• - High polarity
• - Hydrogen bonding
• - Cohesion and adhesion
• - High specific heat capacity
• - High heat of vaporization
• - Hydrophobic exclusion
• - Ionization
• - Lower density of ice
• 10. High polarity: Water is a polar molecule; it has a slightly negative pole and two slightly positive ones.
• 11. Hydrogen bonding: The charged regions on water molecules are attracted to oppositely charged regions on neighbouring molecules, forming weak bonds called hydrogen bonds.
• 12. Cohesion and adhesion: Cohesion is the attraction among water molecules; adhesion is the attraction of water molecules to polar surfaces.
• 13. High specific heat capacity: Water has a high heat capacity due to hydrogen bonding; it works as a temperature stabilizer.
• 14. High heat of vaporization: Water has a high heat of vaporization (574 cal/g); it gives animals an efficient way to release excess body heat.
• 15. Hydrophobic exclusion: Reduction of contact area between water and hydrophobic substances; important for lipid bilayer integrity.
• 16. Ionization: Water dissociates into H⁺ and OH⁻ ions; helps maintain or change pH.
• 17. Lower density of ice: Ice is less dense than water because of its giant structure and maximum hydrogen bonding; ice floats and insulates water below.

Remember

• - Water = most abundant molecule in organisms (~70%)
• - Lowest water content = 20% (seeds and bones)
• - Highest water content = 85–90% (brain cells)
• - **Jellyfish** = 99% water
• - Properties of water:
• 1. High polarity
• 2. Hydrogen bonding
• 3. Cohesion and adhesion
• 4. High specific heat capacity
• 5. High heat of vaporization
• 6. Hydrophobic exclusion
• 7. Ionization
• 8. Lower density of ice
• - High polarity = universal solvent
• - Hydrogen bonding = liquid at suitable temperatures
• - Cohesion and adhesion = transport medium
• - High specific heat capacity = temperature stabilizer
• - High heat of vaporization = efficient cooling
• - Hydrophobic exclusion = lipid bilayer integrity
• - **Ionization** = maintains pH
• - Lower density of ice = life under ice

Common Mistakes

• 1. Thinking water is only a solvent – Water is also a reactant in hydrolysis reactions.
• 2. Confusing cohesion and adhesion – Cohesion = water molecules stick to each other; Adhesion = water molecules stick to other surfaces.
• 3. Thinking water has a low specific heat capacity – Water has a high specific heat capacity.
• 4. Thinking water has a low heat of vaporization – Water has a high heat of vaporization.
• 5. Confusing hydrophobic exclusion with hydrophobic interactions – They are related but not the same. Hydrophobic exclusion is the reduction of contact between water and hydrophobic substances.
• 6. Thinking ice is denser than water – Ice is less dense than water, which is why it floats.
• 7. Forgetting that water ionizes – Water dissociates into H⁺ and OH⁻ ions.
• 8. Thinking all organisms have the same water content – Water content varies: 99% in jellyfish, 85–90% in brain cells, 20% in seeds and bones.
• 9. Confusing the properties of water with its functions – Properties (e.g., high polarity) lead to functions (e.g., universal solvent).
• 10. Thinking water is not important for temperature regulation – Water is crucial for temperature regulation due to its high specific heat capacity and high heat of vaporization.
• 11. Forgetting the importance of ice floating – Ice floating insulates water below, allowing life to survive under the ice.
• 12. Thinking hydrolysis releases energy – Hydrolysis does not release energy like respiration. It is a chemical reaction where water is used to break bonds.

Quick Revision

• Water is one of the main constituents on earth.
• Origin, sustainability, and continuity of life depend on water.
• More than two-thirds of the earth is covered by water.
• Distribution of life on earth is directly proportional to the availability of life, e.g., deserts have few living organisms whereas rivers and oceans are full of life.
• Approximately 70 percent of any organism is formed of water.
• Water is the most abundant component in any organism, the lowest is 20% in seeds and bones and the highest is 85–90% in brain cells.
• Jellyfish has an exceptionally large amount of water, i.e., 99%, making the body of jellyfish transparent.
• The properties of water that make it the medium of life are:

60-Second Revision

• Water = most abundant molecule (~70%)
• Lowest water content = 20% (seeds and bones)
• Highest water content = 85–90% (brain cells)
• **Jellyfish** = 99% water
• Properties of water:
• High polarity = universal solvent
• Hydrogen bonding = liquid at suitable temperatures
• Cohesion and adhesion = transport medium
• High specific heat capacity = temperature stabilizer
• High heat of vaporization = efficient cooling
• Hydrophobic exclusion = lipid bilayer integrity
• **Ionization** = maintains pH
• Lower density of ice = life under ice
• Warning: Water is not just a solvent; it is also a reactant in hydrolysis. Cohesion and adhesion are different. Ice is less dense than water. Water has a high specific heat capacity and high heat of vaporization. Do not confuse the properties with their functions.