Water: The Molecule of Life
Structure of Water Molecules
Chemical formula: H_2O
Each water molecule consists of two hydrogen atoms and one oxygen atom.
Shape: V shape with an angle of 104.5°
The bent shape is crucial to water's properties.
Two hydrogen atoms are joined to one oxygen atom by a single covalent bond.
Covalent bonds involve sharing electrons between atoms.
Polar molecule due to unequal distribution of shared electrons:
Oxygen is more electronegative which results in a partial negative charge (\delta^-), and hydrogen atoms have a partial positive charge (\delta^+).
Electronegativity is the ability of an atom to attract shared electrons in a chemical bond.
This polarity enables water to form hydrogen bonds.
Like tiny magnets, the positive end of one water molecule is attracted to the negative end of another.
Formation of Hydrogen Bonds
Hydrogen bonds form between water molecules due to the attraction between the partially positive hydrogen of one molecule and the partially negative oxygen of another.
Think of it as positive ends attracting negative ends, like magnets.
These bonds are essential for many of water's unique properties.
They help give water its life-supporting characteristics.
One water molecule can form hydrogen bonds with up to four other water molecules, creating a dynamic network.
This creates a sort of "chain" or web of interconnected water molecules.
Hydrogen bonds are individually weaker than covalent bonds, but collectively, they are strong enough to hold water molecules together, influencing water's high surface tension and boiling point.
Although each bond is weak, their large numbers add up to significant strength.
Properties of Water
Universal/Versatile Solvent
Water's polarity allows it to dissolve ionic and polar molecules.
Because water is polar, it can dissolve other polar substances and ionic compounds.
Hydration shells form around ions and polar molecules, keeping them dispersed in water.
These shells prevent the ions from clumping back together.
Provides an aqueous medium for biochemical reactions.
Many metabolic processes occur in water, which acts as a solvent and reactant.
Water helps to break down substances and enables chemical reactions.
Acts as a major transport medium in living organisms.
Transports nutrients and waste products in plants and animals.
For example, blood is mostly water and carries oxygen and nutrients.
High Specific Heat Capacity
Definition: The amount of heat energy required to change the temperature of 1 gram of water by 1°C.
Water can absorb a lot of heat without a big temperature change.
Water's specific heat capacity: 1 cal/g°C or 4.2 J/g°C.
Stabilizes ocean temperature, preventing large fluctuations, and moderates coastal climates.
This means the ocean doesn't heat up or cool down too quickly.
Helps organisms maintain a stable internal temperature, protecting them from thermal shock.
Water in our bodies helps keep
High Latent Heat of Vaporization
Definition: The amount of heat required to convert 1 gram of a substance from its liquid phase to its gaseous phase (vaporization).
Water has a high latent heat of vaporization because hydrogen bonds must be broken for water to evaporate.
A considerable amount of heat is needed to break these bonds.
This keeps the molecules together until enough energy is supplied to break them.
Water’s high heat of vaporization is approximately 540 cal/g.
This is much higher than many other substances.
Cooling effect through evaporation in organisms.
For example, sweating in animals and transpiration in plants.
When sweat evaporates, it absorbs heat from the body, providing a cooling effect.
Transpiration in plants helps regulate their temperature.
Cohesion of Water Molecules
Cohesion refers to the attraction between molecules of the same substance.
Water molecules exhibit strong cohesion due to the extensive network of hydrogen bonds.
These hydrogen bonds link water molecules together.
This is why water droplets form and why some insects can walk on water.
The collective strength of numerous hydrogen bonds creates a high surface tension.
Surface tension is a measure of how difficult it is to stretch or break the surface of a liquid.
Water has a high surface tension compared to many other liquids.
Cohesion is vital for water transport in plants.
Water moves against gravity from the roots to the leaves through tiny tubes called xylem.
Cohesion helps maintain a continuous column of water in the xylem.
As water evaporates from the leaves (transpiration), it pulls on the column of water in the xylem.
The cohesion between water molecules ensures that the entire column moves upward together.
Adhesion also plays a role as water molecules adhere to the walls of the xylem.
Maximum Density at 4°C
Water reaches its maximum density at 4°C.
This is an unusual property because most substances are densest in their solid form.
Water's density changes with temperature, behaving differently from other liquids.
Density is defined as mass per unit volume (\rho = \frac{m}{V}).
The density of water affects its behavior in various environmental conditions.
As water cools from higher temperatures, it becomes denser until it reaches 4°C.
Above 4°C, water acts like most other liquids, contracting and becoming denser as it cools.
The increase in density is due to the molecules packing closer together.
Below 4°C, water becomes less dense.
This is due to the formation of hydrogen bonds, which arrange the water molecules into a crystal lattice structure.
The crystal lattice structure expands the volume of water, thus reducing its density.
This expansion is why ice floats on liquid water.
This is why ice floats.
The density of ice is approximately 917 kg/m^3, while the density of liquid water at 4°C is 1000 kg/m^3.
Because ice is less dense than liquid water, it floats, preventing bodies of water from freezing solid.
The hydrogen bonds in ice form a crystal lattice that spaces the molecules farther apart than in liquid water.
This lattice structure is open and has more empty space compared to liquid water.
The arrangement of molecules in the crystal lattice is responsible for the decrease in density.
Floating ice insulates bodies of water.
Ice forms a protective layer