Water: Structure, Properties, and Global Impact
The Water Molecule
Each water molecule consists of two hydrogen (H) atoms and one oxygen (O) atom. The water molecule has a bent shape, with the H-O-H bond angle being approximately 105°. The two O-H bonds are polar covalent, which makes the entire molecule polar.
Hydrogen Bonding
Hydrogen bonds form when a hydrogen atom, already bonded to an electronegative atom (like oxygen), is attracted to another electronegative atom in a different molecule. These crucial intermolecular links are responsible for many of water’s unique properties.
Water in the Human Body
An average person has a daily water intake of approximately 2.6 liters, distributed as follows:
- 1.3 liters from drinking
- 1 liter from food
- 0.3 liters produced by metabolism
Daily water output includes:
- 1.4 liters in urine
- 1 liter through transpiration and evaporation from the skin and lungs
- 0.21 liters in bowel movements
Global Water Distribution
The total water found on Earth is called the hydrosphere, encompassing oceans, seas, lakes, ice caps, groundwater, rivers, as well as water in living organisms and the atmosphere.
Earth’s surface is approximately 29% land and 71% water. Of the total water on Earth:
- Approximately 97% is saltwater (oceans, seas).
- Approximately 3% is freshwater (ice caps, groundwater, rivers, lakes).
Solid Water: Ice
Water molecules in ice arrange into an open, crystalline structure. Strong hydrogen bonds form between the oxygen atom of one water molecule and the hydrogen atom of another. The density of ice is less than that of liquid water, which is why ice floats.
Water Phase Diagram
Understanding water’s phase changes is crucial:
- At 1 atmosphere (atm) pressure, water freezes at 0°C and boils at 100°C.
- At an altitude of 4500 meters, where pressure is approximately 0.6 atm, water boils at 86°C.
- At the triple point, water coexists in all three states (solid, liquid, gas). This unique condition occurs at 0.01°C and 0.00603 atm.
Electrolysis of Water
Using electrical energy, water can be decomposed into hydrogen and oxygen gas through electrolysis:
2H2O → 2H2 + O2
Key Properties of Water
- Very low electrical conductivity: Pure water is a poor conductor of electricity.
- Excellent thermal regulator: Water can absorb or release significant amounts of heat without substantial temperature changes, due to its high specific heat capacity.
- Neutral pH: Pure water is neither acidic nor basic.
- Excellent solvent properties: Water is often called the “universal solvent” due to its great ability to dissolve many substances.
Water Hardness
Water hardness is determined by the concentration of dissolved mineral salts, primarily calcium and magnesium ions.
- Soft Water: Contains very low concentrations of dissolved ions. Example: distilled water.
- Hard Water: Contains significant concentrations of calcium and magnesium ions. These salts can deposit as scale in electric kettles, industrial boilers, pipes, and other fixtures. Hardness is further classified based on the type of ions present:
- Temporary Hardness: Caused by dissolved bicarbonates of calcium and magnesium. This type of hardness can be removed by boiling, as these compounds precipitate out.
- Permanent Hardness: Caused by sulfates and chlorides of calcium and magnesium. This type of hardness is not removed by boiling.
Heat Capacity of Water
- Heat capacity is the amount of heat required to change the temperature of a substance by one degree Celsius.
- Water has a remarkably high heat capacity compared to most other liquids and solids, with a specific heat of 1 calorie per gram per degree Celsius (1 cal/g°C).
Surface Tension of Water
Water exhibits a very high surface tension. This property makes water molecules ‘sticky’ and allows the surface to behave like an elastic film, causing water to form drops rather than spreading thinly.