Unit 12: Water
Exploring the unique properties, vital roles, and treatment of the most important compound on Earth.
12.12 Unusual Properties of Water (Hydrogen Bonding)
Water ($H_2O$) is a simple molecule, but it has several remarkable properties that make it essential for life. These properties are a direct result of its ability to form hydrogen bonds.
Because oxygen is highly electronegative, the water molecule is polar, with a partial negative charge ($\delta-$) on the oxygen atom and partial positive charges ($\delta+$) on the hydrogen atoms. A hydrogen bond is a strong intermolecular force of attraction between the $\delta+$ hydrogen of one water molecule and the lone pair of electrons on the $\delta-$ oxygen of a neighboring molecule.
Key Unusual Properties:
- High Melting and Boiling Points: For a molecule of its small size, water has an unusually high melting point (0 °C) and boiling point (100 °C). This is because a significant amount of energy is required to overcome the strong hydrogen bonds between the molecules.
- Low Density of Ice: In its solid form (ice), water molecules arrange themselves in a regular, open lattice structure to maximize hydrogen bonding. This structure holds the molecules further apart than in the liquid state, making ice less dense than liquid water. This is why ice floats.
- High Surface Tension: The strong hydrogen bonds at the surface of water create a "skin-like" effect known as surface tension, allowing some insects to walk on water.
Solved Examples:
-
Why does ice float on liquid water?
Solution: When water freezes, the molecules form a crystal lattice with a large amount of empty space to maximize hydrogen bonding. This makes solid ice less dense than liquid water, causing it to float. -
Methane ($CH_4$, Molar Mass = 16 g/mol) is a gas at room temperature, while
water ($H_2O$, Molar Mass = 18 g/mol) is a liquid. Explain this
difference.
Solution: Methane is a non-polar molecule and only has weak van der Waals forces between its molecules. Water is polar and has strong hydrogen bonds in addition to van der Waals forces. Much more energy is needed to overcome the strong hydrogen bonds in water, resulting in its much higher boiling point.
12.13 Water as a Solvent
Water is often called the "universal solvent" because its polar nature allows it to dissolve a wide variety of substances.
- Dissolving Ionic Compounds: The partial positive ends ($\delta+$ H) of water molecules are attracted to the negative ions (anions) of an ionic lattice, while the partial negative ends ($\delta-$ O) are attracted to the positive ions (cations). These attractions can overcome the forces holding the lattice together, causing the compound to dissolve as hydrated ions.
- Dissolving Polar Molecular Compounds: Water can dissolve other polar molecules (like ethanol or sugar) because it can form hydrogen bonds with them.
Non-polar substances, like oil or wax, do not dissolve in water because they cannot form strong attractions with the polar water molecules.
Solved Examples:
-
Explain why salt (NaCl) dissolves in water but not in oil.
Solution: Salt is an ionic compound. The polar water molecules can surround and stabilize the Na⁺ and Cl⁻ ions, pulling them apart. Oil is a non-polar solvent and cannot form these strong attractions, so the ionic lattice of the salt remains intact. -
Why does ethanol ($C_2H_5OH$) mix readily with water?
Solution: Ethanol is a polar molecule and contains an -OH group, which allows it to form strong hydrogen bonds with water molecules, enabling it to dissolve easily.
12.14 Testing for Dissolved Substances in Water
The presence of dissolved substances can be detected in several ways:
- Evaporation: Evaporating a sample of water to dryness will leave behind any non-volatile dissolved solids as a crystalline residue.
- Conductivity: Pure water is a very poor conductor of electricity. If a sample of water conducts electricity, it indicates the presence of dissolved ions (electrolytes).
- pH Measurement: Pure water has a pH of 7. A pH lower than 7 indicates a dissolved acid, while a pH higher than 7 indicates a dissolved alkali.
- Chemical Tests: Specific chemical tests, such as the precipitation reactions for cations and anions discussed in Unit 8, can identify the presence of specific ions.
Solved Examples:
-
A sample of river water is tested with a conductivity meter and gives a high
reading. What does this indicate?
Solution: A high conductivity reading indicates the presence of a significant concentration of dissolved mobile ions, such as salts washed from rocks and soil. -
How would you test a sample of tap water for the presence of chloride
ions?
Solution: Add a few drops of dilute nitric acid, followed by a few drops of aqueous silver nitrate. If a white precipitate forms, chloride ions are present.
12.15 Uses of Water
Water is arguably the most useful substance on Earth, with countless applications:
- Biological: Essential for all known life. It acts as a solvent for biochemical reactions, a transport medium (in blood and plant sap), and a temperature regulator.
- Domestic: Used for drinking, cooking, washing, and sanitation.
- Industrial: Used as a solvent for chemical processes, a coolant in power stations and factories, and a raw material in the production of other chemicals.
- Agricultural: Essential for irrigation to grow crops.
- Recreational: Used for activities like swimming, boating, and fishing.
Solved Examples:
-
Name two industrial uses of water.
Solution: As a coolant in power plants and as a solvent in chemical manufacturing.
12.16 Water Treatment for Town Supply
Water from sources like rivers and reservoirs is not safe to drink directly. It must be treated to remove suspended particles, harmful microorganisms, and pollutants. The main stages are:
- Screening: Water is passed through large screens to remove large debris like leaves and twigs.
- Coagulation/Flocculation: A chemical coagulant (like aluminum sulfate) is added. This causes fine suspended particles (like clay) to clump together into larger, heavier particles called flocs.
- Sedimentation: The water is left in a large tank, allowing the heavy flocs to settle to the bottom as sludge, which is removed.
- Filtration: The water is passed through layers of sand and gravel to filter out any remaining fine particles.
- Disinfection: A small amount of a disinfectant, usually chlorine, is added to kill any remaining harmful bacteria and viruses, making the water safe to drink.
Solved Examples:
-
What is the purpose of adding a coagulant during water
treatment?
Solution: To cause very fine, suspended particles to clump together into larger flocs that can be more easily removed by sedimentation and filtration. -
Why is chlorine added as the final step in water treatment?
Solution: Chlorine is added as a disinfectant to kill any pathogenic microorganisms (like bacteria and viruses) that may have passed through the filtration stage, ensuring the water is safe to drink. - What is the purpose of the sand and gravel beds in a water treatment
plant?
Solution: They act as filters to remove the remaining fine suspended solid particles from the water after sedimentation.