Comprehensive Study Notes on Water, Metals, and Corrosion

Properties and Nature of Water

• Chemical Formula and Abundance: Water is represented by the chemical formula $H_2O$. It is essential to all living things. Approximately $71\%$ of the earth's surface is covered with water.
• Polar Nature: Water is a polar solvent. It is called polar because one end of the molecule is partially negative ($\delta-$) and the other end is partially positive ($\delta+$).
• Universal Solvent: Water is referred to as the universal solvent because it has the ability to dissolve most substances.
• Sources of Water:
  1. Rivers, streams, and rivulets.
  2. Sea.
  3. Lake.
  4. Dam.
  5. Well and spring.
  6. Bore-hole.
  7. Rain.
  8. Pond and Oasis.
  9. Tap water.
• Impurities in Water:
  1. Soil particles.
  2. Living and dead organisms.
  3. Dissolved gases and chemicals.
  4. Germs.

Physical and Chemical Properties of Water

Physical Properties:

• Appearance: It is a clear, colourless, and odourless liquid.
• Standard Constants: It boils at $100\,^\circ C$ and freezes at $0\,^\circ C$ at Standard Temperature and Pressure (STP).
• Density: It has a maximum density of $1\,g/cm^3$ (or $1000\,kg/m^3$) at $4\,^\circ C$.
• Conductivity: It is a poor conductor of electricity due to the absence of dissolved salts or ions.
• Surface Tension: It possesses high surface tension.
• pH: It has a pH of $7$, meaning it is neutral to litmus paper.

Chemical Properties:

• Reaction with Copper Sulphate: It turns white anhydrous copper sulphate blue.
• Reaction with Cobalt (II) Chloride: Water turns blue anhydrous cobalt (II) chloride pink.
• Reaction with Metals: It reacts with some metals to liberate hydrogen gas ($H_2$).
• Reaction with Oxides:
  • Dissolves oxides of non-metals to form acidic solutions.
  • Dissolves oxides of metals to form alkaline (basic) solutions.
• Solvency: It is a polar solvent and dissolves polar substances.

Tests for Water

• Using Copper Sulphate: Adding water to anhydrous copper sulphate changes its colour from white to blue.
  • Equation: $CuSO_4 + 5H_2O \rightarrow CuSO_4.5H_2O$
• Using Cobalt Chloride: Water changes the colour of anhydrous cobalt (II) chloride from blue to pink.
  • Equation: $CoCl_2 + 6H_2O \rightarrow CoCl_2.6H_2O$

Effects of Dissolved Solids and Pressure on Water

• Boiling Point: Dissolved solids raise the boiling point of water.
• Freezing Point: Dissolved solids lower the freezing point of water.
• Density: Dissolved solids change the density of water.
• Atmospheric Pressure: Changes in atmospheric pressure alter both the boiling and freezing points of water.

Uses of Water

Domestic Uses:

1. Drinking.
2. Cooking.
3. Washing.
4. Bathing.

Transportation:

• Ships, boats, and canoes travel on water.

Industrial and Economic Uses:

1. Hydroelectricity production.
2. Irrigation.
3. Dyeing and tanning.
4. Preparation of solutions and medicines.
5. Cooling machine engines.
6. Fishing and fish farming.

Importance to Plants:

1. Transportation and distribution of mineral nutrients.
2. Photosynthesis.
3. Dissolving oxygen for aquatic plants.
4. Assisting in transpiration.
5. Germination of seeds.

Importance to Animals and Humans:

1. Transport and distribution of food nutrients.
2. Serving as a solvent medium for chemical reactions.
3. Maintaining and cooling body temperature.

Hardness of Water

• Definition: Water hardness is the inability of water to form a lather easily with soap, caused by the presence of dissolved salts or ions.
• Causative Agents:
  • Soluble calcium salts/ions ($Ca^{2+}$).
  • Soluble magnesium salts/ions ($Mg^{2+}$).
  • Iron (III) ions ($Fe^{3+}$).
• Anions involved: Bicarbonate ions ($HCO_3^-$), Chloride ions ($Cl^-$), and Tetraoxosulphate (VI) ions ($SO_4^{2-}$).
• Formation Process: Rain water passes through rocks and limestone, dissolving salts of calcium, magnesium, and iron.

Types of Water Hardness:

1. Temporary Hardness: Caused by the presence of carbonates and bicarbonates of calcium or magnesium, such as Calcium bicarbonate ($Ca(HCO_3)_2$) and Magnesium bicarbonate ($Mg(HCO_3)_2$). It can be removed by boiling.
2. Permanent Hardness: Caused by the presence of chlorides or sulphates of calcium or magnesium, such as Calcium chloride ($CaCl_2$), Calcium sulphate ($CaSO_4$), Magnesium chloride ($MgCl_2$), and Magnesium sulphate ($MgSO_4$). It cannot be removed by boiling.

Distinction Between Hard and Soft Water:

• Hard Water: Does not easily lather with soap; forms scum; contains dissolved magnesium or calcium salts; forms scales in boilers. Example: Sea water.
• Soft Water: Lathers easily with soap; does not form scum; lacks dissolved magnesium or calcium salts; does not form scales in boilers. Examples: Rain water, distilled water.

Terminology:

• Scum: An insoluble precipitate of calcium or magnesium salt formed when hard water reacts with soap.
• Scale: Formed when hydrogen carbonates (bicarbonates) of calcium or magnesium decompose upon heating.
  • Equations:
    • $Mg(HCO_3)_2 \xrightarrow{\text{heat}} MgCO_3 + CO_2 + H_2O$
    • $Ca(HCO_3)_2 \xrightarrow{\text{heat}} CaCO_3 + CO_2 + H_2O$

Advantages and Disadvantages of Water Types

Hard Water:

• Advantages: Calcium is good for bones and teeth; pleasant taste for the brewing industry; prevents lead poisoning; reduces heart disease occurrence.
• Disadvantages: Wastes soap; interferes with dyeing and tanning; makes clothes grey when laundered; forms scales in boilers.

Soft Water:

• Advantages: Does not waste soap; does not form scum; does not deposit fur in kettles or scales in boilers; does not interfere with dyeing and tanning.
• Disadvantages: Can lead to lead poisoning; lacks calcium for bones/teeth; lacks a pleasant taste; does not reduce heart disease risk.

Softening of Hard Water

Purification of Water

Small Scale Purification:

• Boiling, Filtration, Sedimentation, Distillation, Decantation.

Large Scale Water Purification (Process):

1. Screening and Straining: Removal of large particles, rubbish, and debris.
2. Aeration: Bringing water into contact with air to remove unpleasant tastes and odours.
3. Coagulation and Flocculation: Alum is added to cause suspended particles to clump together into 'floc'.
4. Sedimentation: Heavy suspended particles fall to the bottom of the chamber.
5. Filtration: Water passes through a filter bed to remove remaining sediments and coagulants.
6. Chlorination: Chlorine is added to kill harmful microbes, germs, and bacteria.
7. pH Control: Chlorine can make water acidic; pH is adjusted by adding lime ($Ca(OH)_2$) or soda ash ($Na_2CO_3$). This also assists in softening.
8. Storage and Distribution: Water is stored in large tanks and distributed to homes.

The Water Cycle (Hydrological Cycle)

• Definition: The continuous movement of water above and below the surface of the earth. It is a cycle with no beginning or end.
• States of Water: Water exists in solid, liquid, and gaseous states within the cycle.
• Stages of the Hydrological Cycle:
  • Evaporation: Water leaves the earth's surface (oceans and other bodies) and enters the atmosphere as gas.
  • Transpiration: Water moves through plants and evaporates from aerial parts.
  • Condensation: Water vapour changes back into liquid in the atmosphere, forming clouds.
  • Precipitation: Condensed water falls to the atmosphere as rain, sleet, snow, or hail ($rain fall$).
  • Interception: Precipitation caught by leaves and branches of plants before reaching the soil.
  • Infiltration: Water on the soil surface enters the soil to sustain plant life.
  • Percolation: Downward movement of water to replenish underground aquifers.
  • Runoff: Movement of water over the land surface into water bodies.
  • Storage: Large amounts of water stored in the ground or surface bodies.
• Importance of the Cycle:
  • Provides cooling for plants and animals.
  • Dissolves oxygen for aquatic life.
  • Provides fresh water.
  • Helps disperse fruits and seeds.
  • Provides water for photosynthesis.
  • Regulates eart's temperature.
• Human Activities Affecting the Cycle: Agriculture/farming, construction of dams, urbanisation, deforestation/afforestation, and altering the chemical composition of the atmosphere.

Metals, Non-metals, and Metalloids

• Earth's Crust Composition: A mixture of metals, semimetals, and non-metals. Component percentages: Oxygen ($45\%$), Silicon ($27\%$), and Aluminium ($8\%$). Oxygen and silicon combine to form sand (silicon dioxide).

Metals:

• Definition: Elements that easily lose electrons to form cations.
• Physical Properties: Solids at room temperature (except Mercury ($Hg$), which is liquid); generally hard; high boiling and melting points; good conductors of heat and electricity (Copper and Silver are excellent); shiny/lustrous; malleable (can be bent/shaped); ductile (can be drawn into wires).
• Chemical Properties:
  • Reaction with Acids: Metals above Hydrogen in the activity series react with acids to liberate hydrogen gas ($Metal + Acid \rightarrow Salt + Hydrogen\,gas$). Example: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$.
  • Reaction with Oxygen: Metals form metal oxides ($Metal + Oxygen \rightarrow Metal\,oxide$). Oxides of metals form basic/alkaline solutions. Examples: $MgO$, $Na_2O$.
  • Reaction with Water: Metals react with water to form metal oxides and hydrogen gas ($Metal + Water \rightarrow Metal\,oxide + Hydrogen\,gas$). Zinc reacts as: $Zn + H_2O \rightarrow ZnO + H_2$.
• Reactivity Variations:
  • $Na, Ca, Mg$: React with cold water.
  • $Al, Zn, Fe$: React with steam.
  • $Pb, Au, Cu, Ag$: Do not react with water.

Non-Metals:

• Definition: Elements that usually gain electrons to form anions.
• Properties: mostly gases, some solids (Carbon) or liquids (Bromine); poor conductors (except Graphite); low melting and boiling points; brittle and non-ductile; dull surfaces; oxides form acidic solutions (e.g., $CO_2$, $SO_2$).

Semi-metals / Metalloids:

• Definition: Elements with properties of both metals and non-metals.
• Conductivity: Higher than non-metals but lower than metals; known as semi-conductors ($Silicon$, $Germanium$).
• Uses: Manufacture of electronic devices such as transistors and integrated circuits for computers and TVs. Conductivity is improved by adding impurities like boron or phosphorus.

The Activity Series

• Order of reactivity (Most reactive to Least reactive):
  1. Potassium ($K$)
  2. Sodium ($Na$)
  3. Calcium ($Ca$)
  4. Magnesium ($Mg$)
  5. Aluminium ($Al$)
  6. Zinc ($Zn$)
  7. Iron ($Fe$)
  8. Lead ($Pb$)
  9. Hydrogen ($H$)
  10. Copper ($Cu$)
  11. Mercury ($Hg$)
  12. Silver ($Ag$)
  13. Gold ($Au$)

Uses of Metals

Alloys

• Definition: A mixture of a metal with other metals or non-metals, mixed in their molten states.
• Common Alloys:
  • Bronze: Copper + Tin. Used for statues, ornaments, medals, coins. (Harder than brass and copper).
  • Brass: Copper + Zinc. Used for ornaments, instruments, screws. (Harder than copper).
  • Duralumin: Aluminium + Copper + Magnesium. Used for aircraft parts. (Light and strong).
  • Nichrome: Nickel + Chromium. Does not rust. Used for electrical cables and cutlery.
  • Steel: Iron + Carbon. Very strong/hard. Used for cars, bridges, ships.
  • Stainless Steel: Iron + Chromium + Carbon + Nickel. Does not rust. Used for cooking utensils and surgical instruments.
  • Solder: Tin + Lead. Low melting point. Used for joining metals and pipes.

Extraction of Metals

Aluminium:

• Ore: Bauxite (impure hydrated aluminium oxide).
• Process: Bauxite is purified to Alumina ($Al_2O_3$). Alumina is mixed with cryolite and melted. The molten alumina is electrolyzed using copper electrodes. Aluminium is discharged at the cathode.

Gold:

• Extraction: Pure state in ore. Crushed into powder, dissolved in sodium cyanide solution with oxygen/water to form gold cyanide slurry. Slurry is filtered, zinc dust added to precipitate gold. Final purification by electrolysis.
• Caution: Cyanide and mercury are toxic; if swallowed, breathed, or absorbed, they cause death.

Small Scale Mining (Galamsey):

• Local name in Ghana for small-scale extraction of gold and diamonds.

Environmental Negatives of Metal Extraction:

1. Water contamination (Mercury, Cyanide leakage).
2. Air contamination (Asenic/Sulphide roasting).
3. Habitat destruction.
4. Land degradation.
5. Noise pollution from machinery.

• Mitigation: Strict laws, reforestation, modern equipment, education of Galamsey workers, and mining royalties.

Corrosion and Rusting

• Corrosion: Wearing away of a metal surface due to reaction with oxygen and water.
• Rusting: Specifically the corrosion of iron or steel. It is a redox reaction forming brown hydrated iron oxide.
• Conditions for Rusting: Oxygen (air) and moisture (water). Factors like acids, alkalis, or salts promote it.
• Prevention Methods:
  • Painting.
  • Greasing or Oiling.
  • Alloying (e.g., Stainless steel).
  • Electroplating.
  • Galvanization (Coating with Zinc).
  • Tin plating.
  • Sacrificial / Cathodic protection.
  • Plastic coating.
• Corrosion Resistance: Aluminium resists corrosion because it forms a thin continuous film of aluminium oxide which protects the underlying metal.

Experiment: Conditions for Rusting

• Setup:
  • Test Tube 1: Nail + Drying agent (Calcium Chloride) + Air = No rust (No water).
  • Test Tube 2: Nail + Boiled water + Oil layer = No rust (No oxygen).
  • Test Tube 3: Nail + Ordinary water + Air = Rusts (Both present).
• Conclusion: Both water (moisture) and air (oxygen) are required for rusting.

Questions & Discussion

• Que: Water is a polar solvent. Explain.
  • Response: It is called polar because one end (Oxygen) is partially negative and the other end (Hydrogen) is partially positive.
• Que: Explain briefly why water is referred to as a universal solvent.
  • Response: Because it is able to dissolve most substances.
• Que: Explain how water comes in contact with chemicals.
  • Response: [Context: Through rainfall passing through rock formations and industrial/human activities].
• Question 45.1: State four differences between metals and non-metals.
  • [Context: Conductivity, state at room temp, ductility/malleability, and nature of oxides].
• Question 45.1 (Aluminium): Name three items in the home made of aluminium. In each case, state the property which makes it suitable.
  • [Context: Cooking pots (conductivity), window frames (malleability/lightness), foil (malleability)].
• Question 47.1: Explain why wooden poles are preferred to steel reinforced concrete poles in erecting transmission lines along the coast.
  • [Context: To prevent corrosion/rusting from the salty sea air].
• Question 47.2: Suggest a method for preventing rust in: sewing machine (oiling/greasing), underground steel pipes (sacrificial protection), food cans (tin plating).
• Que: Explain briefly why steel is preferred to iron in some construction works.
  • Response: Steel is harder, stronger, resists rusting better, and can function as a permanent magnet.