6.0     INTRODUCTION

Water, often referred to as the “universal solvent”, is one of the most essential substances on Earth. It covers approximately 71% of the planet’s surface and is crucial for the survival of all known forms of life. It is a chemical compound with the molecular formula H₂O, consisting of two hydrogen atoms covalently bonded to one oxygen atom. It exists in three states of matter – solid (ice), liquid (water), and gas (steam or water vapor) – depending on temperature and pressure conditions.

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         6.1     SOURCES OF WATER

• Natural water – Rainfall, well, spring, sea etc.
• Treated water – Pipe-borne water, distilled water, and chlorinated water.

         6.2     PHYSICAL PROPERTIES OF WATER

• Water boils at 100^oC and freezes at 0^oC.
• It has a maximum density of 1gcm^-3 at 4^oC.
• It is neutral to litmus.

         6.3     CHEMICAL PROPERTIES OF WATER

• Reacts with electropositive metals to form alkali and liberate hydrogen gas.
• Non-metals like chlorine reacts with water to form an acidic solution.

         6.4     TEST FOR WATER (Indicating the presence of water)

When a few drops of water are added to:

• White anhydrous copper (II) tetraoxosulphate (VI), it turns blue.
• Blue cobalt (II) chloride, it turns pink.

         6.5     WATER AS A PRODUCT OF THE COMBUSTION OF HYDROGEN

When hydrogen burns in the presence of oxygen, it produces water (H₂O). The chemical reaction is 2H₂ + O₂ → 2H₂O, combining two volumes of hydrogen with one volume of oxygen. This reaction releases significant energy, making water in a 2:1 hydrogen-to-oxygen ratio. The water produced is in the vapor state at high temperatures during combustion, but when cooled, it condenses to liquid water.

         6.6     WATER AS A SOLVENT

Water is often referred to as the “universal solvent” because of its ability to dissolve a wide variety of substances than any other liquid. This property is primarily due to the polar nature of water molecules (has both partially positive and negative charges).

         6.7     LABORATORY PEPARATION FOR WATER

In the laboratory, dry hydrogen gas is ignited in the air. It burns with a faint blue flame to give steam, which will condense on contact with any cold surface to form water.

         6.8     ATMOSPHERIC GASES DISSOLVED IN WATER

• Oxygen (O₂) – Source of respiration for aquatic organisms like fish, crucial for maintaining healthy aquatic ecosystems.
• Nitrogen (N₂) – Generally inert, but certain bacteria fix nitrogen into usable forms for plants.  It is also Important for the nitrogen cycle in aquatic environments.
• Carbon Dioxide (CO₂) – Used by aquatic plants for photosynthesisand plays a crucial role in pH regulation.

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         6.9     TYPES OF WATER

• Soft water – Soft water contains low concentrations of dissolved minerals, particularly calcium (Ca²⁺) and magnesium (Mg²⁺) ions. Unlike hard water, it easily forms lather with soap and doesn’t cause scaling in pipes or appliances. It is often preferred for washing and cleaning purposes as it improves the effectiveness of detergents and prevents buildup in plumbing systems.
• Hard water – it contains high levels of dissolved minerals, particularly calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions react with soap to form scum, making it difficult to lather.

         6.10     HARDNESS OF WATER

TEMPORARY HARDNESS – This is caused by the presence of Ca²⁺ and Mg²⁺ in the form of hydrogen trioxocarbonate (iv) i.e Ca(HCO₃)₂.

REMOVAL OF TEMPORARY HARDNESS OF WATER

• Physical method by boiling.
• Use of slake lime (calcium hydroxide solution).

EFFECTS OF TEMPOARY HARDNESS OF WATER

• Furring of kettles and boilers.
• Stalagmite and stalactites.

PERMANENT HARDNESS – This is caused by the presence of Ca²⁺ and Mg²⁺ ions in the form of soluble tetraoxosulphate (vi) and chlorides i.e CaSO₄, MgSO₄, MgCl₂ and CaCl_2.

REMOVAL OF PERMANENT HARDNESS OF WATER

• Addition of caustic soda (NaOH).
• Addition of washing soda (Na₂CO₃).
• Ion exchange resin.

ADVANTAGES OF PERMANENT HARDNESS OF WATER

• Has a better taste than soft water.
• Contains calcium, which help to build strong teeth and bones.
• Provides aquatic animals with CaCO₃ for building their shells.
• Causes no damage, can be supplied in lead pipes.

DISADVANTAGES OF PERMANENT HARDNESS OF WATER

• Furring of kettles and boilers.
• Wastage of soap.
• Cannot be used in dyeing and tanning.

         6.11     TREATMENT OF WATER FOR TOWN SUPPLY

Water treatment for urban supply involves various process to ensure that the water is clean, safe, and free from pathogens or harmful chemicals. These processes include:

• COAGULATION AND FLOCCULATION – Chemicals like alum are added to water in a large settling tank to form sticky particles (flocs) that attract and bind impurities.
• SEDIMENTATION – The flocs and coagulated particles settle to the bottom of the tank, forming sediments.
• FILTRATION – Water is passed through a filter bed (layers of sand, gravel, and charcoal) to remove the remaining smaller particles.
• CHLORINATION (Disinfection) – Chlorine or other disinfectants are added to kill any remaining microorganisms. Iodine and fluorine are also added as food supplements to prevent goitre and tooth decay respectively.
• STORAGE AND DISTRIBUTION – The treated water is stored in a reservoir and distributed to the town.

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         6.11     USES OF WATER

Water is essential for drinking, sanitation, agriculture, industrial processes, transportation, energy generation, cooking, cleaning, recreation, and supporting ecosystems, as well as in chemical reactions, irrigation, and maintaining public health.

• WATER OF CRYSTALIZATION – This is a phenomenon that occurs when water molecules present in a particular substance becomes an integral part of its crystalline structure. These water molecules are essential to maintaining the compound’s crystalline form and are typically released when the substance is heated. Examples of substances/compounds with water of crystallization include CuSO₄·5H₂O, MgSO₄·7H₂O etc.
• EFFLUORESCENCE – This is a phenomenon where a compound loses all or part of its water of crystallization when exposed to the atmosphere, resulting to a powdery residue.
• DELIQUESCENCE – This is a phenomenon where a compound absorbs a large amount of water on exposure to the atmosphere, so they turn into a solution. Deliquescent substances include NaOH, KOH, CaCl₂, FeCl₃ etc.
• HYDROSCOPY – This is a phenomenon where a compound absorbs a large amount of water on exposure to the atmosphere, but do not turn into a solution. Silica gel is a good example of hydroscopic substance.

NB – Hydroscopic substances are used as desiccators (drying agent), due to their strong affinity for water/moisture.