Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Water
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Water
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Subject: Chemistry
Class: Senior Secondary 2
Term: 1st Term
Week: 2
Theme: Chemistry And Environment
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draw and explain the structure of water define the followingconcepts: solute, solventsolution define solubility and,state the rules of solubilityin water explain factors thataffect solubility Explain the causes of hardness of water Explain the methodsemployed in the removalof hardness Explain methods usedin purifying water
Water, with the chemical formula H2O, consists of two hydrogen atoms covalently bonded to one oxygen atom.
Molecular Geometry (Structure): The oxygen atom in water has two bonding pairs and two lone pairs of electrons. According to VSEPR (Valence Shell Electron Pair Repulsion) theory, these four electron domains arrange themselves tetrahedrally around the central oxygen atom.
However, because the lone pairs exert greater repulsive forces than bonding pairs, the H-O-H bond angle is compressed from the ideal tetrahedral angle of 109.5° to approximately 104.5°. This gives water a bent (or angular) molecular geometry.
Diagram 1: Bent Structure of Water ``` H / O \ H (Angle ≈ 104.5°) ``` Polarity: Oxygen is significantly more electronegative than hydrogen. This means the shared electrons in the O-H covalent bonds are pulled closer to the oxygen atom, creating a partial negative charge (δ-) on the oxygen and partial positive charges (δ+) on each hydrogen atom. Because of its bent structure, these bond dipoles do not cancel each other out. The overall molecule has a net dipole moment, making water a polar molecule. This polarity is responsible for many of water's unique properties, including its ability to dissolve a wide range of substances.
Hydrogen Bonding: The partial positive charge on the hydrogen atoms of one water molecule is attracted to the partial negative charge on the oxygen atom of an adjacent water molecule. This strong intermolecular force is called a hydrogen bond. Each water molecule can form up to four hydrogen bonds with neighbouring water molecules. Hydrogen bonding gives water its high boiling point, high specific heat capacity, and surface tension, and explains why ice floats.
Diagram 2: Hydrogen Bonding between Water Molecules ``` H-O-H ... O-H | / H H ``` (Dotted lines represent hydrogen bonds) Water, with the chemical formula H2O, consists of two hydrogen atoms covalently bonded to one oxygen atom.
Molecular Geometry (Structure): The oxygen atom in water has two bonding pairs and two lone pairs of electrons. According to VSEPR (Valence Shell Electron Pair Repulsion) theory, these four electron domains arrange themselves tetrahedrally around the central oxygen atom.
However, because the lone pairs exert greater repulsive forces than bonding pairs, the H-O-H bond angle is compressed from the ideal tetrahedral angle of 109.5° to approximately 104.5°. This gives water a bent (or angular) molecular geometry.
Diagram 1: Bent Structure of Water ``` H / O \ H (Angle ≈ 104.5°) ``` Polarity: Oxygen is significantly more electronegative than hydrogen. This means the shared electrons in the O-H covalent bonds are pulled closer to the oxygen atom, creating a partial negative charge (δ-) on the oxygen and partial positive charges (δ+) on each hydrogen atom. Because of its bent structure, these bond dipoles do not cancel each other out. The overall molecule has a net dipole moment, making water a polar molecule. This polarity is responsible for many of water's unique properties, including its ability to dissolve a wide range of substances.
Hydrogen Bonding: The partial positive charge on the hydrogen atoms of one water molecule is attracted to the partial negative charge on the oxygen atom of an adjacent water molecule. This strong intermolecular force is called a hydrogen bond. Each water molecule can form up to four hydrogen bonds with neighbouring water molecules. Hydrogen bonding gives water its relatively high boiling point, high specific heat capacity, and surface tension, and explains why ice floats. These properties are crucial for life and the environment.
Diagram 2: Hydrogen Bonding between Water Molecules ``` H-O-H ... O-H | / H H ``` (Dotted lines represent hydrogen bonds) Water, with the chemical formula H2O, consists of two hydrogen atoms covalently bonded to one oxygen atom.
Molecular Geometry (Structure): The central oxygen atom in water possesses two bonding pairs of electrons (with hydrogen atoms) and two lone pairs of electrons. According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, these four electron domains (two bonding, two lone) arrange themselves in a tetrahedral electron geometry around the central oxygen atom to minimize repulsion.
However, because lone pairs exert stronger repulsive forces than bonding pairs, the H-O-H bond angle is compressed from the ideal tetrahedral angle of 109.5° to approximately 104.5°. This gives water a bent (or angular) molecular geometry.
Diagram 1: Bent Structure of Water Molecule ``` H / O \ H (Angle ≈ 104.5°) ``` Polarity: Oxygen is significantly more electronegative than hydrogen. This means that the shared electrons in each O-H covalent bond are pulled closer to the oxygen atom. This uneven sharing creates a partial negative charge (δ-) on the oxygen atom and partial positive charges (δ+) on each hydrogen atom. Due to water's bent molecular structure, these individual bond dipoles do not cancel each other out. The overall molecule therefore possesses a net dipole moment, making water a polar molecule. This inherent polarity is the fundamental reason behind many of water's unique and vital properties, most notably its ability to act as an excellent solvent for a vast range of substances.
Hydrogen Bonding: The partial positive charge (δ+) on the hydrogen atoms of one water molecule is strongly attracted to the partial negative charge (δ-) on the oxygen atom of an adjacent water molecule. This powerful intermolecular force is known as a hydrogen bond. Each water molecule can form up to four hydrogen bonds with neighbouring water molecules (two through its hydrogen atoms and two through its lone pairs on oxygen). Hydrogen bonding is responsible for water's unusually high boiling point (compared to other hydrides of Group 16 elements), high specific heat capacity, high heat of vaporisation, high surface tension, and the anomalous expansion of water upon freezing (explaining why ice floats). These properties are crucial for regulating Earth's climate and supporting aquatic life.
Diagram 2: Hydrogen Bonding between Water Molecules ``` H-O-H --- O-H | / H H ``` (Dotted lines represent hydrogen bonds)
Solution: A homogeneous mixture of two or more substances, where one substance is uniformly dispersed in another at a molecular or ionic level. It consists of a single phase.
Example:* Dissolved sugar in water (sugar solution), common in making "zobo" or kunu drinks.
Solvent: The component of a solution that is present in the larger quantity and dissolves the other component(s). Water is often referred to as the "universal solvent" due to its ability to dissolve many substances.
Example:* Water in a sugar solution.
Solute: The component of a solution that is present in the smaller quantity and is dissolved by the solvent.
Example:* Sugar in a sugar solution. Types of Solutions (based on state of matter): Solutions can exist in various states, though this topic primarily focuses on solids dissolved in liquids.
Solid in Liquid: Sugar in water, salt in water.
Liquid in Liquid: Ethanol in water (e.g., in local gin production).
Gas in Liquid: Carbon dioxide in water (e.g., in soft drinks like Fanta or Coke).
Gas in Gas: Air (nitrogen, oxygen, etc.).
Solid in Solid: Alloys (e.g., brass – copper and zinc).
Types of Solutions (based on saturation): Unsaturated Solution: A solution that contains less solute than the maximum amount that can be dissolved at a given temperature. More solute can still be dissolved.
Example:* A pinch of salt in a glass of water, where the salt quickly dissolves.
Saturated Solution: A solution that contains the maximum amount of solute that can be dissolved in a given amount of solvent at a specific temperature. Any additional solute added will not dissolve and will typically precipitate out.
Example:* Adding salt to water until no more dissolves, and some salt remains at the bottom of the container.
Supersaturated Solution: A solution that contains more solute than a saturated solution at a given temperature. These solutions are typically unstable and are prepared by dissolving solute at a higher temperature and then carefully cooling the solution without precipitation. Any disturbance (e.g., scratching the container, adding a seed crystal) can cause the excess solute to rapidly crystallise out.
Example:* A carefully prepared hot sugar syrup that crystallises when cooled or disturbed.
Ensuring Safe Drinking Water and Public Health: Application: Understanding water purification methods (like boiling, chlorination, filtration) is crucial for preventing waterborne diseases such as cholera, typhoid, and dysentery, which are prevalent in some parts of Nigeria. Knowledge of water hardness helps in managing domestic water sources (e.g., boreholes) to ensure their suitability for various uses.
Integration: Students can relate the municipal water treatment process to the source of their tap water (if available) or understand the technology behind sachet and bottled water production, which is a major industry in Nigeria. They can also explore simple household purification techniques like SODIS used in rural areas.
Industrial Processes and Economic Impact: Application: Water quality, particularly hardness, significantly impacts various Nigerian industries. Breweries (e.g., Nigerian Breweries, Guinness Nigeria) require very soft water to maintain product taste and prevent scale formation in their equipment. Textile industries (e.g., Kaduna Textiles) need soft water for dyeing processes to ensure consistent colour quality. Power generation plants and oil refineries (e.g., NNPC refineries) require extensive water treatment to prevent scaling and corrosion in boilers and cooling systems, which can lead to costly breakdowns and reduced efficiency.
Integration: Students can research local industries that depend on water and discuss how water quality issues could affect their operations and the broader Nigerian economy. This links chemistry to industrial growth and sustainability.
Agriculture and Environmental Management: Application: Water is critical for irrigation in agriculture, a cornerstone of the Nigerian economy. The solubility of fertilisers and pesticides in water is vital for their effective application. Understanding the interaction of pollutants with water (solubility, dissolution) is essential for addressing environmental issues like water pollution from industrial effluents or agricultural run-off into rivers (e.g., River Niger, River Benue, Lagos Lagoon).
Integration: Students can discuss the role of water quality in crop yield, the environmental impact of chemical runoff on local ecosystems, and community efforts in protecting water sources from pollution. This connects chemistry to food security and environmental conservation.