1.1.1Unit 1: Lifestyle, Transport, Genes and Health
The Importance of Water — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 8 of 29 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Structure of the Water Molecule
Everything biologically useful about water follows from its polar molecules and the hydrogen bonds between them.
Covalent bonds
- Covalent bonds Each water molecule has one oxygen atom sharing electrons with two hydrogen atoms through covalent bonds.
Polarity
- Polarity Oxygen attracts the shared electrons more strongly,
- so it carries a slight negative charge (δ−) and each hydrogen a slight positive charge (δ+).
Dipole
- Dipole Because the charge is unevenly distributed across the molecule, water is described as a polar molecule or dipole.
Hydrogen bonds
- Hydrogen bonds The δ+ hydrogen of one molecule is attracted to the δ− oxygen of another, forming weak but numerous hydrogen bonds.
Part 2 of 4Properties and Their Importance
Properties and Their Importance
Solvent
- Solvent Polar water molecules surround ions and polar molecules such as glucose,
- so reactions can occur in solution and substances can be transported in blood and sap.
Cohesion and surface tension
- Cohesion and surface tension Hydrogen bonds hold molecules together, enabling columns of water to rise in xylem and giving a surface on which small organisms can live.
High specific heat capacity
- High specific heat capacity A lot of energy is needed to raise the temperature of water,
- so aquatic habitats and cell contents resist rapid temperature changes.
High latent heat of vaporisation
- High latent heat of vaporisation Much energy is taken from the surface as water evaporates,
- so sweating and transpiration give an effective cooling effect.
Part 3 of 4Further Roles of Water
Metabolite
- Metabolite Water takes part in reactions such as photosynthesis and hydrolysis, where it is used as a reactant.
Transport medium
- Transport medium Blood plasma and phloem sap are mainly water, which carries dissolved nutrients, hormones and wastes around organisms.
Density of ice
- Density of ice Ice is less dense than liquid water because of the lattice of hydrogen bonds, so it floats and insulates the water below it.
Support
- Support Water inside plant cells creates turgor pressure that keeps non-woody stems upright, and gives a hydrostatic skeleton in some animals.
Part 4 of 4Exam Checklist & Tips
1.1.1 – Key Points for the Exam
- Polar molecule A molecule with an uneven distribution of charge, giving partially positive and negative regions.
- Hydrogen bond A weak attraction between a δ+ hydrogen atom and a δ− atom on another molecule.
- Cohesion The attraction between molecules of the same substance, such as water to water.
- Solvent A liquid in which other substances dissolve.
- Specific heat capacity The energy needed to raise the temperature of 1 kg of a substance by 1 °C.
Common Mistakes
- Saying that hydrogen bonds are inside the water molecule; they are between neighbouring molecules.
- Describing water as a charged molecule; it is overall neutral but polar.
- Writing that water has a low specific heat capacity; it is high, which buffers temperature.
- Confusing cohesion (water with water) with adhesion (water with another surface).
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