Water

• Water is the most abundant substance on Earth's surface
• Water is essential for life
• Water is a polar molecule
• Water bears


  Tiny invertebrates that live in aquatic habitats such as damp moss
• Water bears


  • Require water to obtain oxygen by gas exchange
  • Can enter a shrivelled dormant state to survive dry conditions for decades
  • Can be found almost anywhere on Earth
  • Have survived the vacuum, radiation blasts, and freezing temperatures of space
• In 2007, scientists placed water bears into a satellite and shot them into space. They survived radiation blasts 700 times stronger than the sun's rays on Earth.
• Water bears are helping scientists rethink what they know about what it takes to survive in extreme conditions and what humans might need to survive on another planet.
• Charles Darwin wrote about the first organism appearing "in some warm little pond"


  1871
• Most hypotheses today place the first cells in the oceans rather than a pond.
• During the formation of the first cells, a small volume of water became enclosed in a membrane. Substances were dissolved in this water and chemical reactions could occur between the solutes.
• After billions of years of evolution, most molecules of life are still dissolved in water.
• With water in a liquid state, molecules can move around and interact, allowing the process of life happening.
• Water


  The substance that makes life possible (70-95% of cells is water)
• Water is present on the Earth in its three states of matter (liquid, solid and water), being the most common the liquid state.
• Water molecule


  Formed by covalent bond (sharing electrons) between oxygen atom and two hydrogen atoms
• Polar molecule


  The electrons of the covalent bond spend more time closer to oxygen than to hydrogen (an unequal sharing of electrons)
• Dipolar


  The two ends of the water molecule have opposite charges (oxygen region has a partial negative charge, hydrogens have a partially positive charge)
• Partially charged particles (positive ions) and negatively charged particles (negative ions) attract each other and form ionic bonds, however water just have a partially negative and partially positive charge, so the attraction is small.
• Hydrogen bond


  Type of intermolecule force that forms when a hydrogen in one polar covalent molecule is attracted to the slightly negative atom of another covalent molecule
• Hydrogen bonds are an interaction between molecules, not a bond.
• When water is in its liquid state, its hydrogen bonds are extremely fragile, 1/20 as strong as a covalent bond. The hydrogen bonds break and reform with great frequency.
• The extraordinary properties of water are emergent properties resulting from the hydrogen bonding.
• Cohesion


  Intermolecular forces between water molecules caused by the polar nature of water responsible for surface tension (water molecules sticking together)
• Cohesion due to hydrogen bonding contributes to the transport of water and dissolved nutrients against gravity.
• Examples of cohesion


  • Conduction of water in xylem
  • Use of water surface as a habitat

  
• Adhesion


  The attraction between water molecules and other molecules
• Hydrogen bonds can form between water and the surface of a solid composed of polar molecules, causing water to stick to the surface of the solid.
• Capillarity


  Movement of water through narrow tubes or spaces due to adhesion
• Capillary action due to adhesion is useful for plants. Water adheres to cellulose molecules in cell walls, so any wall that starts to dry out is automatically rewetted as long as there is a source of water available.
• If water evaporates from the cell walls in leaves and is lost to the atmosphere, adhesion can draw more water up from the roots to rewet the cell walls.
• Capillary action


  Attraction of water to many chemical substances in soil, causing water to be drawn up through dry soil
• Capillary action due to adhesion is useful for plants, as water adheres to cellulose molecules in cell walls, rewetting dry areas
• Paraphyllia in mosses


  • Narrow hair-like structures on stems that attract water from fog or dew and store it, helping keep the moss hydrated
• If water evaporates from cell walls in leaves


  Adhesive forces cause water to be drawn out of the nearest xylem vessel, keeping walls moist for photosynthesis and generating low pressure to draw water up
• If a xylem vessel becomes air-filled


  Adhesion between water and vessel wall can help the vessel refill with water, e.g. in deciduous trees in spring

  
• Cohesion


  Water molecules are more strongly attracted to each other than to surrounding material, causing them to bead up
• Adhesion


  Water molecules are more strongly attracted to some other material, causing them to spread out and get close to that material
• Solution


  A liquid that is a homogenous mixture of two or more substances, e.g. sweetened water
• Solvent


  The dissolving agent in a solution, water is the most versatile solvent known
• Solute


  A substance that is dissolved in water, e.g. sugar