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GSCI 163

GSCI 163. Lecture 9. Water. What is so special about water? Abundance Solubility Density vs temperature curve High specific heat High latent heat. Water – essential to life. Fruits and vegetables 95% Meat 50% Human body 70% Earth 70.9%.

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GSCI 163

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  1. GSCI 163 Lecture 9

  2. Water What is so special about water? • Abundance • Solubility • Density vs temperature curve • High specific heat • High latent heat

  3. Water – essential to life • Fruits and vegetables 95% • Meat 50% • Human body 70% • Earth 70.9% Scientists looking for aliens consider liquid water "the Holy Grail, the thing that people really want to find," Cowan said. "Water is the main requirement we can see that life on Earth seems to have." http://www.space.com/7342-water-water.html The visible camera image showing the ejecta plume at about 20 seconds after LCROSS's impact on the moon.NASA

  4. Solubility • Known as universal solvent – ability to dissolve things Why is this important? In living things water is the medium by which molecules are transported, either by diffusion, capillary forces or circulatory system.

  5. Unusual properties • Unusual density vs temperature curve: ice floats – this makes life under cold water bodies possible • High specific heat – ability to absorb a lot of heat without corresponding large change in temperature. Thermal stabilization of the planet. • Unusually high latent heat – ability to cool off by evaporating small amount of water. Thermal control of mammals by sweat.

  6. Why ice floats in water? It has to do with its molecular structure • Water molecule has a polar covalent bond between H and O. The higher number of protons on O attract the shared electrons away from the H atoms. • This makes water a polar molecule with slightly negative and positive regions

  7. Why ice floats in water? The molecules of water are attracted to each other via a hydrogen-oxygen bonds. When water freezes, the molecules organize themselves with lots of space in between (low density). When ice melts, thermal agitation breaks the bonds, and the structure, bringing water molecules closer together (higher density). Theses bonds are also responsible for the wonderful shapes of snowflakes.

  8. Specific heat How much heat do you need to raise the temperature of an object? • It depends on the mass of the material • It depends on the material (heat capacity) • It depends on the difference in temperature

  9. High specific heat of water • Water has a high specific heat. What does that mean? Why is this good? It regulates atmospheric temperature. Ocean water

  10. Changing phases • Start with ice and increase the temperature. At what temperature will ice melt? As ice is melting, the temperature of pure ice/water does not change. Why? After ice has melted, continue increasing the temperature. At what temperature will water boil? As pure water is boiling the temperature does not change. Why? What happens if I add solute (compound that dissolves in water) to the water during phase changes?

  11. Solutions • What is a solution? Key words: Homogeneous Mixture Two substances • Why some substances dissolve in water and some other substances do not? solute solvent

  12. Solubility Depends on molecular structure • Substances that easily dissolve in water: • Substances made of ions – table salt NaCl (but not all ionic compounds are soluble. Why?) (demonstration) • Substances made of similar polar molecules. Ethanol, a polar molecule is soluble in water; oil, a non-polar molecule is not soluble in water

  13. How do we measure solubility The concentration of the solution can be measured in many ways: • Weight percentage %w or w/w • Parts per million (ppm) or parts per billion (ppb) • Molarity: number of moles of solute per liter of solvent.

  14. Saturated solutions It is evident that we cannot add infinite amounts of solute to solvent • Why can’t we? • What can be done to improve solubility? Explain. Side topic: • How soap/detergent “dissolves” oil in water?

  15. The nature of a amphiphilic molecule • How surfactants dissolve oils in water

  16. Back to phase change The addition of solute in solution changes the freezing point and the boiling point of water. • Why do people put salt on streets ahead of a winter storm? • Would eggs cook faster in a salty water?

  17. Next class • Acids & bases • Crystals

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