water

• Water Resources

  • Groundwater
  • Rainwater
• Groundwater


  Water from rocks that trap water underground (aquifers), contains dissolved minerals like Mg, Ca, Na and K
• Rainwater


  Contains dissolved CO2, which affects the pH of the water
• Water Pollutants


  • Natural pollutants
  • Man-made pollutants (pesticides, facts, household and industrial waste)
• Water can contain natural pollutants like bacteria and viruses
• Reasons for Demand for Sustainable Water

  • Fresh water is crucial for humans (not just for drinking, but also for farming)
  • Global population is increasing, so greater need for water
  • Water is expected to become more scarce with water shortages being possible
  • Climate change increases risk of droughts, making water shortages more likely
  • As demand for water goes up, so will the price, so we need to reduce water consumption and collect/treat water in environmentally-friendly ways
• Water Purification in Treatment Plants

  1. Sedimentation (large solid particles settle)

  2. Filtration (small soluble particles removed by filtering through sand and gravel)

  3. Chlorination (chlorine gas bubbled through to kill harmful bacteria and microbes)
• Adding fluoride to water is controversial - it can prevent tooth decay in children but high doses may be linked to health issues
• Distillation is a way to desalinate seawater by separating the water from the salt through evaporation and condensation
• Simple Distillation

  1. Pour sample into distillation flask

  2. Connect condenser to cold tap

  3. Gently heat flask

  4. Water vapour condenses in condenser and collects as pure water in beaker

  5. Salt left behind in flask
• Distillation is an energy-intensive process, making it impractical in poorer countries
• Solubility


  Measure of how much solute will dissolve in a solvent
• Solubility of most solids increases as temperature increases
• Solubility curves show the relationship between solubility and temperature
• Plotting a Solubility Curve

  1. Measure volume of water and mass of solute

  2. Add solute to water and heat until dissolved

  3. Cool solution and record temperature when crystals appear

  4. Calculate solubility at that temperature

  5. Repeat with different volumes of water

  6. Plot solubility values on a graph to create a solubility curve
• Investigating Solubility

  1. Repeat the experiment several times

  2. Use a slightly greater volume of water each time

  3. Always add the same mass of solute that you used in the first experiment

  4. Find the solubility at each temperature

  5. Plot the values on a graph to make a solubility curve
• Solubility

  The amount of a substance that can dissolve in a given volume of solvent
• Calculating Solubility

  1. Mass of solute dissolved = mass of solid added - mass of dried excess solid

  2. Solubility = (mass of solute dissolved / mass of water) x 100
• Hard water makes scum and scale
• Hard water

  Water that contains lots of calcium ions (Ca2+) and magnesium ions (Mg2+)
• Rain falling on some types of rocks (eg. limestone, shalk and gipsum) can dissolve compounds like magnesium sulfate and calcium sulfate
• Calcium and magnesium ions are good for healthy teeth and bones
• People who live in hard water areas are at less risk of developing heart disease than people who live in soft water areas
• Temporary hardness


  Caused by dissolved calcium hydrogencarbonate or magnesium hydrogencarbonate
• Permanent hardness


  Caused by dissolved calcium sulfate or magnesium sulfate
• Removing temporary hardness


  1. Boiling water in a kettle

  2. The metal hydrogencarbonate decomposes on heating to form an insoluble metal carbonate (limescale)
• Removing both temporary and permanent hardness

  1. Adding sodium carbonate (washing soda)

  2. The carbonate ions react with the Ca2+ and Mg2+ ions to make an insoluble precipitate of calcium carbonate or magnesium carbonate (limescale)
• Removing hardness using ion exchange columns

  1. The columns have resin molecules with sodium ions (or hydrogen ions) attached

  2. They 'exchange' these for calcium or magnesium ions in the water that runs through them

  3. The columns can be regenerated by rinsing in a sodium chloride solution
• Measuring Water Hardness


  1. Use titration to compare the hardness of water samples

  2. Add soap solution to the water sample until a lasting lather is formed

  3. Record the volume of soap solution needed

  4. Repeat with other water samples (distilled, boiled, etc.)
• Distilled water contains little or no hardness
• Imported water contains more hardness than local water
• Local water contains temporary hardness only, which is removed by boiling
• Imported water contains both temporary and permanent hardness
• Local water and imported water contain the same amount of temporary hardness