Chemistry- water

• About 71% of the Earth's surface is covered in water. All life depends on water. It is needed for the correct functioning of our bodies. Water is by far the most common natural compound on Earth. Water is used to generate electricity, in cooling, transporting things, to irrigate crops, cleaning and as a medium for many chemical reactions.
• Pure water


  Water that has nothing else dissolved in it
• Universal solvent


  Water dissolves most common substances, even if for some, it only dissolves them slightly
• Solution


  The mixture that results when water dissolves soluble substances in it
• Most covalently bonded substances are insoluble in water. However, some such as, glucose and sucrose are very soluble in water.
• Water does not exist as pure water in nature. It has many dissolved substances in it.
• Water (H2O)


  • It is a neutral oxide (pH = 7)
  • It is odourless, tasteless and colourless
  • It boils at 100oC (at standard pressure, 1 atmosphere) and freezes at 0oC
  • The solid form of water (ice) is less dense that the liquid form, which is why ice floats on water
  • Its maximum density is 1g/cm3 or 1000kg/m3 at 4oC
  • It is a very good solvent
  • Pure water does not conduct electricity
  • It is neutral to litmus paper (red/blue)

  
• Impurities in water


  • Bacteria/Viruses
  • Dissolved solids
  • Solid substances
  • Dissolved gases
• Water pollution is the contamination of water sources (e.g. lakes, rivers, oceans and groundwater) by substances which make the water unusable for drinking, cooking, cleaning, swimming and other activities. All forms of pollution eventually make their way to water. Water pollution affects plants and organisms living in these bodies of water; and, in almost all cases the effect is damaging not only to individual species and populations, but also to the natural biological communities.
• Pollutants of water


  • leaves and grass clippings
  • swimming pool chemicals
  • hazardous chemicals
  • soaps and detergents
  • thrash and litter
  • animal waste
  • dirt
  • oil and other chemicals from cars
  • air pollution from cars
  • pesticides and fertilizers
• Sources of water pollution

  • Atmospheric pollutants
  • Land pollutants
  • Human waste
  • Industrial waste
• Fertilisers


  Chemical fertilisers (such as ammonium nitrate and ammonium phosphate) and pesticides washed off surrounding land (runoff) will add nitrate and phosphate ions to water

  
• Industrial chemicals


  Oils, metal compounds, acids, alkalis, dyes, etc. from factories
• Pesticides


  Chemicals sprayed on crops to kill pests
• Oil


  Oil from refineries and ship-wrecked tankers
• Detergents


  Chemicals from factories, offices, homes, etc.
• Effects of water pollutants


  • Bacteria and algae grow faster and therefore use up all the dissolved oxygen and cause fish to die
  • Toxic to animals, plants, and bacteria in the water
  • Poisonous chemicals accumulate in the bodies of larger animals
  • Covers sea birds with oil; pollutes water bodies
  • Causes water to foam; poisonous to aquatic organisms
• Water purification methods


  • Filtration
  • Boiling
  • Distillation
  • Reverse osmosis
  • Chlorination
• Filtration


  Removes suspended solids, used for water used in industry
• Boiling


  Removes dissolved gases, used for removal of oxygen from water used in beverages
• Distillation


  Removes dissolved solids, used for distilled water to be used in the lab
• Reverse osmosis


  Reduces the salt content of sea or brackish water to obtain drinking water

  
• Chlorination


  Used to make drinking water safe for drinking and for sanitizing the water in swimming pools
• In Malta, water was pumped directly from the natural aquifers to be distributed domestically, but this is no longer possible due to climate change. Malta now relies on reverse osmosis to desalinate seawater to obtain drinking water. Wastewater is also treated and used for agricultural purposes.

  
• Osmosis


  The movement of water from a solution of low concentration (high water content) to a solution of high concentration (low water content)
• Reverse osmosis


  The reverse of osmosis, where seawater (concentrated salt solution) is forced through a semi-permeable membrane to get rid of its salt content and make it drinkable

  
• Reverse osmosis is very costly as it consumes a lot of electricity.
• Solvent action of water

  Water allows many substances (solute) to dissolve in it (though not all), forming a solution that can be described as dilute or concentrated depending on the ratio of solute to solvent
• Factors affecting solubility

  • Nature of the solute and solvent
  • Temperature
  • Pressure
• Factors affecting rate of dissolution

  • Size of the particles
  • Stirring
  • Amount of solute already dissolved
  • Temperature
• Saturated solution


  A solution which cannot dissolve any more solute at a given temperature
• Dilute solution


  A solution with a relatively small amount of solute dissolved
• Concentrated solution


  A solution with a relatively large amount of solute dissolved
• The solubility of a substance in a solvent is usually shown graphically by plotting the amount of solute in grams that dissolves in 100 g of solvent against temperature. Different substances have different solubility in water, and the solubility of solids generally increases with increasing temperature, while the solubility of gases decreases with increasing temperature.
• Solubility


  The maximum quantity of solute that can dissolve in a certain quantity of solvent (100g usually) at a specified temperature
• The graphs of solubility against temperature are known as solubility curves. By using curves of this type you can find the solubility of the solute at any temperature.
• Different salts have different solubilities in water. Note that the solubility shown by the graph represents the point at which 100 g of water cannot take in any more solute at a particular temperature and so all points on the graph represent saturated solutions!
• The amount of solid solute that a solvent will dissolve increases with increasing temperature but it is not always the case – gases!
• Solubility curves


  Graphs of solubility against temperature that can be used to find the solubility of a solute at any temperature
• Different salts have different solubilities in water