Water

5.1 Air and oxygen

5.3 Rusting

Water

  1. Overview

    1. Natural water contains dissolved salts, which can be removed by distillation to obtain pure water
    2. Desalination is an important method of obtaining water for domestic use in some countries
    3. The boiling point of pure water is 100 °C at standard atmospheric pressure: 101325 Pa or 1 atm

  2. Production of drinking water

    1. Drinking water should have sufficiently low levels of dissolved salts and microbes1
    2. It can be produced by passing suitably-sourced water through filter beds to remove solids and sterilising with chlorine

  3. Filters

    1. Water filters containing carbon, silver and ion-exchange resins can remove some dissolved substances from tap water to improve the taste and quality2
    2. Ion-exchange resins exchange ions dissolved in water for hydrogen & hydroxide ions, which recombine to form water3

  4. Sterilisation

    1. Chlorine may be added to reduce the level of microbes

    2. Fluoride may be added to improve dental health; however, it is very controversial:

      For Against
      Some areas have had it for about 50 years now and nobody has proved that there are any harmful effects, that is, apart from fluorosis Excess fluoride in children causes white streaks of calcium fluoride to appear on their teeth, which are porous and can become stained; this reflects changes in their bones and can be a sign of other problems, such as weakening bones, increasing the number of fractures, and bone cancer
      The latest studies show that it has caused a 30% reduction in teeth cavities as well as causing the bacteria that cause tooth decay to die out and that not fluoridating makes it five times more likely to get tooth decay The benefit is not significant, and toothpastes & dental care have improved, so why take the risk?
      It helps protect the teeth of those who do not have good dental hygiene habits, including not visiting the dentist regularly It is ethically wrong to give people treatments that they have not consented to since they have a right to choose
      It also protects from the heart diseases caused by the same bacteria as tooth decay Some studies show that excess fluoride affects the brain, producing learning difficulties, and has also been associated with Alzheimer’s disease in older people
      The fluoride is only added in tiny amounts, about one part per million It cannot be ensured that people stick to safe limits of fluoride because people’s intakes cannot be controlled
  1. Though there is no universal drinking water quality standard, comparisons of different ones can be found on Wikipedia ↩
  2. Carbon, especially activated carbon which has low-volume pores to increase surface area, reduces the level of chlorine as well as pesticides and other organic impurities
    Silver nanoparticles discourage microbial growth in the filter by killing bacterial cells by affecting their structure & morphology ↩
  3. An ion-exchange resin is an insoluble matrix, typically small yellow-white beads, made from an organic polymer, that can remove ions including, but not limited to: calcium, magnesium, lead, copper, aluminium, chloride and sulfate ↩