Global challenges Study Guide

Potable water is water that is safe for human consumption, meaning it is free from harmful microorganisms and dissolved substances. The methods used to produce potable water depend heavily on the source of the raw water. For fresh water sources, such as rivers and lakes, the main challenge is removing insoluble solids and microbes. This is typically achieved through sedimentation, filtration, and chlorination. Sedimentation involves allowing large, insoluble particles to settle at the bottom. Filtration then removes smaller insoluble particles by passing the water through beds of sand and gravel. Finally, chlorine is added to kill bacteria and other harmful microorganisms. For salty water sources, like seawater, the primary challenge is removing the high concentration of dissolved salts. Desalination is the process used for this, with distillation being a common method. Distillation involves boiling the salty water, converting it into steam, and then condensing this steam back into liquid water. The dissolved salts do not evaporate with the water and are left behind, effectively separating them. Reverse osmosis is another desalination technique where water is forced through a semi-permeable membrane at high pressure, allowing water molecules to pass through but retaining salt particles. These separation techniques are crucial for making water safe and available for drinking.

Addition polymerisation is a fundamental process in organic chemistry where many small monomer molecules, typically alkenes, join together to form a very large polymer molecule. This type of polymerisation is called 'addition' because no atoms are lost during the reaction; all atoms present in the monomers are incorporated into the polymer. The key characteristic of monomers suitable for addition polymerisation is the presence of a carbon-carbon double bond (C=C), which is a functional group. During the polymerisation process, this double bond breaks, allowing individual monomer units to link up in a long chain. The product, a polymer, consists of repeating units. Each repeating unit corresponds to the original monomer molecule, but with the double bond replaced by single bonds on either side, connecting it to the next repeating unit in the chain. For example, ethene monomers (CH₂=CH₂) polymerise to form poly(ethene), where the repeating unit is (-CH₂-CH₂-). The repeating unit is essentially the part of the monomer that forms the polymer backbone. By understanding the functional group (C=C double bond) in the monomer, one can deduce the structure of the repeating unit in the resulting polymer and vice versa. This process is crucial for producing many common plastics like poly(ethene), poly(propene), and poly(chloroethene) (PVC).