Global challenges Study Guide

Potable water is water that is safe for human consumption, meaning it is free from harmful microorganisms and dissolved salts in high concentrations. The methods used to produce potable water depend heavily on the source of the water. For fresh water sources like rivers and lakes, the primary challenge is removing insoluble solids and bacteria. This typically involves a multi-stage process. First, screening removes large debris. Then, sedimentation and flocculation are used to allow smaller insoluble particles to settle out, often aided by adding chemicals that make them clump together. Filtration, usually through sand and gravel beds, removes remaining suspended particles. Finally, sterilisation, often using chlorine or ultraviolet light, kills harmful microorganisms. For seawater or other highly saline water sources, a much more energy-intensive process called desalination is required. The main method of desalination is distillation, where water is boiled to produce steam, which is then condensed back into liquid water, leaving dissolved salts behind. Reverse osmosis is another desalination technique where water is forced through a partially permeable membrane, separating it from dissolved salts. Both distillation and reverse osmosis are energy-intensive and therefore costly methods but are essential in regions with limited freshwater availability.

Crude oil is a complex mixture of hydrocarbons, which are compounds made of hydrogen and carbon atoms. These hydrocarbons have different chain lengths and thus different boiling points. To separate this mixture into more useful fractions, a process called fractional distillation is employed. This process takes advantage of the different boiling points of the components in crude oil. First, crude oil is heated to a very high temperature in a furnace, turning most of it into hot vapour and liquid. This hot mixture is then pumped into the bottom of a tall, cylindrical fractional distillation column. The column has a temperature gradient, meaning it is hottest at the bottom and gradually cooler towards the top. As the hot vapours rise up the column, they cool. When a particular hydrocarbon vapour cools to its boiling point, it condenses back into a liquid. Hydrocarbons with high boiling points (longer carbon chains) condense lower down the column, where it is hotter. These are collected as heavy fractions like bitumen and fuel oil. Hydrocarbons with low boiling points (shorter carbon chains) continue to rise higher up the column before they cool enough to condense. These are collected as lighter fractions like gasoline, naphtha, and refinery gases at the top of the column. The fractions are collected at different levels of the column based on their boiling point ranges, allowing for the separation and collection of many valuable products from crude oil, each with specific uses.