Earth's Resources & Potable Water

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Using the Earth's resources

Humans use the Earth's resources for warmth, shelter, food and transport. Natural resources, supplemented by agriculture, provide food, timber, clothing and fuels
Finite resources will run out: fossil fuels, metal ores. Renewable resources can be replaced as fast as they are used: timber, crops, wind
Natural products are supplemented or replaced by synthetic ones: rubber from trees is supplemented by synthetic polymers; cotton and wool by polyester and nylon; wood by plastics and composites
Sustainable development meets the needs of current generations without compromising the ability of future generations to meet their own needs. Chemistry helps by improving agricultural and industrial processes and by developing new products
Reading resource data: compare the rate of use with the amount left to estimate how long a resource will last, and use orders of magnitude to judge whether a difference matters

Potable water is water that is safe to drink: low enough levels of dissolved salts and microbes
It is not pure water in the chemical sense, because it contains dissolved substances. Pure water is only $\text{H}_2\text{O}$
In the UK, rain provides fresh water with low levels of dissolved substances, collecting in the ground, lakes and rivers. Most potable water is produced by:
- Choosing an appropriate source of fresh water
- Passing it through filter beds to remove solids such as sand and twigs
- Sterilising to kill microbes, using chlorine, ozone or ultraviolet light
Where fresh water is scarce, sea water or salty water is desalinated by distillation or by membranes such as reverse osmosis. Both need large amounts of energy, so they are expensive

	Ground or fresh water	Sea water
Dissolved salts	low	very high
Treatment	filtration, then sterilisation	desalination (distillation or reverse osmosis), then sterilisation
Energy and cost	low	high

Flow chart of UK water treatment: source (reservoir) to filter beds to sterilisation (chlorine, ozone or UV) to the tap, with what each stage removes

Test the pH of samples from different sources with universal indicator or a pH probe
Find the dissolved solids: weigh an empty evaporating basin, add a measured volume of the sample, evaporate to dryness over a water bath, and weigh again. The increase in mass is the mass of dissolved solids
Distil a salty sample: heat it in a flask, the water boils and the steam is cooled in a condenser and collected as pure distillate, leaving the salts behind
Test the distillate: its pH should be 7, and evaporating a sample should leave no residue, showing the dissolved solids have been removed