#5070 #0602 water #igcsechemistry0620 #gcsechemistry #5070chemistry

Water is an essential resource used for domestic, agricultural, and industrial purposes. In chemistry, it is important to distinguish between water that is "pure" and water that is "potable". *1. Pure vs. Potable Water* *Pure water* contains only $H_2O$ molecules with no other substances dissolved in it. *Potable water* is water that is safe to drink. It is not necessarily pure, as it often contains low levels of dissolved salts and minerals. To be potable, water must have a *pH between 6.5 and 8.5* and contain no harmful microorganisms like bacteria or fungi. *2. Sources of Water* *Fresh water**: Collected as **surface water* (lakes, rivers, reservoirs) or *groundwater* stored in underground rocks called **aquifers**. *Seawater**: Used in areas with low rainfall through **desalination**. This is done via **distillation* (boiling and condensing) or *reverse osmosis* (passing water through a membrane to trap salt ions). Both methods are expensive due to high energy requirements. *3. Water Treatment Processes* To make fresh water potable, it undergoes three main stages: 1. **Screening/Filtration**: Water passes through a wire mesh to remove large debris (twigs, plastic) and then through beds of sand and gravel to filter out smaller solids. 2. **Sedimentation**: Water stands in tanks where heavier particles sink to the bottom to form a sediment layer. Carbon (charcoal) may be used to remove unpleasant tastes and odours. 3. **Sterilisation**: To kill harmful microbes, the water is treated with **chlorine gas**, ozone, or ultraviolet (UV) light. *4. Chemical Tests and Purity* Students must know how to identify water and verify its purity in a lab: **Chemical Tests**: **Cobalt(II) chloride**: Blue anhydrous paper turns **pink**. $CoCl_2 (s) + 6H_2O (l) ⇌ CoCl_2·6H_2O (s)$ **Copper(II) sulfate**: White anhydrous powder turns **blue**. $CuSO_4 (s) + 5H_2O (l) ⇌ CuSO_4·5H_2O (s)$ *Testing Purity**: Pure water has a **sharp boiling point of 100°C* and a **melting point of 0°C**. Impurities will increase the boiling point and decrease the melting point. *5. Waste Water Treatment* Waste water comes from domestic (sewage), agricultural (nutrient runoff), and industrial (chemicals) sources. *Process**: Following screening and sedimentation, the waste is separated into **effluent* (liquid) and *sludge* (solid). *Digestion**: The effluent is broken down by microbes using **aerobic digestion* (with oxygen), while the sludge undergoes *anaerobic digestion* (without oxygen). *Byproducts**: Sludge digestion produces **methane gas* for energy and waste that can be used as **fertiliser**. *6. Substances in Natural Water* Natural water sources contain both beneficial and harmful substances. **Beneficial**: Dissolved oxygen (for aquatic life) and metal compounds like calcium and magnesium (essential minerals). *Harmful**: Toxic metals (lead, mercury), untreated sewage (causing diseases like cholera), and nitrates/phosphates from fertilisers which can lead to **eutrophication* and deoxygenation of water. #chemistry #0620 #9701 #5070 #5070chemistry #p1 #alevelchemistry #caie #p3