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Water

Environmental Chemistry, Hardness and Water Treatment

QuestionAnswer
What makes the water in Ireland so hard? As it passes over limestone and other rocks it picks up a lot of Ca+2 and Mg+2 ions
What is hard water? Hard water is water that contains Ca+2 and Mg+2 ions and forms a scum with soap.
What are the 2 forms of water hardness? Temporary hardness and Permanent hardness
What causes temporary hardness? What is the easiest way to remove it? Temporary hardness is caused by Calcium Carbonate being dissolved by Carbonic Acid to form Calcium Hydrogen Carbonate. The easiest way to remove temporary hardness is by boiling the water.
What causes limescale? Calcium Hydrogen Carbonate is soluble in water. However when water is boiled it reverts back to Calcium Carbonate which is insoluble in water and causes Limescale.
What causes permanent hardness? Permanent hardness is caused by Calcium Sulphate or Magnesium Sulphate in water. It cannot be removed by boiling.
Techniques to remove permanent hardness? 1: Distillation removes all ions from solutions but is expensive. 2: Washing Soda contains Sodium Carbonate which removes the Calcium Ions as insoluble Calcium Carbonate. 3: Ion Exchange: removal of Ca or Mg ions with Na and is expensive.
Advantages and disadvantages of hard water. Advantages: Tastes better, provides Ca for bones and teeth and is good for brewing and tanning. Disadvantages: Wastes soap, blocks pipes with limescale and produces a scum.
What is EDTA short for? Ethylene diamene tetra acetic (acid)
How is the concentratino of water hardness express? mg/L of CaCO3 (ppm)
What indicator is used in the calculation of water hardness? What is the colour change? Solochrome black or Erichrome Black T. Colour change is wine red to blue. The end point is very hard to detect. We call this a complexometric titration.
What is the ratio of EDTA to Calcium ions? 1:1
What are the different degrees of hardness in terms of ppm of CaCO3? Soft 0-75ppm. Moderately Hard 75-150ppm. Hard 150-300ppm Very Hard: >300
What are the seven different stages of water treatment? 1: Screening. 2: Flocculation 3: Sedimentation 4: Filtration 5: Chlorination 6: Fluoridation 7: pH adjustment
What chemical is added in the flocculation stage of water treatment? Aluminium Sulphate
In what form is Fluorine added to the water in water treatment? Sodium Fluoride and Hexaflourosilicic
What is added to the water if it is too acidic or too basic? Too acidic: Calcium Hydroxide is added. Too basic: dilute Sulphuric acid or Carbon Dioxide
Why is oxygen only slightly soluble in water? Water is a polar molecule and oxygen is a non-polar molecule
What happens the solubility of oxygen as temperature increases? The solubility of oxygen decreases as the temperature of the water increases.
What is BOD? Biochemical Oxygen Demand is defined as the amount of dissolved oxygen consumed by biological action when a sample of water is kept at 20 Celsius in the dark for 5 days.
Why is a BOD sample kept in the dark? To prevent photosynthesis from adding oxygen to the sample
Eutrophication Eutrophication occurs when there is an excess of NItrate and Phosphate ions present. This increases the number of algae who will consume all the available oxygen in the water. This is known as algal bloow.
What reagents are needed to test a sample of water for dissolved oxygen? Manganese Sulphate, Alkaline Potassium Iodide and Sulphuric Acid
What are the colour changed involved in the calculation of dissolved oxygen? When Manganese Sulphate and Alkaline Potassium Iodide are added a white precipitate is formed. This then reacts with the oxygen present (if any) to produce a brown colour. When the acid is added Iodine is liberate to bring a reddish/brown colour.
What is the water reacted with when calculating dissolved oxygen? What indicator is added? Sodium Thiosulphate is used as there is Iodine present in the water sample. Indicator used is Starch. It is added when the sample goes straw yellow.
What is the colour change involved in the titration used to calculate the dissolved oxygen. The solution goes blue/black when the indicator is added and is titrated until it goes colourless.
What is sewage? Sewage is the name given to water that is delivered to waste treatment plants. It include water from homes, businesses and factories. It contains organic wastes, inorganic wastes and gases.
Why must sewage be treated? Its high BOD would reduce the dissolved Oxygen in rivers or lakes.
What are the 3 stages of sewage treatment? Primary Treatment (removal of solids). Secondary Treatment (removal of suspended and dissolved organic material. Tertiary Treatment (removal of Nitrates and Phosphates)
What is the Activated Sludge Process? The sewage is pumped into a large aeration tank where the sewage provides nutrients for micro-organisms. TO help break down the sewage the tank is constantly agitated to draw in air and oxygen, or has compressed air injected in.
What use can be made of the sludge made in the secondary stage of sewage treatment. It can be used as a fertilliser.
How are Nitrates and Phosphates removed in Tertiary Treatment? They are removed by the addition of Aluminium Sulphate which causes them to stick together and settle to the bottom of the tank.
What is free chlorine? Free chlorine is chlorine found in water as hyperchlorous acid or the hypochlorite ion.
How can we measure the amount of free chlorine in a water sample? (2 techniques) A comparator or a colorimieter.
What chemical reagent is needed for a comparator? DPD Diethyl Phenylene Diamine
What is the relationship between light absorption and chlorine concentration? The amount of light absorbed is directly proportional to the amount of chlorine present.
What does chlorine form when it reacts with water? Hypochlorous and Hydrochloric Acid. Hypochlorous acid is a weak acid that dissociates in water to form the hypochlorite ion.
Created by: cbschemistry