S3 Chemistry Chapter 3 The Ocean Revision Notes PDF

Marymount Secondary School S3 Chemistry Chapter 3 The Ocean (Revision Notes) 1. Composition of Sea Water  Sea water is a mixture of salts, dissolved gases, water,...

Marymount Secondary School S3 Chemistry Chapter 3 The Ocean (Revision Notes) 1. Composition of Sea Water  Sea water is a mixture of salts, dissolved gases, water, etc.  The approximate composition of sea water (by mass): Composition Percentage by mass (%) Water 96.5 Sodium chloride 2.4 Magnesium chloride 0.5 Sodium sulphate 0.4 Calcium chloride 0.1  Sodium chloride (table salt) and water are two important components of sea water.  Sea water is a solution:  Dilute, Concentrated & Saturated Solution o Dilute solution: A small amount of solute in a given volume of solution. o Concentrated solution: A large amount of solute in a given volume of solution. o Saturated solution: A concentrated solution containing the maximum amount of solute (sodium chloride) at a certain temperature.  Progress Check (CE 05 II Q19) Which of the following correctly describes the sequence of procedures to separate sand, salt and water from a mixture of sand and salt solution? A. filtration, evaporation B. filtration, distillation C. crystallisation, filtration D. crystallisation, filtration, distillation  Progress Check (CE 07 II Q2) Which of the following substances has a sharp boiling point? A. Petrol B. Red wine C. Molten wax D. Liquid ammonia Answers: B, D S3/Chemistry/Chapter 3/The Ocean (Revision Notes) 1 2. Extraction of Common Salt from Sea Water  To extract sodium chloride (common salt) from sea water, decantation, filtration, evaporation and crystallization are employed. Method (All diagram should be labelled properly) Decantation is a quick method for separating a much denser solid from a liquid, e.g. separating coarse sand from sea water. Filtration is a method for separating insoluble solid from a liquid, e.g. separating mud from sea water: Residue: (e.g. mud) Solid remains on the filter paper. Filtrate: (e.g. sea water) Liquid that passes through the filter paper. Evaporation is a method for separating dissolved solid from a solution. When sea water is evaporated to dryness, sodium chloride is left. The sodium chloride obtained is in powder form, these powders may have impurities. Crystallization is another method for separating a solute from a solution. It is the process of forming crystals. By crystallization, more pure dissolved solids (solute) can be obtained from the solution.  By cooling a hot concentrated solution: o Hot saturated solution can be prepared by heating a dilute solution to remove solvent. o As solute has higher solubility in solvent at higher temperature, the cooled solvent cannot hold all solutes. Then extra solutes separate out as crystals. S3/Chemistry/Chapter 3/The Ocean (Revision Notes) 2  By slow evaporation of a solution at room temperature: o As solvent evaporates form the solution, the reduced amount of solvent cannot hold all the solutes. Then extra solutes separate out as crystals. o Large crystals are obtained if longer time for crystals to form.  Progress Check (CE 08 I Q11) Copper(II) sulphate crystals (CuSO4 5H2O) can be prepared in a laboratory by the following: Step 4: Allow the hot saturated solution to cool down to obtain copper(II) sulphate crystals. Explain why crystals would be obtained when the saturated solution is allowed to cool down. Answers: The solubility of CuSO4 decreases when temperature of solution drops. 3. Isolation of Pure Water from Sea Water  Simple distillation is a method for separating liquid from a solution. When sea water is heated, the water vapour escapes and passes through the inner tube of the condenser → The vapour is cooled and condensed to form liquid water →The pure water is collected in the conical flask is called distillate. using simple apparatus using ‘Quickfit’ apparatus Distillation apparatus:  Thermometer is used to measure the temperature of the vapour actually distilled over. Hence the thermometer bulb should be placed at the opening of the delivery tube.  Anti-bumping granules are added to ensure smooth boiling.  Condenser is used to condense the vapour to form liquid.  Water should enter the condenser from the bottom to get a better cooling effect.  The end of the delivery tube should be placed above the distillate. Otherwise, sucking back of cold distillate into the hot boiling tube will occur if the heat source is removed. This may crack the hot boiling tube. S3/Chemistry/Chapter 3/The Ocean (Revision Notes) 3 4. Flame Test  Flame test is used to identify the metallic element in a compound by the flame colour produced. o Different metallic element give different flame colours. o Procedures of the flame test: 1. Dip the platinum wire (or nichrome wire) in concentrated hydrochloric acid. 2. Heat the platinum wire strongly in the flame in order to vaporize the impurities. 3. Dip the cleaned platinum wire in concentrated hydrochloric acid again. 4. Dip the moistened platinum wire in a small amount of sample tested. 5. Heat the platinum wire with the sample in a non-luminous Bunsen flame. 6. Note the flame colour. Compounds containing Flame colour Potassium Purple Sodium golden flame Calcium Brick-red Copper Bluish green 5. Silver Nitrate Test  Acidified Silver Nitrate Solution is used to test whether a compound contains chloride o Procedures for showing the presence of chloride in a solution: 1. Dissolve a sample in deionized water. 2. Excess dilute nitric acid is added to the solution. 3. Silver nitrate solution is then added to the solution. (Remarks: Step 2 and Step 3 can be interchanged) o We can also mix excess dilute nitric acid and silver nitrate solution first to give acidified silver nitrate solution. Then, the acidified silver nitrate solution is added to the solution. o Excess dilute nitric acid prevents formation of other white insoluble compounds (e.g. silver carbonate and silver sulphite) which may interfere the results.  In the presence of chloride, white precipitate is formed. Silver ions + Chloride ions → Silver chloride colourless colourless white precipitate  insoluble in water S3/Chemistry/Chapter 3/The Ocean (Revision Notes) 4 6. Test for Water  The presence of water in a sample can be tested by using anhydrous copper (II) sulphate (CuSO4) or dry cobalt (II) chloride (CoCl2) paper. Chemical used Colour Colour (absence of water) (presence of water) Cobalt(II) chloride paper Blue Pink Anhydrous copper (II) sulphate White Blue o The colour of (chemical used) changes from (colour A) to (colour B).  Note: o This test only shows the presence of water in a sample. It does not show that the sample is pure water. o To show that the liquid sample is pure water, you can determine its melting point (0 oC) or boiling point (100 oC).  Progress Check The setup shown below is used to study the effect of heating copper(II) sulphate crystals. (a) Suggest the observation of the experiment. (b) Suggest how you can show that the crystals containing water. (c) Explain why the boiling tube is held in a downward slanting position shown. Answers: (a) Upon heating, the blue crystals turn white and colourless liquid condenses on the upper part of the test tube. (b) Test the colorless liquid that condenses on the upper part of the test tube using a piece of cobalt(II) chloride paper. It will turn from blue to pink if the liquid contains water. (c) If the test tube is held with its mouth pointing upwards, the water condensed on the upper part of the test tube will flow back to the hot test tube. The tube will crack.  Progress Check A student carried out some tests on a compound X which is a white solid. The results obtained were summarized in the following flow diagram: Based on the results obtained, deduce what solid X might be. Answers: From Test 1, solid X contains calcium ions. From Test 2, solid X contains chloride ions. Hence, solid X might be calcium chloride. S3/Chemistry/Chapter 3/The Ocean (Revision Notes) 5 7. Electrolysis of Sea Water  Electrolysis of sea water produces three useful chemicals. They are: * Hydrogen gas * Chlorine gas * Sodium hydroxide  Equation: electricity Sea water → chlorine gas + hydrogen gas + sodium hydroxide solution (NaCl + H2O) (Cl2) (H2) (NaOH)  Experimental Setup :  Common Uses of Products from Electrolysis of Sea Water: Sea water Hydrogen Chlorine Sodium hydroxide Hydrochloric acid Bleaching solution Fuel Plastic Waste water treatment Ammonia Sterilizing Soap (to make fertilizer) drinking water S3/Chemistry/Chapter 3/The Ocean (Revision Notes) 6