Unit 4 WATER 4.1 Occurrence and Physical Properties

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1 Chemistry Form 3 Page 42 Ms. R. Buttigieg Unit 4 WATER 4.1 Occurrence and Physical Properties Water is essential for many fundamental life processes. It is however, very difficult to get pure because it is so good at dissolving other substances. A clean water supply is however essential for good health. Many diseases like cholera are a consequence of an impure water supply. Pure water does not exist in a natural state, but supplies of water can be obtained all over the world, varying in degrees of purity from rain water (0.0005% pure) to sea water, in which the impurities reach a high proportion of 3.6%. Physical properties of Pure water: - 1. It is a clear colourless liquid. 2. It is a very good solvent. Most solids are more soluble in hot water. Gases are less soluble in hot water than in cold. 3. It freezes at 0 o C and when it freezes it expands. 4. It boils at 100 o C at a pressure of 760 mm Hg (mercury). 5. Its maximum density is 1 g cm -3 at 4 o C. 6. It is neutral to litmus. 7. It is a liquid at room temperature. Tests for water: a. It turns white anhydrous copper (II) sulphate blue. b. It turns blue cobalt chloride paper, pink. REVERSE OSMOSIS: A way of purifying water As already indicated above it is very hard to actually obtain pure water. A lot of water on our planet can be found in the sea, but it contains a lot of dissolved impurities. Even in Malta, given the lack of rain, the main source of water is from the sea. But before it can be good for drinking this has to be treated so as to purify the water from the brackish water. What is Osmosis? Osmosis is the movement of water form a solution of low concentration to a solution of high concentration. The two solutions are connected by a partially permeable membrane which only allows water to pass through. Osmosis Impure Solution water moves by osmosis Pure water Solution Reverse Osmosis water forced to move this way Impure Solution Pure water Solution In the process of reverse osmosis as the name suggests, this is done in reverse. Water is forced to move from a low concentration of water (in the sea water) to a higher concentration of pure water. This is done by exerting pressure on the solution, so that only the water passes through the semi permeable membrane. Given it is the reverse of osmosis it is called reverse osmosis and is a very suitable way in which sea-water can be changed into tap water that can be used for many things.

2 Chemistry Form 3 Page 43 Ms. R. Buttigieg Water Pollution and its effects POLLUTING SOURCE EFFECT SUBSTANCE Sewage Homes etc. If sewage is pumped directly into the sea, it can cause health hazards. If the amount of sewage is small, bacteria can cope with it and break it up into carbon dioxide, water and nitrates. But if the amount of sewage is too big, the bacteria use all the oxygen in the water. Most organisms living in the water die including the bacteria. So the water becomes cloudy and smelly. Heat (Thermal pollution) Power Stations Hot water from power stations contains less oxygen dissolved in it. So some fish may not live in this hot water. Fertilizers Pesticides Oil Detergents Industrial chemicals and heavy metals Answer the following: Washed off from fields after being placed there by farmers Spraying of crops with chemicals From refineries and from shipwrecked tankers Factories, home, offices Metal compounds, acids, alkalis, dyes, etc from factories Rain washes the fertilizers into the water-table; and into rivers and lakes. Water pumped from the water table may not be suitable for drinking. Bacteria and algae grow faster, use up all the oxygen dissolved and then die. Poisonous chemical accumulate in the bodies of larger animals causing them harm. Covers sea birds with oil and pollutes beaches Causes water to foam, poisonous to organisms living in the water. Poisonous to animals, plants and bacteria living in the water and eventually may also harm human beings who eat those organisms. 1. Samantha finds a colourless liquid. She wants to find out if this liquid contains water. She adds anhydrous copper (II) sulphate to the liquid. a. What colour change would she expect if the liquid contains water? She then adds universal indicator to the liquid and it turns green (ph7). She then evaporates a sample to dryness when a white residue is left and notes that boiling occurs at 102 o C. b. Which of her observations suggest that the liquid could be pure water? c. Which observation could suggest that the liquid is an aqueous solution (therefore not pure)? d. What do you think is the nature of the white residue left after evaporating?

3 Chemistry Form 3 Page 44 Ms. R. Buttigieg 4.2 Solvent Action of Water Solution & Solubility Solvent A liquid which dissolves a substance (solute) Solute The substance that dissolves in the solvent. Solution Occurs when a solute dissolves in a solvent. Saturation When the solution contains as much solute as can be dissolved at a particular temperature. Water is a very good solvent and dissolves a wide range of solutes. The solubility of a solute in water at a given temperature is the number of grams of that solute which can be dissolved in 100g of water to produce a saturated solution at that temperature. Usually the amount of solute that a solvent will dissolve increases with increasing temperature but it is not always the case Interpreting Solubility Curves (See GCSE Chemistry pg. 106 Solubility Curves) These are graphs of solubility against temperature. By using such curves you can find the solubility of any solute at a given temperature. For which salt does solubility increase most rapidly with rise in the temperature? How does the solubility of NaCl change with temperature? Which salt dissolves the most solute at 40 o C? Which salt is the most soluble at 5 o C? On a graph paper plot the following solubility curve for KNO3. and work also pg. 106 (GCSE) no. 2-3 Temperature 10 o C 15 o C 30 o C 40 o C 50 o C 57 o C Solubility/g per 100g of water a. From the graph plotted what is the solubility of Potassium Nitrate at 35 o C? b. What is the maximum number of grams that can be dissolved at 20 o C?

4 Chemistry Form 3 Page 45 Ms. R. Buttigieg Water of crystallization The water of crystallization is that definite amount of water, which remains associated (joined) in the crystal when a substance crystallizes out of the solution. It is necessary for the crystalline shape of the crystal, and the crystals (that contain the water of crystallization) cannot form without the presence of water. Water of crystallization is sometimes called water of hydration. When a few crystals of Copper (II) sulphate are heated gently in a test-tube, water vapour is given off. The colour and shape of the crystals disappears and instead of blue crystals, a white powder remains. When water is added to the white powder (anhydrous copper (II) sulphate), a hissing sound is heard, and steam is given off. This happens because heat energy is liberated and a chemical change takes place. A blue solid is left. Hydrated Copper (II) sulphate Anhydrous Copper (II) Sulphate + Water CuSO 4.5H 2 O (s) CuSO 4 (s) + 5H 2 O (g) BLUE WHITE The heat generated is called heat of hydration. The reaction shown above as can be seen by the direction of the arrows can be reversed. Hydrated Salts Salts that contain water of crystallization Anhydrous Salts Salts which are heated so the water of crystallization is driven off. Efflorescence, deliquescence and hygroscopy A deliquescent solid absorbs water-vapour from the atmosphere and dissolves in it. E.g. Sodium hydroxide (NaOH), Calcium chloride (CaCl 2 ) An efflorescent substance loses its water of crystallization to the atmosphere. E.g. Sodium sulphate (Na 2 SO 4 ), sodium carbonate decahydrate (Na 2 CO 3.10H 2 O) A hygroscopic substance is one which absorbs water from the atmosphere. E.g. Concentrated Sulphuric acid (H 2 SO 4 ) (not called deliquescent as not a solid!) Note that: Deliquescent and hygroscopic substances are used as drying agents. Silica gel absorbs water vapour very readily and is used for drying gases on an industrial scale. When spent up (cannot hold more water vapour), the gel can be re-activated by heating to a suitable temperature, so it looses the water and can be used again hydration & dehydration

5 Chemistry Form 3 Page 46 Ms. R. Buttigieg 4.3 Synthesis of Water An experiment in which water can be formed from hydrogen. Water is an oxide of hydrogen. If hydrogen is burnt in oxygen, only water is produced. 2H 2 (g) + O 2 (g) 2H 2 O (l) This also forms the basis of the idea of having hydrogen fuelled cars. No polluting gases would be produced as only water is produced. Care must be taken as hydrogen is highly flammable. Test for water: Chemical test White anhydrous copper sulphate turns blue if water is present. Blue cobalt chloride turns pink Physical test If water is pure it should boil at 100 o C see Chemistry for You pg Chemical properties of Water Reaction of Water with non-metals Carbon - Carbon when heated reacts with steam to form carbon monoxide and hydrogen. C (s) + H 2 O (g) CO (g) + H 2 (g) Chlorine - Chlorine reacts with water to form 2 acids: Hydrochloric acid (HCl) and Hypochlorous acid (HClO) Cl 2 (g) + H 2 O (l) HCl (aq) + HClO (aq) Reaction of Water with non-metal oxides: In these cases an acid is formed (ph < 7) E.g. SO 3 (g) + H 2 O (l) H 2 SO 4 (aq) sulphuric acid Reaction of Water with metal (basic) oxides: A metal hydroxide is formed (ph > 7) E.g. Na 2 O (s) + H 2 O (l) 2 NaOH (aq) Sodium hydroxide The reaction with basic and acidic oxides was included in Topic 3.5 See Handouts pg

6 Chemistry Form 3 Page 47 Ms. R. Buttigieg Answer the following: - 1) Water reacts with carbon to form carbon monoxide and hydrogen. a) From the above information write a word equation and a balanced chemical equation. (3) b) Under what condition/s does this happen? (1) 2. Chlorine in water renders it acidic and a bleach. Write an equation for the reaction between chlorine and water. (2) 3. Complete the following: (6) Most metal oxides are (acidic, basic) Most non-metal oxides are (acidic, basic) Some metal oxides are amphoteric meaning An example is Some non-metal oxides are neutral meaning An example is 4. Complete the following table: (6) Word Meaning Example Deliquescent substance Efflorescenct substance Hygroscopic substance 5a) Label the diagram. (4) b) Write an equation for the reaction (3) c) How can you test for the presence of the prepared oxygen. (2)

7 Chemistry Form 3 Page 48 Ms. R. Buttigieg Reaction of Water with metals Metal Method Observation and equation involved Note Potassium A pea sized amount of potassium is placed in a trough half filled with cold water Reacts violently with cold water Potassium hydroxide solution (alkaline) and hydrogen gas are produced. Potassium melts, moves about on the water surfacwe and catches fire (burns with a lilac flame) 2K (s) + 2H 2 O (l) 2KOH (aq) + H 2 (g) Sodium As above Reacts vigorously with cold water Sodium hydroxide solution (alkaline) and hydrogen gas are produced. Melts into a silvery ball and moves on the water surface. 2Na (s) + 2H 2 O (l) 2NaOH (aq) + H 2 (g) Lithium As above Reacts smoothly with cold water Lithium hydroxide solution (alkaline) and hydrogen gas are produced. Moves about on the water surface 2Li (s) + 2H 2 O (l) 2LiOH (aq) + H 2 (g) Calcium A small piece of calcium is placed in a beaker full of water. Calcium sinks to the bottom and gives off a steady stream of hydrogen. The solution becomes alkaline and cloudy due to the formation of calcium hydroxide which is soluble in water Ca (s) + 2H 2 O (l) Ca(OH) 2 (aq) + H 2 (g) These 4 metals react with cold water. Potassium has the greatest reactivity. Calcium is the least reactive of the 4. Products are always, metal hydroxide & hydrogen. Magnesium A coil of magnesium is put in a beaker full of water. An inverted test tube full of water is placed over it to trap any gas given off and left for several weeks. Sodium Some sand is placed at the bottom of a hard glass tube and wetted. Some Mg powder is placed in the middle of the tube and the tube is closed with a bung, which has a hole and is connected to a glass pipe. Sand is heated and water changes to steam. Heating is continued till all air is expelled. Mg is then heated, ensuring steam is still passing over Mg. Lead No reaction Copper After several weeks some hydrogen gas gathers in a test tube Mg (s) + 2H 2 O (l) Mg(OH) 2 (aq) + H 2 (g) A reaction occurs between magnesium and steam Mg flares up and gives out white light and the hydrogen formed burns from the opening of the pipe. MgO ( ), a white powder remains in the test tube. Mg (s) + H 2 O (g) MgO (s) + H 2 (g) 3Fe (s) + 4H 2 O (g) Fe 3 O 4 (s) + 4H 2 (g) Reacts very, very slowly with cold water but vigorously with steam. These react with steam but not with cold water. Mg reaction is the most vigorous, with iron the least. Products are metal oxide & hydrogen

8 Chemistry Form 3 Page 49 Ms. R. Buttigieg 4.5 Hardness of Water & its effects See Chemistry for You pg and GCSE Chemistry pg a. Effects of hardness. b. The cause of temporary and permanent hardness of water; c. Methods of removing both types of hardness. d. The formation of scum with soap. e. The formation of stalactites. Furring of kettles and scaling in water pipes. f. Advantages and disadvantages of hard water. Soft water is water that readily forms a lather with soap. Hard water is water that does not readily form a lather with soap. Water is rendered hard by magnesium and calcium compounds which are dissolved in the water mainly: Calcium hydrogen carbonate Magnesium hydrogen carbonate Calcium sulphate Magnesium sulphate So hard water is hard because it contain Calcium ions (Ca ++ (aq)) and Magnesium ions (Mg ++ (aq)) dissolved in it. Soft water does not have these ions. Where hardness comes from: The most common cause of hardness is Calcium hydrogen carbonate. Ca(HCO 3 ) 2 Rainwater is not pure but contains carbon dioxide dissolved in it, which forms a weak acidic solution. This attacks the calcium carbonate (such as limestone and chalk) to form calcium hydrogen carbonate. Stalactites and Stalagmites CaCO 3 (s) + H 2 O (l) Ca(HCO 3 ) 2 (aq) Often found in underground caves in limestone areas. When water containing calcium hydrogen carbonate is warmed or boiled, the Ca(HCO 3 ) 2 decomposes to calcium carbonate, carbon dioxide and water. This is the reverse of what happens when water becomes hard. Ca(HCO 3 ) 2 (aq) CaCO 3 +H 2 O (l) + CO 2 (g) The temperature inside the caves is warm enough for some hard water to decompose. As drops form on the roof of the cave, a deposit of calcium carbonate is left behind. A deposit is also left when the drop hits the floor. That s why stalactites and stalagmites form opposite to each other. As time passes, stalactites grow down and stalagmites grow up.