Mark the Following Statements as True or False

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1 G J Z A H R A B. E D ( H O N S ) P A G E 1 G J Z A H R A B. E D ( H O N S ) S T A L B E R T T H E G R E A T C O L L E G E W A T E R St Albert the Great College Y E A R 9, C H A P T E R 5 Mark the Following Statements as True or False Water is made up of hydrogen and oxygen Burning hydrogen results in pure water Pure water is quite common in nature A substance that boils at 110 C and melts at 5 C is not, and contains no traces of, water Hard water does not form lather easily with soap The solubility of gases increases with an increase in pressure A saturated solution is one that contains the maximum amount of solute No metal can catch fire in water On heating blue copper (II) sulfate, water is obtained Copper and silver react slowly with water Hydrogen is always obtained when a metal reacts with water or steam Silica packets (used in packaging of shoes and clothes) contain a hygroscopic substance An efflorescent substance absorbs water of crystallization from the atmosphere

2 P A G E 2 G J Z A H R A B. E D ( H O N S ) General Properties of Water Water, H 2 O, is the most common molecule on Earth. It makes up 71% of the planet s surface and from 55% to 78% of the human body. It is found as a liquid and solid in the oceans and as a gas in the atmosphere. However, natural water is rarely pure. Water is a colourless, tasteless, odourless liquid at rtp (room temperature and pressure) Pure water has a freezing point of 0 C and boiling point of 100 C. However the boiling point and the melting point change as impurities are added. Pure water is an insulator of electricity since water molecules have no net charge. However, as solutes are added conductivity increases and thus conductivity can be used (together with melting point and boiling point determination) as a measure of water purity. Water can dissolve so many substances that it is often called the universal solvent. It can be produced in the lab by the combustion of hydrogen gas. The water vapour formed is condensed on a cold surface and collected. Water can be purified by repeated distillations. This water is called distilled water. The chemical test for the presence of water is that it turns grey anhydrous copper(ii) sulfate blue. (It also turns blue anhydrous cobalt(ii) chloride pink and white anhydrous iron(ii) sulfate green.) Exercise A, Water Properties 1. Give three physical properties of water (3marks) 2. How can you show that air contains water vapour? Include a chemical equation. (4marks) 3. Name 2 physical tests that can be done to show that a sample of water is pure. (2marks) 4. Name and draw a diagram for the separation technique used to: a. Obtain pure water from a suspension of zinc carbonate in water(4marks) b. Obtain pure water from an oil/water mixture (4marks) c. Obtain pure water from a solution of copper sulfate (4marks) d. Obtain pure water from an alcohol/water mixture (4marks)

3 G J Z A H R A B. E D ( H O N S ) Various chemicals dissolve in water and their solubility changes with temperature. In general: Solubility Curves The solubility of solids increases with temperature. The rate (speed) of solubility of a solid solute can be increased by heating, stirring and crushing the solid. The solubility of gases decreases with temperature but increases with pressure. Solubility is always measured in g/100g of solvent (water) Since water has a density of 1g/ cm 3, 100g of water are equivalent to 100cm 3 of water. P A G E 3 Exercise B, Solubility By using the solubility curve in this page estimate: 1. The solubility of sodium nitrate at 35 C (1mark) 2. The solubility of ammonia at 65 C (1mark) 3. The mass of potassium nitrate that dissolves in 200g of water at 50 C (2marks) 4. The mass of hydrogen chloride that dissolves in 50g of water at 45 C (2marks) 5. The mass of ammonium chloride that crystallizes from a 100cm 3 saturated solution which is cooled from 70 C to 30 C (3marks) 6. The mass of ammonia that escapes from a 50cm 3 saturated solution which is heated from 10 C to 90 C (3marks) 7. What happens when a 100cm 3 solution containing 30g of sodium chloride and 30g of potassium nitrate is cooled to 10 C? (2marks)

4 P A G E 4 G J Z A H R A B. E D ( H O N S ) Solubility Rules Ionic Solids are held together by ionic bonds. These are weaker than covalent bonds; however there are a huge number of these in an ionic lattice. Energy is needed to break these forces of attraction to form ions which will go in solution. If there is not enough energy, more energy can be supplied by heating: heating usually increases the solubility of solids. Gases need to form new bonds to enter solutions. Energy increases particles movement making it easier for gas particles to exit a solution. Thus the solubility of gases decreases at high temperatures. High pressures can be used to push gas particles in solution. There exist a number of rules to predict whether an ionic compound is soluble or not in water: Salts containing group 1 metals and ammonium cations are soluble. Nitrates are soluble. Chlorides, bromides and iodides are soluble (except those of silver, lead). Sulfates and sulfites are soluble (except those of calcium, lead and barium). Oxides, hydroxides, sulfides and carbonates are insoluble (except those of group 1 metals and ammonium cations) Exercise C, Solubility of Ionic Compounds Write the formula of these ionic compounds and predict whether they are soluble or insoluble in water Carbon dioxide is dissolved in sparkling wines, sparkling water and carbonate drinks under pressure. a) Sodium chloride b) Silver bromide c) Potassium nitrate d) Calcium sulfate e) Magnesium sulfate f) Iron(II) hydroxide g) Sodium sulfide h) Ammonium carbonate i) Copper(II) oxide j) Aluminium sulfate k) Barium carbonate l) Silver nitrate

5 G J Z A H R A B. E D ( H O N S ) Hydrated and anhydrous copper(ii) sulfate Water of Crystallization P A G E 5 When blue crystals of copper(ii) sulfate are heated it turns grey and steam is released. This vapour can be condensed on a cold surface in order to show that it is pure water. The blue copper(ii) sulfate is said to be hydrated, because it contains water molecules attached to it. The water molecules held within such a structure are called water of crystallization. When hydrated copper(ii) sulfate is heated, it loses its water of crystallization to form anhydrous copper(ii) sulfate. Hydrated and anhydrous cobalt(ii) chloride CuSO 4 5H 2 O (s) > CuSO 4(s) + 5 H 2 O (g) Other examples of hydrated compounds are cobalt(ii) choride, which is pink when hydrated (CoCl 2 4H 2 O) and blue when anhydrous and iron(ii) sulfate pentahydrate, which turns from yellow to white. FeSO 4 5H 2 O (s) > + Substances like anhydrous copper(ii) sulfate and anhydrous cobalt(ii) chloride readily absorb water vapour from the atmosphere to form the hydrated crystals. Such substances are called hygroscopic. + > CoCl 2 4H 2 O (s) Compounds like calcium chloride, sodium hydroxide and potassium hydroxide absorb so much water from the atmosphere that they form a solution. These are called deliquescent. Other substances like hydrated calcium sulfate lose their water of crystallization to form the anhydrous salt. Such compounds are called efflorescent. CaSO 4 2H 2 O (s) > +

6 P A G E 6 G J Z A H R A B. E D ( H O N S ) Potassium Sodium Reactions of Water with Metals Potassium is a soft, silvery metal. It quickly tarnishes on exposure to air and is kept under oil. When placed in water, a piece of potassium catches fire and sometimes explodes. It forms hydrogen gas and potassium hydroxide in solution. Calcium Magnesium Sodium looks like potassium, but is a bit harder. It can be cut with a knife and is kept under oil. When put in water it sometimes catches fire and gives hydrogen and aqueous sodium hydroxide. Aluminiuim Zinc Calcium is not soft like potassium and sodium, however it also tarnishes on exposure to air. It is not kept under oil. Calcium sinks in water and produces hydrogen and calcium hydroxide. Unlike the hydroxides of sodium and potassium, calcium hydroxide is only slightly insoluble. Iron Tin Magnesium looks like calcium. It reacts very slowly with cold water to produce magnesium hydroxide. However it burns in steam (hot water) to give hydrogen gas and ashes of magnesium oxide. Lead Copper Silver Aluminium seems to be quite unreative as used in everyday life. This is because aluminium reacts with oxygen to form aluminium oxide. This oxide layer covers aluminium rendering it very unreactive. If this oxide layer is removed (by scratching) aluminium would react with steam to give aluminum oxide and hydrogen. Gold Zinc and iron also react with steam at very high temperatures in similar ways as aluminium. Tin, lead, copper, silver and gold do not react with water.

7 G J Z A H R A B. E D ( H O N S ) Water Pollution P A G E 7 Fertilizers contain anions such as nitrates and phosphates which are nutrients for aquatic plants and algae. Excessive growth of these results in blockage of sunlight in the deeper parts of the water. As these plants and algae die and decompose, they use up dissolved oxygen. An excessive number of these organisms results in an excessive usage of dissolved oxygen which badly affects the breathing ability of fish and other aquatic organisms which normally breathe dissolved oxygen. Soaps, detergents and sewage also result in excessive growth of algae. Apart from this, sewage might also contain bacteria and viruses which might be harmful to some aquatic organisms Pesticides can be harmful to some organisms. They build up through a food chain. For example, if a particular fish get contaminated with a chemical in a pesticide, the organisms who eat the fish will also consume this chemical each time this type of fish is eaten. Oil spills do not only occur when an oil tanker has an accident. Underwater drilling for oil results in a small loss of crude oil in the sea. Oil is a cause for pollution for the shore lines, shore and aquatic animals and as it prevents light and oxygen from entering the water beneath it.

8 P A G E 8 G J Z A H R A B. E D ( H O N S ) Heavy metals are metals with relatively high density when compared to other metals. These are poisonous, sometimes even in very low concentrations. Examples include lead, chromium, copper and mercury. These are sometimes discharged illegally by industries and accumulate up a food chain. Thermal pollution is due to the discharge of extremely hot or cold water which results in a temperature change in a particular area. Organisms are very susceptible to small changes in temperature and heat pollution might kill a number of aquatic organisms. Exercise D, Reactions of Water From the following list of substances choose, giving relevant balanced chemical equations where possible, one which is: Copper (II) Sulfate Sodium hydroxide Copper Calcium a) Burns to produce pure water in the lab (2marks) b) Deliquescent (1mark) c) Hygroscopic (1mark) d) A metal that reacts with cold water (2marks) Hydrogen Aluminium e) A metal that reacts with steam but not at all with cold water (2marks) f) A metal that does not react with water or steam (2marks) g) Formed by the reaction of sodium with water (2marks) h) Highly reactive with steam to give an oxide (2marks) i) A substance that can be used to test for the presence of water (1mark) j) Less soluble at higher temperatures (1mark) Magnesium Silver

9 G J Z A H R A B. E D ( H O N S ) Water Hardness P A G E 9 If tap water in a watch glass is evaporated, a white deposit of solids is left at the bottom of the watch glass. This is due to solid impurities (in Malta this is mainly calcium carbonate) which have dissolved in water. A kidney stone recovered from a patient after medical intervention Solid impurities of calcium and magnesium compounds dissolved in water make the water hard. Hard water is water that does not easily form lather with soap. While soft water forms lather when soap is added, hard water forms scum when soap reacts with the solid impurities. Hard water forms lather once all the solid impurities have reacted with the soap. Therefore hard water needs a larger amount of soap to form lather, and hence our definition that hard water is water that does not readily form lather with soap. Hard water leaves fur on boiling. This reduces the efficiency of kettles and boilers Hard Water Advantages Calcium compounds are good for bones and teeth. A thin coating of calcium compounds in pipes prevents corrosion. Disadvantages Coating of calcium compounds makes boilers, pipes and radiators less efficient. More energy is needed to boil hard water. More soap is needed to produce lather. Leaves scum and is less efficient when used for cleaning Too much calcium causes kidney of bladder stones Sodium staerate (NaSt) is the ingredient of soap responsible for forming lather. If water contains calcium or magnesium ions, sodium staerate will react with these forming calcium or magnesium stearate. Calcium and magnesium staerate are insoluble and visible as scum. 2 NaSt (aq) + CaSO 4(aq) > CaSt 2(s) + Na 2 SO 4(aq) 2 NaSt (aq) + Mg(HCO 3 ) 2(aq) > MgSt 2(s) + 2 NaHCO 3 (aq) The table below shows the advantages and disadvantages of hard water Soft Water Advantages Lathers easily and gets clothes cleaner. Laundry requires less soap and lower temperatures (less energy). Disadvantages Can contain more sodium ions, depending on how the water was softened, which is linked to heart disease.

10 P A G E 10 G J Z A H R A B. E D ( H O N S ) Stalactites and stalagmites: As water passes through rocks it becomes hard. When this hard water enters a cave, some of the calcium hydrogencarbonate converts back to calcium carbonate leaving a deposit. Deposits on the ceilings of caves are called stalactites while those on the ground are called stalagmites. Permanent and Temporary Hardness Hardness is mainly due to carbonates and sulfates Carbonates are usually insoluble, but they can dissolve in water that has been acidified due to carbon dioxide from the atmosphere. This solution of carbon dioxide reacts with calcium or magnesium carbonate to form the hydrogencarbonate which is soluble in water: CaCO 3(s) + H 2 O (l) + CO 2(g) > Ca(HCO 3 ) (aq) Calcium and magnesium hydrogencarbonate decompose on heating to give back the carbonate, carbon dioxide and water: Ca(HCO 3 ) (aq) > CaCO 3(s) + H 2 O (l) + CO 2(g) As hardness due to hydrogencarbonates can be removed simply be heating, it is called temporary hardness. Calcium and magnesium sulfates are present in types of rocks and soil as gypsum. CaSO 4 and MgSO 4 are only slightly soluble in water, and thus dissolve slightly when a stream of water passes over gypsum deposits. Unlike the hydrogencarbonate, calcium and magnesium sulfate do not precipitate on heating and are called permanent hardness. Removing Hardness Boiling water removes temporary hardness only: Ca(HCO 3 ) (aq) > CaCO 3(s) + H 2 O (l) + CO 2(g) Adding bath salts (sodium carbonate) removes any form of hardness, since the carbonate produced by this reaction is insoluble: Na 2 CO 3(aq) + MgSO 4(aq) > MgCO 3(s) + Na 2 SO 4(aq) Na 2 CO 3(aq) + Ca(HCO 3 ) 2(aq) > CaCO 3(s) + 2 NaHCO 3 (aq) Ion exchange resins are used to soften large amounts of water from both types of hardness. Ion exchange resins consist of a complex structure which has sodium ions. When hard water is passed through, sodium ions replace the calcium or magnesium which solidify in the resin instead of the sodium ions. The supply of sodium in the exchange resin can be renewed by passing a sodium chloride solution through the ion exchange resin.

11 G J Z A H R A B. E D ( H O N S ) P A G E 11 Reverse Osmosis Reverse osmosis can also be used to soften water. A semipermeable membrane is a material that has tiny holes that allow water molecules to pass through but not solute particles. When a concentrated solution and a dilute solution are separated using a semipermeable membrane, water particles flow from the concentrated solution to the less concentrated one. In such way, two solutions of equal concentration are obtained. This is called osmosis. Osmosis can be reversed by applying pressure on the concentrated solution, thus forcing water molecules through the semipermeable membrane. Reverse osmosis is the passage of water molecules from a concentrated solution to a less concentrated one caused by applying pressure. This technology is used to obtain drinking water from sea water in dry countries such as Malta. Reverse osmosis plants also use filtration to remove residues and disinfection using chlorine or some of its compounds prior to reverse osmosis. Exercise E, Reactions of Water Explain, including chemical equations, how the following substances react with water: a) Iron b) Aluminium c) Copper d) Zinc e) Magnesium f) Calcium g) Sodium h) Potassium i) Anhydrous copper(ii) sulfate j) Caesium, Cs: by looking at sodium and potassium and their position in the periodic table

12 P A G E 12 G J Z A H R A B. E D ( H O N S ) Questions 1. Define a) Water of crystallization b) Hydrated c) Anhydrous d) Hygroscopic e) Deliquescent f) Efflorescent 2. In an experiment, known masses of some compounds are left in air for a period of time. The following changes were observed. Deduce, giving reasons, which of the substances is most likely to be: Substance Mass Before Mass After Observations A 10.00g 13.20g Substance turned from white to blue B 10.00g 16.30g Substance turned from white to colourless solution C 10.00g 10.00g n/a E 10.00g 8.02g n/a a) Copper(II) sulfate b) Calcium sulfate c) Sodium chloride d) Calcium chloride 3. What is the difference between permanent hardness and temporarily hardness in terms of a) The substances present b) The ways by which they can be removed 4. Soft water is said to lather easily with soap, while hard water does not. Explain this observation by providing a balanced chemical equation of hard water with soap. 5. Name the following compounds and predict whether each is soluble or not: a) NaCl b) AgCl c) CuSO 4 d) PbSO 4 e) K 2 O f) (NH 4 ) 3 PO 4 g) CaCO 3 h) NaOH