Exhaustive IGCSE Chemistry Paper 6 Revision Guide

 # Chemical and Physical Testing for Water and Purity

Chemical tests for water include the use of white anhydrous copper(II) sulfate, which changes from white to blue in the presence of water, and blue anhydrous cobalt(II) chloride, which changes from blue to pink. It is important to note that the effect of heat on hydrated copper(II) sulfate is the formation of anhydrous copper(II) sulfate, causing the blue powder to turn white. For testing the purity of a substance, physical tests are employed. For solids, a pure substance displays a sharp, fixed melting point ($m.p.$), whereas an impure substance will have a lower melting point over a broader temperature range. For liquids, a pure substance displays a sharp, fixed boiling point ($b.p.$), while an impure substance will have a higher boiling point over a range.

Identification of Organic Compounds, Acids, and Alkalis

To test for alkenes, which are unsaturated hydrocarbons containing a $C=C$ bond, bromine water is used; it changes from brown to colorless. Alcohols can be identified using a burning splint, as they burn with a clean yellow or blue flame (indicating flamability), and they turn acidified potassium manganate(VII) from purple to colorless. Hydrocarbons, by contrast, burn with a smoky or sooty flame. Acids turn blue litmus paper red and have a $pH < 7$, appearing red, orange, or yellow with universal indicator. Alkalis turn red litmus paper blue, have a $pH > 7$, and appear blue, indigo, or violet with universal indicator. Most salt solutions and pure water are neutral ($pH = 7$), turning universal indicator green.

Characteristics and Classification of Oxides

Acidic gases, which are typically non-metal oxides, are tested using damp universal indicator paper: $CO_2$ (yellow/weak), $SO_2$ (red), $NO_2$ (red), and $HCl$ (red). The alkaline gas ammonia ($NH_3$) turns universal indicator blue. Most metal oxides are basic. Group I metal oxides are water-soluble alkalis that completely dissolve, while Group II metal oxides are white solids that partially dissolve. Transition metal oxides are often colored and insoluble. Some oxides are amphoteric, such as $Al_2O_3$ and $ZnO$, reacting with both $HCl$ and $NaOH$. Magnesium oxide ($MgO$) is strictly basic, reacting with $HCl$ but not $NaOH$, while Silicon dioxide ($SiO_2(s)$) is acidic, reacting with $NaOH$ but not $HCl$.

Indicators and Laboratory Safety Precautions

Laboratory safety requires wearing eye goggles, using a fume cupboard for toxic gases, and employing gloves and masks. When collecting gas over water, the system must be disconnected before heating stops to avoid back-suction and tube cracking. Flammable substances should be heated in a water bath. Indicator colors vary by medium: Litmus is red in acid, violet in neutral, and blue in alkali. Phenolphthalein ($Ph.Ph.$) is colorless in acid and neutral but pink in alkali. Methyl orange is red in acid, orange in neutral, and yellow in alkali. Thymol phthalene is colorless in acid and neutral but blue in alkali.

Gas Collection Methods and Substance Colors

Upward delivery is used for gases less dense than air, such as $H_2$ and $NH_3$. Collection over water is suitable for gases with a density similar to air that are insoluble in water, like $O_2$. Downward delivery is used for gases more dense than air, including $Cl_2$, $NO_2$, $CO_2$, $HCl$, and $SO_2$. Non-metals have distinct colors: Carbon is black, Sulfur is yellow, Phosphorus is pale yellow, Iodine is a dark grey solid (purple vapor), Bromine is a red liquid, and Chlorine is a yellowish-green gas. Metals are generally silvery-grey, except for Copper (reddish-brown) and Gold (yellow). Compounds likewise show specific colors: Iron(III) and Iodine solution are brown; Iron(II), chlorine water, and copper(II) carbonate are green; copper salts (sulfate, chloride, hydroxide) are blue; potassium permanganate is purple; and manganese dioxide and copper oxide are black.

Redox Reactions and Drying Agents

Oxidizing agents gain electrons and lose oxygen; examples include acidified potassium dichromate(VI), which changes from orange to green, and acidified potassium permanganate(VII), which changes purple to colorless. Reducing agents lose electrons and gain oxygen, examples being Carbon, $CO$, $H_2$, $SO_2$, Ethanol, and metals. Oxidation involves the gain of $O_2$, loss of $H_2$, or loss of electrons ($OIL$), resulting in an increased oxidation state. Reduction involves the loss of $O_2$, gain of $H_2$, or gain of electrons ($RIG$), resulting in a decreased oxidation state. Ethanol reacts with $O_2$ to form ethanoic acid, or undergoes combustion with excess oxygen and heat to produce $CO_2$ and $H_2O$. Drying agents include solids like anhydrous calcium chloride ($CaCl_2$), calcium oxide ($CaO$), silica gel, and soda lime, as well as liquids like concentrated sulfuric acid ($H_2SO_4$). A drying agent must not react with the gas it is drying.

Displacement Reactions and Observational Chemistry

In exothermic displacement reactions, a more reactive element replaces a less reactive one in its salt solution. For example, adding $Zn(s)$ to blue $CuSO_4(aq)$ produces a temperature rise, a colorless solution, and reddish-brown deposits of $Cu(s)$. Chlorine displacement of Bromine from $KBr$ results in a color change from colorless to reddish-brown. Group I metals in water float and skid, cause fizzing, and create an alkaline solution ($pH = 13$). Group II metals in water sink, fizz, and create a cloudy alkaline solution ($pH = 11$). Sublimation of Iodine involves dark grey crystals turning into purple vapors when heated.

Electrolysis and Industrial Applications

Electrolysis is an endothermic process converting electrical energy to chemical energy. In molten lead bromide with inert electrodes, reddish-brown bubbles of $Br_2$ form at the anode, while silvery $Pb$ deposits at the cathode. In aqueous sodium chloride, $Cl_2$ gas (yellowish-green) forms at the anode, $H_2$ gas (colorless) at the cathode, and alkaline $NaOH$ remains. Electrolysis of copper sulfate with inert electrodes yields $O_2$ at the anode and $Cu$ at the cathode, while the solution fades from blue to colorless. Acidified water yields a $2:1$ ratio of $H_2$ to $O_2$ volume. Electroplating involves coating a metal (cathode) with another metal using an active anode and a corresponding metal salt electrolyte. Refining uses an impure metal anode and a pure metal cathode.

Solubility and Salt Preparation Techniques

All nitrates are soluble. Sulfates are soluble except for Calcium, Lead, and Barium ($Ca$, $Pb$, $Ba$). Chlorides are soluble except for Silver, Lead, and Mercury ($Ag$, $Pb$, $Hg$). Carbonates are insoluble except for Ammonium and Group I metals. Soluble salts are prepared via the excess method (acid plus excess metal/oxide/carbonate) or titration (acid plus soluble base). Insoluble salts are prepared via precipitation by mixing two soluble salts, followed by filtration, washing with distilled water, and drying.

Laboratory Gas Preparation and Property Details

Hydrogen ($H_2$) is prepared from $Zn$ and $H_2SO_4$; it is colorless, odorless, flammable, and less dense than air. Carbon dioxide ($CO_2$) is prepared from $CaCO_3$ and $HCl$; it is colorless, acidic, more dense than air, and soluble. Oxygen ($O_2$) is prepared by the decomposition of hydrogen peroxide using a manganese(IV) oxide ($MnO_2$) catalyst; it is more dense than air and sparingly soluble. Hydrogen chloride ($HCl$) gas is prepared from $NaCl$ and $H_2SO_4$; it is dense, pungent, and forms white fumes with ammonia. Ammonia ($NH_3$) is prepared from a base and an ammonium salt; it is alkaline, lighter than air, and also forms white fumes with $HCl$.

Reaction Kinetics and Thermal Measurement

Rate of reaction is measured as $Volume/min$ or $Mass/min$. In exothermic reactions, the initial temperature ($T_i$) is less than the final temperature ($T_f$), as heat is released. In endothermic reactions, T_i > T_f as heat is absorbed. Polystyrene cups are used for insulation. Errors include recording mistakes, timing errors, heat loss, and contamination. Accuracy is improved by using burettes or pipettes ($50.0 ext{ cm}^3$ accuracy) instead of measuring cylinders, crushing solids for surface area, and stirring frequently for heat distribution.

Qualitative Analysis Tables and Flame Tests

Anion tests: Carbonate ($CO_3^{2-}$) reacts with acid to give $CO_2$; Chloride ($Cl^-$), Bromide ($Br^-$), and Iodide ($I^-$) react with silver nitrate to give white, cream, and yellow precipitates respectively. Sulfate ($SO_4^{2-}$) gives a white precipitate with barium nitrate. Cation tests with aqueous $NaOH$ or ammonia: Aluminium, Zinc, and Calcium give white precipitates (Al and Zn are soluble in excess NaOH). Copper(II) gives a light blue precipitate (dark blue in excess ammonia). Iron(II) gives a green precipitate that turns brown, and Iron(III) gives a red-brown precipitate. Gas tests: $H_2$ pops with a lighted splint; $O_2$ relights a glowing splint; $SO_2$ turns potassium manganate(VII) colorless. Flame tests: Lithium is red, Sodium is yellow, Potassium is lilac, Calcium is orange-red, Barium is light green, and Copper(II) is blue-green.

Experimental Protocols and Planning

Experimental designs often require specific comparative setups. To determine the order of reactivity for Mg, Zn, and Cu, one measures the volume of gas produced over time when each metal reacts with acid. To obtain lead metal from Cerussite ($PbCO_3$), the sample is crushed and heated to form lead oxide, then reduced with carbon. Evaluation of saucepan materials (Cu, Al, steel) involves measuring mass loss after exposure to acid. Calculating the frequency of double bonds in oils is done using titration with bromine water. For investigations of soil pH, barium sulfate is used to clump particles together before filtration and measurement of the filtrate's pH. Chromatography is used to identify pigments in lemon drinks or detect forged banknotes by comparing the height and number of spots against a control sample. Energy production in fuels depends on the temperature rise of a known mass of water in a copper can during the combustion of a specific mass of fuel.

Qualitative Analysis: Questions & Discussion

Question 4: Finding the volume of CO2 in lemonade. Answer: (a) Connect a heated conical flask of lemonade to a gas syringe using delivery tubes. (b) To ensure all gas is removed, verify that the fizzing has stopped and the volume in the gas syringe has become constant.

Question 6: Comparing energy in Methanol and Ethanol. Answer: Place $25 ext{ cm}^3$ water in a copper can. Record the initial temperature. Burn the alcohol in a spirit burner under the can until complete combustion occurs. Record the final temperature. The fuel causing a higher temperature rise per gram produces more energy.

Question 7: Comparing alkali concentration in oven cleaners. Answer: Use a pipette to take $25 ext{ cm}^3$ of cleaner into a flask with phenolphthalein. Titrate with acid from a burette until the color changes from pink to colorless. The cleaner requiring a greater volume of acid is more concentrated.

Question 12: Solubility of FASTGROW fertilizer. Answer: Add a known mass of fertilizer to $100 ext{ cm}^3$ water in a beaker, warm to $30^ ext{┢C}$, and stir. Filter the mixture, dry and weigh the residue. The mass of the dissolved fertilizer is the original mass minus the residue mass. Solubility is expressed as $g/100 ext{ cm}^3$.

Question 32 (b): Danger of pouring fizzy drinks into sinks with drain cleaner. Answer: Drain cleaners often contain sodium hydroxide or bleach. Carbon dioxide in fizzy drinks is acidic and may react with the cleaner components to release toxic chlorine gas.