Elements, compounds, Mixtures

Transcription

1 Elements, compounds, Mixtures Model Answers 1 Level IGCSE(9-1) Subject Chemistry Exam Board Edexcel IGCSE Module Double Award (Paper 1C) Topic Principles of Chemistry Sub-Topic Booklet Elements, Compounds, Mixtures 1 Model Answers 1 Time Allowed: Score: Percentage: 63 minutes /52 /100 Grade Boundaries : >90% 80% 70% 60% 50% 40% 30% 20% 10% Model Answer Key Red = Answer This is what you need to write to get the mark Blue = Explanation - This is here to help you understand the answer - You DON'T need to write this to get the marks 1 Model answers are copyright Save My Exams Ltd. All rights reserved.

2 1 A student wants to find out if the green colouring in grass is a mixture of dyes. He uses a solvent to dissolve the green colouring from some grass. He then separates the solution of the green colouring from the remaining grass. (a) Which of these methods is used to separate the solution of the green colouring from the remaining grass? A boiling B C D condensation evaporation filtration Filtration is used to separate solid from solutions. The green solution will be able to pass through the filter paper (the filtrate) but the grass won t and will be left on the filter paper as the residue. 2 Model answers are copyright Save My Exams Ltd. All rights reserved.

3 (b) The student uses a dropping pipette to place a drop of the green solution onto a piece of chromatography paper and produces a chromatogram. The diagram shows his results. (i) Add three more labels to the diagram to show the solvent the chromatography paper the original position of the spot of the green solution (3) (ii) Explain how many different dyes are present in the green colouring. The chromatogram shows 4 different spots. This means there are 4 different dyes contained in the solution. Each spot represents a different dye that has travelled up the chromatography paper. (Total for Question 1 = 5 marks) 3 Model answers are copyright Save My Exams Ltd. All rights reserved.

4 2 Techniques used in the separation of mixtures include A B C D crystallisation filtration fractional distillation simple distillation For each separation, select the most suitable technique, A, B, C or D, used to obtain the first named substance from the mixture. Each letter may be used once, more than once or not at all. (a) Pure water from sea water D - simple distillation: the salt in sea water is soluble so this will need to be separated from the water. However, you want to collect the water not the salt so you cannot use crystallisation. Simple distillation will enable you to collect pure water as it will be collected at 100 o C, the boiling point of pure water. 4 Model answers are copyright Save My Exams Ltd. All rights reserved.

5 (b) Ethanol from a mixture of ethanol and water C - fractional distillation: ethanol and water are both liquids and so the only way to separate them by fractional distillation. This enables them to be separated because they have different boiling points. (c) Calcium carbonate from a mixture of calcium carbonate and water B filtration: calcium carbonate is a solid. If you filter the mixture, the water will pass through the filter paper and the calcium carbonate will be left on the filter paper. 5 Model answers are copyright Save My Exams Ltd. All rights reserved.

6 (d) CuSO 4.5H 2 O(s) from CuSO 4 (aq) A crystallisation: CuSO4.5H2O(s) are hydrated crystals of copper sulfate. CuSO4 is the formula for copper sulfate solution. In order to obtain crystals from the solution you must first heat the solution to evaporate off some of the water. Allow the heated solution to cool until crystals form. Filter off the crystals and pat dry. This process is called crystallisation. (Total for Question 2 = 4 marks) 6 Model answers are copyright Save My Exams Ltd. All rights reserved.

7 3 A student investigates the pigments found in some vegetables and fruit. She obtains some coloured vegetable and fruit extracts from carrots, tomatoes and sweet potatoes. She places a spot of each extract on chromatography paper, along with spots of the three pigments beta-carotene, chlorophyll and lycopene. Her teacher provides a solvent containing volatile, flammable organic compounds for the experiment. The diagram shows the apparatus at the start of the experiment. lid tank starting line solvent V1 V2 V3 P1 P2 P3 Key to vegetable and fruit extracts and pigments V1 = carrots P1 = beta-carotene V2 = tomatoes P2 = chlorophyll V3 = sweet potatoes P3 = lycopene (a) (i) Explain why it is important for the solvent level to be below the spots. Chromatography separates mixtures by using a solvent that the components of the mixture are soluble in. If the solvent goes beyond the spots being tested the spots will dissolve and you will not get any results. The solvent line must be below the spots to prevent them from dissolving. 7 Model answers are copyright Save My Exams Ltd. All rights reserved.

8 (ii) State two potential problems that are prevented by fitting the tank with a lid. (2) Any two from: Evaporation or loss of solvent Risk of fire Fumes may be toxic or poisonous (b) The diagram shows the chromatogram at the end of the experiment. solvent front V1 V2 V3 P1 P2 P3 Key to vegetable and fruit extracts and pigments V1 = carrots P1 = beta-carotene V2 = tomatoes P2 = chlorophyll V3 = sweet potatoes P3 = lycopene Which three of the statements A, B, C, D and E are supported by the chromatogram? Place a cross in three boxes to indicate your choice. (3) 8 Model answers are copyright Save My Exams Ltd. All rights reserved.

9 A B C D E Chlorophyll is not present in carrots, sweet potatoes or tomatoes. Beta-carotene is present in carrots but not present in tomatoes. Both beta-carotene and lycopene are present in sweet potatoes. Lycopene is present in tomatoes but not present in carrots. Both carrots and tomatoes contain a pigment other than beta-carotene, chlorophyll and lycopene. A cross in box A: chlorophyll is not present in carrots, sweet potatoes or tomatoes - carrots, sweet potatoes nor tomatoes have a spot on the final chromatogram at the same level as the spot for P2 chlorophyll. A cross in box C: both beta-carotene and lycopene are present in sweet potatoes the final chromatogram shows that sweet potatoes have spots at the same levels as P1 beta-carotene and P3 lycopene. A cross in box E: both carrots and tomatoes contain a pigment other than beta-carotene, chlorophyll and lycopene both carrots and tomatoes have spots on the chromatogram at different levels to P1, P2 or P3 and must therefore contain other pigments. 9 Model answers are copyright Save My Exams Ltd. All rights reserved.

10 (c) One of the pigments present in the vegetable extracts is not shown in the chromatogram. It appears as a very faint spot 1.3 cm above the starting line. Calculate its R f value using the expression R f = distance travelled by pigment distance travelled by solvent (2) You will need to measure the distance travelled by the solvent using a ruler. Distance travelled by the solvent front: 6cm Rf = distance travelled by the pigment distance travelled by the solvent RRRR = = R f = (d) Suggest a reason why there is a spot on the starting line in the chromatogram for sweet potatoes. There is a substance in sweet potatoes that is insoluble in the solvent. (Total for Question 3 = 9 marks) 10 Model answers are copyright Save My Exams Ltd. All rights reserved.

11 4 A student adds dilute sulfuric acid to a beaker containing calcium chloride solution. He obtains a mixture containing a precipitate of calcium sulfate in a solution of hydrochloric acid. (a) Complete the equation for this reaction by inserting state symbols. CaCl 2 (aq) + H 2 SO 4 (aq) CaSO 4 (s) + 2HCl(aq) (b) The student uses this apparatus to separate the mixture into a residue and a filtrate. folded filter paper filter funnel conical flask Draw a diagram to show how he should assemble the apparatus for the filtration. (2) You must show the filter paper in the funnel and the funnel in the neck of the conical flask. 11 Model answers are copyright Save My Exams Ltd. All rights reserved.

12 (c) The student carries out a flame test on the filtrate he obtains and observes a brick-red colour. (i) Identify the ion responsible for this colour. Calcium is the element responsible for the brick-red flame colour. Exam Tip: CHEMICAL TESTS Ensure that you know the tests for different anions and cations. You should know what chemicals need to be added and the result of a positive test. These are simple recall questions so marks are easily gained. (ii) Suggest why this ion is present in the filtrate. The calcium chloride was added in excess and therefore there would have been some that is unreacted which will have passed through the filter paper contaminating the filtrate. 12 Model answers are copyright Save My Exams Ltd. All rights reserved.

13 (d) The student tests the filtrate for chloride ions by adding silver nitrate solution. (i) State what he would observe in this test. When chloride ions react with silver nitrate, silver chloride is produced which is a white precipitate. (ii) State the name of the substance responsible for this observation. silver chloride (iii) He reads in a textbook that dilute nitric acid should be added before the silver nitrate solution in the test. Suggest why the student does not need to add dilute nitric acid in the test. HCl is another product which is acidic. The solution is already acidic so there is no need to add the nitric acid. (e) The calcium sulfate residue he obtains is impure because it contains some hydrochloric acid. Describe how he can obtain a pure dry sample of calcium sulfate from this residue. (2) The solid needs to be washed with water and then left to dry. (Total for Question 4 = 10 marks) 13 Model answers are copyright Save My Exams Ltd. All rights reserved.

14 5 Illegal drugs are sometimes used to affect the performance of racehorses. These drugs can be detected in horse urine using chromatography. a concentrated sample of urine from each horse is spotted onto the start line of a sheet of chromatography paper known illegal drugs are also spotted onto the same paper ethanol is used as the solvent The chromatogram shows urine samples, A, B, C and D, and the two illegal drugs lasix and bute. solvent front lasix bute A B C D start line (a) Explain which urine sample contains an illegal drug. (2) The substance containing an illegal drug is C. This is because it has a spot on the chromatogram at the same level as bute, one of the illegal drugs. No other substances have spots at the same level as either of the two illegal drugs. 14 Model answers are copyright Save My Exams Ltd. All rights reserved.

15 (b) What is the meaning of the term solvent? A substance that dissolves another substance. (c) The results for known drugs are given as R f values. R f value = distance travelled by the drug distance travelled by the solvent Calculate the R f value for lasix. (2) distance travelled by the substance Rf value = distance travelled by the solvent = 62 = R f value for lasix =... (d) Suggest how the solubility of the drug in the solvent affects the distance travelled by the substance. The more soluble the substance the further it will travel. (Total for Question 5 = 6 marks) 15 Model answers are copyright Save My Exams Ltd. All rights reserved.

16 6 The diagram shows the apparatus used to form a compound containing iron and chlorine. iron powder open tube dry chlorine gas heat compound (a) (i) State the colour of chlorine gas. Chlorine is green. (ii) Suggest why it is necessary to have an open tube fitted to the apparatus. To allow excess gas to escape so there is not a pressure build up. (iii) For safety reasons, this reaction should be carried out in a fume cupboard. Explain why this is necessary. Chlorine gas is toxic. 16 Model answers are copyright Save My Exams Ltd. All rights reserved.

17 (b) A mass of g of iron reacts with g of chlorine. (i) Calculate the empirical formula of the compound formed. (3) The empirical formula is FeCl3 empirical formula = FeCl3... (ii) Suggest a name for this compound. Iron (III) chloride (c) When chlorine gas is bubbled into aqueous sodium hydroxide, a mixture of bleach (NaClO), sodium chloride and water is formed. Write a chemical equation for this reaction. (2) Firstly, write out all the correct formulae for the reactants and products: Cl 2 + NaOH -> NaCl + NaClO + H 2 O Then balance the equation so that there are equal numbers of each atom on both sides: Cl 2 + 2NaOH -> NaCl + NaClO +H 2 O (Total for Question 6 = 9 marks) 17 Model answers are copyright Save My Exams Ltd. All rights reserved.

18 7 This apparatus can be used to prepare a sample of hydrogen. dilute hydrochloric acid hydrogen water zinc (a) Write a chemical equation for the reaction between zinc and hydrochloric acid. Include state symbols. First, write out all the correct formulae for the reactants and products: (2) Zn(s) + HCl(aq) ZnCl 2(aq) + H 2(g) Then balance the equation so that there are equal numbers of each atom on both sides. Zn(s) + 2HCl(aq) ZnCl 2(aq) + H 2(g) (b) State two observations you would make when hydrochloric acid reacts with zinc in the conical flask. (2) The solid zinc will disappear and you would see bubbles of gas. 18 Model answers are copyright Save My Exams Ltd. All rights reserved.

19 (c) A student carries out two experiments to find the volume of dilute hydrochloric acid required to completely react with 0.5 g of zinc powder. Experiment 1 She fills a burette to the 0.00 cm 3 mark with dilute hydrochloric acid. She places 0.5 g of zinc powder into a conical flask and then slowly adds the acid to the zinc until the reaction is complete. The diagram shows the final reading on the burette Experiment 2 She then repeats the experiment with 0.5 g of zinc powder from the same source, but with a different sample of dilute hydrochloric acid. The diagram shows the initial and final burette readings for this experiment initial reading final reading 19 Model answers are copyright Save My Exams Ltd. All rights reserved.

20 (i) Use the burette readings to complete the table, recording the volumes to the nearest 0.05 cm 3. (3) Exam Tip: READING BURETTES Remember that burettes are read from the top! Volumes are also measured from the bottom of the meniscus, as shown by the red line. Burettes can measure to an accuracy of 2 decimal places, so your answers need to be to 2 decimal places too. (ii) The concentration of the acid in experiment 1 was 0.74 mol/dm 3. Explain how the concentration of the acid in experiment 2 can be calculated. (2) As the volume required has doubled, the concentration is half. (Total for Question 7 = 9 marks) 20 Model answers are copyright Save My Exams Ltd. All rights reserved.