Cambridge IGCSETM ChemistryAnswers to the Student book*Supplement questions are indicated with an asteriskCambridge Assessment International Education bears no responsibility for the example answers to questionstaken from its past question papers which are contained in this publication.Exam-style questions and sampleanswers have been written by the authors.In examinations,the way marks are awarded may be different.References to assessment andlor assessment preparation are the publisher's interpretation of the syllabus1 States of matterTest Yourself questionsPage 41 The particles in copper are vibrating about a fixed point.As the temperature increases,they gain kinetic energy and vibrate more,colliding with neighbouring particles.Thishas the effect of increasing the overall volume of the metal.In other words,it expandsPage 7*2 The particles in gases have much weaker forces of attraction between their particles asthe particles are already further away from each other compared with those in a solid.These weaker forces of attraction are more easily broken as temperatures rise,allowingthe particles to move away from one another more easily.*3 There is friction between the ice and the air as the car moves through it.This frictionproduces heat which causes the ice to melt.*4 Adding salt to ice lowers the freezing point of the ice.As long as the temperature is notlower than the freezing point of the mixture of salt and ice,the ice will melt.5110100gas gas (liquidI water andI watervapour)I all0I(liquidsolid and liquidsolidwater)I(ice)time/minutesThe graph should show that when the changes of state occur the temperature does notchange until the change of state is complete.So there should be a flat line at 100C asthe steam condenses to form water and at 0C as the water freezes to form ice.Cambridge IGCSE Chemistry Fourth editionBryan Earl and Doug Wilford 2021Cambridge IGCSE Chemistry Student's book answersPage 86 Particles from the coffee diffuse from the jar around the room as a result of collisionswith particles in the air.The smell of coffee would be stronger nearer to the jar becauseof the higher concentration of particles.It would take longer for people further awayfrom the coffee to be able to smell it.●0food colouringparticles0water moleculesInitially there would be a darker green colour at the bottom of the beaker.As the watermolecules collide and mix with the nickel sulfate particles the green colour wouldspread throughout the beaker.8 In gases,the particles have weaker forces of attraction between their particles and aremoving much more quickly than the particles in a liquid.This enables the gas to diffusemore rapidly*9 Different gases diffuse at different rates because,at a given temperature,gas particleswith a lower relative molecular mass are able to move faster than the particles of a gaswith a higher relative atomic mass.At a given temperature all gases will have,onaverage,the same kinetic energy.The gas particles with the lower mass will movemore quickly.Practical Skills questions(Changes of state,page 5)1 So that the stearic acid wasn't being kept at a higher temperature by the water.2 So that the temperature in the tube was the same throughout/to evenly mix the hot andcold parts of the stearic acid.3 The more readings there are the easier it is to spot anomalous results4 &5 Temperature should be drawn on the y-axis ideally using a scale from 55-80C.Cooling curve for stearic acid80655504681012time/minutesCambridge IGCSE Chemistry Fourth edition2Bryan Earl and Doug Wilford 2021Cambridge IGCSE Chemistry Student's book answers6a69℃b It became flat.c As the change of state occurs,stronger bonds begin to form between the stearic acidmolecules,which releases heat,and prevents the temperature from dropping untilthe change of state has occurred.Exam-style questions(page 9)1 a i0Db In solid oxygen,the particles vibrate about a fixed point but cannot move past oneanother.[1]In liquid oxygen,the particles are able to move past one another.[1]Inoxygen gas,the particles are far away from one another,moving randomly andcolliding with other particles.[1]*c As the particles in gaseous oxygen are cooled they slow down [1]and the attractiveforces between them grow stronger,pulling them closer together,changing theminto a liquid state.[1]As the temperature drops,these forces become so strong thatthe particles are unable to move past one another and vibrate about a fixed pointand are changed into a solid state.[1]Cambridge IGCSE Chemistry Fourth edition3Bryan Earl and Doug Wilford 2021