Edexcel IGCSE Chemistry Elements, Compounds & Mixtures PDF

Head to www.savemyexams.com for more awesome resources Edexcel IGCSE Chemistry Your notes Elements, Compounds & Mixtures Contents Element, Compound or Mixture Pure Substance vs Mixture Separation Techniques Int...

Head to www.savemyexams.com for more awesome resources Edexcel IGCSE Chemistry Your notes Elements, Compounds & Mixtures Contents Element, Compound or Mixture Pure Substance vs Mixture Separation Techniques Interpreting Chromatograms Practical: Investigating Paper Chromatography Page 1 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Element, Compound or Mixture Your notes Element, compounds and mixtures All substances can be classified into one of these three types Elements Compounds Mixtures What is an element? A substance made of atoms that all contain the same number of protons and cannot be split into anything simpler There are 118 elements found in the Periodic Table E.g. copper, iron, magnesium What is a compound? A pure substance made up of two or more elements chemically combined There is an unlimited number of compounds Compounds cannot be separated into their elements by physical means E.g. copper(II) sulfate (CuSO4), calcium carbonate (CaCO3), carbon dioxide (CO2) What is a mixture? A combination of two or more substances (elements and/or compounds) that are not chemically combined Mixtures can be separated by physical methods such as filtration or evaporation E.g. sand and water, oil and water, sulfur powder and iron filings Particle diagram showing elements, compounds and mixtures Page 2 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources All substances can be classified as an element, compound or mixture Your notes Examiner Tip As well as learning the definitions for an element, compound and mixture, you must be able to classify any given substance. Page 3 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Pure Substance vs Mixture Your notes Pure substance vs mixture In everyday language, we use the word pure to describe when something is natural or clean and to which nothing else has been added In chemistry, a pure substance may consist of a single element or compound which contains no other substances For example, pure water contains only H2O molecules and nothing else Drinking water would be classed as a mixture and not a pure substance because it contains H2O molecules and additional substances like dissolved ions and chlorine Pure substance v mixture Pure water consists of only H2O molecules whereas tap water is a mixture How can purity be distinguished? Pure substances melt and boil at specific and sharp temperatures E.g. pure water has a boiling point of 100 °C and a melting point of 0 °C Impure substances have a range of melting and boiling points as they consist of different substances Generally, impure substances have lower melting points and higher boiling points than the pure substance Melting and boiling point data can therefore be used to distinguish pure substances from mixtures Melting point analysis is routinely used to assess the purity of drugs This is done using a melting point apparatus which allows you to slowly heat a small amount of the sample, making it easier to observe the exact melting point This is then compared to data tables Page 4 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The closer the measured value is to the actual melting or boiling point then the purer the sample is Measuring purity is also important in foodstuffs Your notes Worked example Sulfur has a melting point of 114 oC. A student tests the melting point of a sample of sulfur. It begins to melt at 100 oC and finishes melting at 113 oC. Explain whether the substance is pure or impure. Answer: The substance is impure because its melts over a range of temperatures. Page 5 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Separation Techniques Your notes Simple distillation Simple distillation is used to separate a liquid and soluble solid from a solution (e.g., water from a solution of salt water) or a pure liquid from a mixture of liquids The solution is heated, and pure water evaporates producing a vapour which rises through the neck of the round bottomed flask The vapour passes through the condenser, where it cools and condenses, turning into the pure liquid that is collected in a beaker After all the water is evaporated from the solution, only the solid solute will be left behind Simple distillation Diagram showing the distillation of a mixture of salt and water Examiner Tip If asked to draw or label a diagram of simple distillation, make sure that the water goes in at the bottom of the condenser near the collecting beaker, and comes out at the top near the column. Page 6 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Fractional distillation Your notes Fractional distillation is used to separate two or more liquids that are miscible with one another (e.g., ethanol and water from a mixture of the two) The solution is heated to the temperature of the substance with the lowest boiling point This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture For water and ethanol Ethanol has a boiling point of 78 ºC and water of 100 ºC The mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distills out of the mixture and condenses into the beaker When the temperature starts to increase to 100 ºC heating should be stopped. Water and ethanol are now separated Fractional distillation apparatus Page 7 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Fractional distillation of a mixture of ethanol and water Page 8 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Filtration Your notes Filtration is used to separate an undissolved solid from a mixture of the solid and a liquid / solution ( e.g., sand from a mixture of sand and water) Centrifugation can also be used for this mixture A piece of filter paper is placed in a filter funnel above a beaker A mixture of insoluble solid and liquid is poured into the filter funnel The filter paper will only allow small liquid particles to pass through as filtrate Solid particles are too large to pass through the filter paper so will stay behind as a residue The filtration process Filtration of a mixture of sand and water Page 9 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Crystallisation Your notes Crystallisation is used to separate a dissolved solid from a solution, when the solid is much more soluble in hot solvent than in cold (e.g., copper sulphate from a solution of copper (II) sulphate in water) The solution is heated, allowing the solvent to evaporate, leaving a saturated solution behind Test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution If the solution is saturated, crystals will form on the glass rod The saturated solution is allowed to cool slowly Crystals begin to grow as solids will come out of solution due to decreasing solubility The crystals are collected by filtering the solution, they are washed with cold distilled water to remove impurities and are then allowed to dry The process of crystallisation Diagram showing the process of crystallisation Page 10 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Paper chromatography Your notes Paper chromatography is used to separate substances that have different solubilities in a given solvent (e.g. different coloured inks that have been mixed to make black ink) A pencil line is drawn on chromatography paper and spots of the sample are placed on it Pencil is used for this as ink would run into the chromatogram along with the samples The paper is then lowered into the solvent container The pencil line must sit above the level of the solvent so the samples don´t wash into the solvent container The solvent travels up the paper by capillary action, taking some of the coloured substances with it Different substances have different solubilities so will travel at different rates This causes the substances to separate Those substances with higher solubility will travel further than the others This will show the different components of the ink / dye Page 11 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 12 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Analysis of the composition of ink using paper chromatography Worked example Sodium chloride is a soluble solid. Name the technique used to separate a soluble solid from a solution? A filtration B simple distillation C crystallisation D chromatography Answer The correct answer is C Soluble solids are separated by gently heating the solution until the solvent evaporates and the solid crystallises Notice the question asks for the separation of the solid, not the liquid from the solution. If it asked for the liquid, B would be the correct answer Examiner Tip Paper chromatography is the name given to the overall separation technique while a chromatogram is the name given to the visual output of a chromatography run. This is the piece of chromatography paper with the visibly separated components after the run has finished. Page 13 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Interpreting Chromatograms Your notes Interpreting chromatograms We can use a chromatogram to compare the substances present in a mixture to known substances and make assumptions Pure substances will produce only one spot on the chromatogram Impure substances will produce more than one spot on the chromatogram If two or more substances are the same, they will produce identical chromatograms If the substance is a mixture, it will separate on the paper to show all the different components as separate spots It is common practice to include a known compound as a reference spot This can help match up to an unknown spot or set of spots in order to identify it Interpreting chromatograms Chromatogram diagram showing the analysis of a mixture and pure substances using chromatography We can draw several conclusions from this chromatogram: Page 14 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The brown ink is a mixture as there are three dots Red, yellow and blue are pure as there is only one dot for each The brown ink contains red, blue and yellow as the dots are in line with one another horizontally Your notes Examiner Tip Chromatograms in exams will be in black and white so to identify whether a mixture contains a known sample, the dots need to be in line with one another. Page 15 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Rf Values Your notes Rf values are used to identify the components of mixtures The Rf value of a particular compound is always the same However, it does depend on the solvent used If the solvent is changed then the Rf value changes Calculating the Rf value allows chemists to identify unknown substances because it can be compared with the Rf values of known substances under the same conditions The retention factor, Rf, is calculated by the equation: R f = distance moved by substance distance moved by solvent The Rf value: Is a ratio Has no units Will always be less than 1 Page 16 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes A student obtained the following chromatogram when carrying out chromatography. Calculate the Rf value of the substance. Answer: The Rf value of the substances in the chromatogram above can be calculated by: distance moved by substance 3 Rf = = = 0.5 distance moved by solvent 6 Examiner Tip When you calculate Rf values in exams, make sure to use your ruler carefully to measure the distance moved by the solvent and the substance as mark schemes can be strict about the values accepted for these. For both measurements, the distance should be measured from the baseline to the centre of the dot. Page 17 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Practical: Investigating Paper Chromatography Your notes Practical: Investigate Paper Chromatography Using Inks & Food Colourings Aim: Investigate how paper chromatography can be used to separate and identify a mixture of food colourings Apparatus: A 250 cm3 beaker A wooden spill A rectangle of chromatography paper Four known food colourings labelled A–D An unknown mixture of food colourings labelled U Five glass capillary tubes Paper clip Ruler & pencil Diagram of the apparatus needed for paper chromatography Page 18 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Method: 1. Use a ruler to draw a horizontal pencil line 2 cm from the end of the chromatography paper Your notes 2. Use a different capillary tube to put a tiny spot of each colouring A, B, C and D on the line 3. Use the fifth tube to put a small spot of the unknown mixture U on the line 4. Make sure each spot is no more than 2-3 mm in diameter and label each spot in pencil 5. Pour water into the beaker to a depth of no more than 1 cm and clip the top of the chromatography paper to the wooden spill. The top end is the furthest from the spots 6. Carefully rest the wooden spill on the top edge of the beaker. The bottom edge of the paper should dip into the solvent 7. Allow the solvent to travel undisturbed at least three quarters of the way up the paper 8. Remove the paper and draw another pencil line on the dry part of the paper as close to the wet edge as possible. This is called the solvent front line 9. Measure the distance in mm between the two pencil lines. This is the distance travelled by the water solvent 10. For each of food colour A, B, C and D measure the distance in mm from the start line to the middle of the spot Practical Tip: The pencil line must never be below the level of the solvent as the samples will be washed away Results: Record your results in a suitable table Distance moved by spot Distance Moved by Food colouring Rf value (mm) solvent (mm) A B C D The Rf values of food colours A, B, C and D should be compared to that for the unknown sample as well as a visual comparison being made Substances with matching Rf values are the same substance and will move the same distance up the paper Page 19 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers