IGCSE (9-1) Edexcel - Chemistry Principles of Chemistry Element, Compounds and Mixtures NOTES
1.8: Understand how to classify a substance as an element, compound or mixture Classifications: S Class Element Definition A substance made up of atoms that all contain the same number of protons (one type of atom) and cannot be split into anything simpler. There are limited types of element. Example: Hydrogen, Oxygen, Carbon Compound A pure substance made up of two or more elements chemically combined together. There are unlimited types of compound. Cannot be separated by physical methods of separation. Example: Copper (ii) Sulphate, Calcium Carbonate Mixture A combination of two or more substances (elements and / or compounds) that are not chemically joined together. Can be separated by physical methods of separation (see 1.10). Example: sand and water, oil and water Example: S
1.9: Understand that a pure substance has a fixed melting and boiling point, but that a mixture may melt or boil over a range of temperatures Pure substance: Contains only one substance, with no other substances mixed together. Has fixed melting and boiling point as they consist of only one substance so all molecules will have the same melting and boiling points E.g. Water has a boiling point of 100 C and a melting point of 0 C. Mixture: A combination of two or more substances (elements and / or compounds) that are not chemically joined together. Has a range of melting and boiling point as they consist of different substances that melt or boil at different temperatures This can be used to test if a substance is pure. For example, for water if it melts at exactly 0 C and boils at exactly 100 C the water if pure. If melting and boiling points have changed the water is impure and is a mixture.
1.10: Describe these experimental techniques for the separation of mixtures: simple distillation, fractional distillation, filtration, crystallization, paper chromatography Simple distillation Diagram showing the simple distillation of a mixture of salt and water Use: To separate a liquid and soluble solid from a solution (e.g. water from a solution of salt water) Explanation: Solution is heated and water vapours will rise and evaporate Water vapours will pass through the condenser, where it cools and condenses, turning into a liquid that will be collected in a beaker After all the water is evaporated from the solution, the solute will be left behind
Fractional distillation Diagram showing the fractional distillation of a mixture of ethanol and water Use: To separate two or more liquids that are miscible with one another (e.g. ethanol and water from a mixture of the two) Explanation: Solution is heated at temperature of substance with the lowest boiling point This substance will rise and evaporate, and vapours will pass through a condenser, where it cools and condenses, turning into a liquid that will be collected in a beaker All of this substance is evaporated and collected, leaving behind a mixture or a substance For water and ethanol: Ethanol has a boiling point of 78 C and water of 100 C. The mixture is heated until it reaches 78 C, the ethanol boils and distils out of the mixture and condenses into the beaker. When temperature starts to increase to 100 C heating should be stopped. Water and ethanol are separated.
Filtration Diagram showing the filtration of a mixture of sand and water Use: 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) Explanation: Filter paper is placed in a filter funnel above another beaker Mixture of insoluble solid and liquid is poured into the filter funnel Filter paper will only allow small liquid particles to pass through as the filtrate Solid particles are too large to pass through the filter paper so will stay behind as a residue
Crystallization Diagram showing the process of crystallization Use: 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) Explanation: Solution is heated, allowing the solvent to evaporate to leave a saturated solution. Test the solution is saturated by dipping a clean, dry, cold glass rod into the solution. If the solution is saturated, crystals will form in the glass rod. Saturated solution is allowed to cool and solids will come out of the solution, as the solubility decreases, and crystals will grow Crystals are collected by filtering the solution. Then the crystals are washed with cold, distilled water to remove impurity. Dry the crystals.
Paper chromatography Diagram showing the paper chromatography of ink and plant dye Use: To separate substances that have different solubilities in a given solvent (e.g. different coloured inks that have been mixed to make black ink) Explanation: Pencil line is drawn on chromatography paper and spots of ink / dye is placed on it. Paper is lowered into a bucket of solvent, allowing the solvent to travel up the paper, taking some of the coloured substances with it. Different substances will have different solubilities so will travel at different rates, causing the substances to spread apart. Those with higher solubility with spread more than the others. This will show the different components of the ink / dye.
1.11: Understand how a chromatogram provides information about the composition of a mixture Paper Chromatography - Composition of a mixture Diagram showing the composition of an ink using paper chromatography Composition of a mixture: Chromatography paper is taken. A sample (unknown) is placed along the pencil line. Other known elements are placed side by the sample in the same line. Chromatograms will show the composition of a mixture as the different coloured substances (components) will spread apart as they will have different solubilities so will travel at different rates A pure substance will only produce one spot on the chromatogram during paper chromatography In the diagram above, red, blue and yellow are three pure substances, whilst the sample on the left is a mixture of all three
1.12: Understand how to use the calculation of R f values to identify the components of a mixture Retardation factor (R f )values: Used to identify the components of mixtures S The R f value of a particular compound is always the same Calculating the R f value allows chemists to identify unknown substances because it can be compared with R f values of known substances under the same conditions. Equation: S R f value = distance moved by compound distance moved by solvent
1.13: Practical: Investigate paper chromatography using inks / food colourings Paper Chromatography Diagram showing the paper chromatography of ink and plant dye Use: To separate substances that have different solubilities in a given solvent (e.g different coloured inks that have been mixed to make black ink). Method: Pencil line is drawn on chromatography paper and spots of ink / dye is placed on it. Paper is lowered into a bucket of solvent, allowing the solvent to travel up the paper, taking some of the coloured substances with it. Allow chromatography to occur until the solvent reaches the top of the paper. Results: As the solvent travels up the paper, different substances will have different solubilities so will travel at different rates, causing the substances to spread apart. This will show the different components of the ink / dye.