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Chemistry IGCSE Chapter 2 – Experimental Techniques

 Methods of Purification

Filtration

·         Used to mainly separate suspensions

·         E.g. chalk and water

·         Filter paper is aligned around the inner surface of a filter funnel and the solution is passed through.

·         The solute (suspension) particles are trapped by the filter paper as residue

·         The solvent passes through the filter paper and collects as filtrate

Evaporation

·         Used to separate solutions usually with a solute that consists of particles which have been spread throughout the solvent and are too small to be obtained in filtration.

·         Solution heated and solvent evaporates leaving solid behind.

·         Salt obtained from solution by this method

Crystallization

·         Solids from solution can be separated by letting them form crystals (E.g. copper(II) sulphate)

·         When a solution of copper (II) sulphate is cooled, then crystals of the salt form because it is less soluble at a lower temperature.

·         In crystallization, a solution of copper (II) sulphate is heated so that some of the water evaporates leaving a more concentrated solution of the salt.

·         The solution can be checked to see if it is ready by placing one drop on a microscopic slide, and crystals should form on the cool glass

·         The solution is then left to cool and crystallize. The crystals are removed by filtering, rinsed with water and dried with filter paper.

Distillation

·         Method used to obtain pure solvent from a solution

·         E.g. obtaining pure water from salt water

·         Solution heated in flask, it boils and steam rises into condenser, leaving salt behind

·         Condenser is cold and steam condenses into water which drips into beaker completely pure (distilled water).

Separating Funnel

·         Used to separate liquids that are immiscible (form layers on one another)

·         When a mixture of oil and water is poured into the funnel, the oil floats on top. When tap opened, water runs out and closed when layer of water finished.

Fractional Distillation

·         Used to separate two miscible liquids that have different boiling points

·         E.g. a mixture of ethanol and water

·         Mixture is heated; at 78°C ethanol begins to boil. Some water evaporates too. This mixture of ethanol vapour and water vapour condenses in the cool glass beads in the column, making them hot

·         When the glass beads reach 78°C, ethanol vapour no longer condenses on them, only water vapour does. The water vapour then drips back into the flask, whilst the ethanol rises through into the condenser.

·         The cool condenser forces ethanol to condense (liquid ethanol) and drip into the beaker

·         When the thermometer rises above 78°C, it is a sign that ethanol has been separated to heating can stop.

Paper Chromatography

·         Used when chemists want to analyze a mixture (find out what substances are present in it).

·         E.g. finding out what dyes and preservatives added to food

·         Method of separating solutes in a solution

·         When a drop of solution applied to paper, the paper absorbs solute/binds it to surface. As the solvent rises, some solute stays put and others dissolve in the solvent and travels through the paper.

·         A paper very soluble travels through the paper faster than one which is slightly soluble

·         When the solvent reaches the top of the paper, the process stops and different spots are left on the paper.

·         Each spot represents another solute, this way they are separated

·         Different solutes travel different distances depending on solubility

·         Many solvents used, ethanol, ethanoic acid, propanone

·         With a solvent other than water, a closed container should be used so that the

Chromatography paper is surrounded by the vapour of the solvent.

·         Also separate a mixture of coloured substances (e.g. in black ink)

·         On a circular filter paper, substances more soluble will form larger circles with least soluble forming smaller circles

Purity based on melting/Boiling Points

·         Substances can be identified using their boiling and melting points

·         Pure substances change state at a constant temperature whilst impure substances change over a range of temperature.

·         If a solid is not pure, its melting point will be low and its boiling point will be high.

·         Impurities widen the range over which the substance is liquid.

·         The surrounding pressure can increase and decrease boiling and melting points. If the surrounding pressure falls, the boiling point falls. An increase in pressure raises the boiling point.

·         No two substances have the same boiling point and same melting point