Scientific practical techniques Separating and estimating Purity Unit 4: 2 Scientific practical techniques Separating and estimating Purity

Grading criteria P2 Demonstrate use of scientific techniques to separate substances. P3 Estimate the purity of samples using scientific technique M2 Describe the factors that influence purity D2 Evaluate the accuracy of the methods used to estimate the purity of the samples.

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Distillation Saltwater Pure water How do we know that we have separated the salt and water? What test could we use?

Salt Water - Sodium Chloride Solution Anions and Cations In solid salt ions are in fixed positions + How could we test to show the presence or absence, of these ions in our water sample? H2O molecules

Test for Chloride anion - add to silver nitrate AgNO3 (aq) + NaCl (aq) → AgCl (s) + AgNO3 (aq) Why does it go cloudy?

Precipitation reactions An insoluble solid that forms during an aqueous reaction is called a precipitate. A reaction which forms a precipitate is called a precipitation reaction. The limewater test for carbon dioxide is a precipitation reaction. Limewater is actually a dilute solution of calcium hydroxide. The calcium hydroxide reacts with carbon dioxide to form calcium carbonate, which is insoluble in water: calcium hydroxide carbon dioxide calcium carbonate + + water Ca(OH)2 + CO2 CaCO3 + H2O

Uses of precipitation reactions Most precipitation reactions are very fast reactions that occur between ions. This makes them very useful for identifying specific ions based on the type of precipitate formed. Precipitation reactions have a number of other uses: production of coloured pigments for paints and dyes removal of toxic chemicals from water separation of reaction products. Photo credit: © 2009 Shutterstock, Lindsey Moore A lead iodide precipitate.

Isolating the precipitate The precipitate from a precipitation reaction can be separated from the reaction mixture by filtration. A Buchner funnel and flask can be used to accelerate the process. Buchner funnel vacuum pump filter paper This apparatus uses a vacuum pump to draw the mixture through the filter. Buchner flask The filtrate is finally washed and dried.

Precipitation Task: To make an insoluble salt and separate by filtration NaCO3 (aq) + MgSO4 (aq) → MgCO3 (s) + NaSO4(aq)