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

2. 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
D Evaluate the accuracy of the methods used to estimate the purity of the samples.

3. Video clip

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

5. 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

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

7. 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

8. 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.

9. 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.

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