Separation and analysis

October 2025

Define a pure substance

Where are the particles are the same in a substance

Define a mixture

Two or more elements or compounds not chemically bonded

Define an element

Substance made from only one type of atom

Define compound

Substance made from different types of atoms joined together

How to tell mixture from pure?

Mixture melts/boils over a range of temperatures

What material is used to make gold alloy

silver/copper/zinc

How do calculate percentage of gold

number of carats/24 × 100

What materials are used to make steel alloys

Carbon

How to make bronze alloys

copper + tin

How to make aluminium alloys

Aluminium + magnesium

How to make brass alloys

copper + zinc

What does distillation separate and how do you do it?

It separates one substance with a significantly different boiling point

1. heat mixture

2. substance with the lowest boiling point evaporates first

3. The vapour travels into a cold leibig condenser and condenses back into a liquid

4. liquid runs down and is collected into a separate container

In a condenser for distillation, why is the water input from the bottom?

To ensure jacket is full of cold water to maximise cooling

What does fractional distillation separate and how do you do it?

Miscible liquids with similar boiling points

1. Heat distillation flask

2. Mixture A vapour hits the thermometer and travels through to condenser after boiling

3. Replace flask

4. Impurities are left behind

Describe the method of crystallisation

1. Pour solution into evaporating basin and heat gently to evaporate some solvent

2. Stop heating when solution becomes concentrated and small crystals start to form

3. You can dip a cold glass rod and if it crystallises it is ready to remove

4. Leave solution to cool

5. Crystals form as the solubility of the solute decrease with temperature

6. Filter the crystals to remove remaining solution and wash with distilled water

7. Dry using filter paper or in an oven

Describe the method of evaporation

1. Pour solution in evaporating basin

2. Heat strongly until all solvent has evaporated

3. Allow basin to cool

Why does evaporation work and what is it used for?

Separation depends on volatility - solvent evaporates as it is more volatile than the solute, which remains as a solid residue.

Used to recover dry, soluble solid (like salt)

Describe the method of filtration

1. Dissolve soluble solid in water to separate from insoluble solids

2. Filter solution using funnel and filter paper

3. Wash residue with water

4. Dry residue in warm oven

5. Filtrate heated to partially evaporate

6. Allow to crystallise

Describe the method of separating immiscible liquids

1. Pour mixture into separating funnel

2. Allow mixture to stand undisturbed — two distinct later will form as this depends on density

3. Remove stopper and open tap to let bottom layer flow

4. close

5. collect top layer in separate container

What is chromatography used for?

To separate mixtures made up from different chemicals with different solubilities

How do you calculate the Rf value?

distance travelled by substance (mm)/distance travelled by solvent (mm)

Why do some substances move at different speeds in chromatography

• More soluble substances rise up paper further as they are attracted to the mobile phase more than the stationary phase

• The less soluble substances rise up less fair and move slower as they have a higher affinity to the stationary phase

How do you test for ammonia?

Turns damp red litmus blue 

How do you test for chlorine?

Bleaches damp blue litmus paper

What colour does lithium turn in flame test

red/crimson

What colour does sodium turn in flame test

yellow

What colour does potassium turn in flame test

Lilac

What colour does calcium turn in flame test

orange

What colour does copper turn in flame test

green

How do you test for carbonate atoms?

Add dilute acid which forms carbon dioxide and bubble gas through limewater which will turn cloudy

What is the cloudy precipitate in limewater test?

CaCO3 (calcium carbonate)

How do you test for sulphate ions?

Add HCl to remove ionic impurities

Add Barium chloride (BaCl2) and a white ppt will form if sulphate ions are present

How do you test for halide ions? What colour are the results

Add nitric acid to remove ionic impurities

Then add silver nitrate (AgNO3) and precipitate will form

Iodide - Yellow

Bromide - Cream

Chloride - White

How to test for copper (ii) ions?

Add sodium hydroxide, blue precipitate forms

What happens when you add calcium hydroxide to aluminium ions? Does the ppt dissolve?

White ppt

Dissolves if you add more

What happens when you add calcium hydroxide to Iron (II) ions?

Green ppt

What happens when you add calcium hydroxide to Iron (III) ions?

Orange-brown ppt

How to test for ammonium ions?

Add sodium hydroxide to form ammonia gas
warm alkali to increase rate of reaction