Chemistry: Chemical analysis (Paper 2)

What is a pure substance?

Single compound or element

Not mixed with any other substance

Melts at a specific fixed temperature

Has a specific fixed boiling point

(impure melt & boil at a range of temps)

How can we measure purity?

If the melting and boiling point are a horizontal line, the substance is melting/boiling at a fixed point therefore pure.

The image is impure

What is a formulation?

A complex mixture that had been designed as a useful product

The quantity of components is measured

E.g. Fuels, cleaning products, paints, medicines

What is the process of chromatography?

1. Draw pencil line at the bottom

2. Dot the ink on the line

3. Dip the paper in a solvent

4. Solvent travels up the paper and carries the ink

What is the stationary and mobile phase in chromatography?

• Paper is stationary phase (Doesn’t move)

• Solvent is mobile phase (does move)

What result will occur in chromatography?

Pure compound - single spot

Compounds in a mixture separate into different spots

More soluble substances travel further up paper

How does chromatography help identify unknown substances?

Calculate the Rf value

What are some disadvantages of chromatography?

Could have the same Rf value

Use a different solvent

Rf value might not be in the database

How to test for gases?

Chlorine - Bleach litmus paper

Oxygen - Relights splint

Hydrogen - ‘Squeaky pop’

Carbon Dioxide - Turns limewater (calcium hydroxide) cloudy

Results of flame tests

Process of flame test

1. Sterilise wire in water & in bunsen burner flame

2. Place sample onto wire

3. Put wire into blue bunsen burner flame

4. Observe colour result

What makes flame tests unreliable? What can be done for an accurate result?

• Difficult to distinguish colour

• Sample contains mix of metal ions - mask flame colour

• Flame emission spectroscopy is more accurate

What is flame emission spectroscopy?

1. Metal ion in solution placed into flame

2. Light is passed through machine (Spectroscope)

3. Converts into line spectrum

Advantages of flame emission spectroscopy

(Concentration is visible)

• Rapid

• Accurate

• Sensitive

Result of Cations in NaOH solution

Balanced equations of Cations in NaOH

Forms sodium nitrate and hydroxide

1 2 → 2 1

Aluminium 1 3 → 3 1

How to test for Carbonates

1. Add dilute acid

2. Forms CO2 (fizzing)

3. Bubble through limewater (cloudy)

How to test for Halides?

How to test for SO₄^2- or non-metal ions

1. Add hydrochloric acid & barium sulphate

2. White precipitate