C12 - Chemical Analysis

Question 1

What is a pure substance

Answer 1

A pure substance is one that is made up of only one substance. It can either be an element or a compound

Question 2

Examples of pure substances

Answer 2

• Distilled water
• Helium
• Pure ethanol

Question 3

What is special about a pure substances melting and boiling points?

Answer 3

They are fixed and exact

Question 4

What do impurities do to a substance?

Answer 4

They generally lower the melting point and raise the boiling

Question 5

How can you identify an impure substance?

Answer 5

They don’t have a set melting/ boiling point; they melt/boil over a range

Question 6

What is a formulation

Answer 6

A formulation is a mixture that has been designed to give a useful product

Question 7

Examples of formulations

Answer 7

• Medicinal drugs
• Paint
• Washing up liquid
• Cosmetics
• Fertilisers

Question 8

What are the two different phases in chromatography

Answer 8

• The mobile phase (solvent)
• The stationary phase (usually paper )

Question 9

What does a high affinity to the mobile phase do

Answer 9

The substance moves further up

Question 10

What does a high affinity to the stationary phase do

Answer 10

The substance doesn’t move up as far

Question 11

How to calculate the Rf (retention factor)

Answer 11

Distance moved by substance / distance moved by solvent

Question 12

Test for hydrogen

Answer 12

Lightened splint will make a squeaky pop with hydrogen

Question 13

Test for oxygen

Answer 13

• Use hydrogen peroxide to make oxygen
• Glowing splint will be relit

Question 14

Test for carbon dioxide

Answer 14

Limewater (calcium hydroxide) goes cloudy due to carbonate precipitate forming

Question 15

Test for chlorine

Answer 15

Damp blue litmus paper will turn white (get bleached)

Question 16

How to carry out a flame test

Answer 16

• Dip nichrome wire in acid and flame to clean and in acid again
• Dip in metal
• The flame may change colour

Question 17

Why is it hard to identity some colours in a flame test

Answer 17

Colours like potassium are weak colours and can be masked by stronger colours such as sodium

Question 18

What ions form white precipitates with sodium hydroxide

Answer 18

• Aluminium ions
• Magnesium ions
• Calcium ions

Question 19

What happens to an aluminium precipitate in excess NaOH

Answer 19

It dissolves unlike a calcium precipitate or a magnesium precipitate

Question 20

What colour precipitate do copper (II) ions form in NaOH

Answer 20

Light blue

Question 21

What colour precipitate do iron (II) ions form in NaOH

Answer 21

Green

Question 22

What colour precipitate do iron (III) ions form in NaOH

Answer 22

Brown

Question 23

Test for carbonates

Answer 23

• Add dilute HCl to sample
• It will fizz (due to CO₂) if it is a carbonate
• Gas can then be bubbled through limewater

Question 24

Test for halides

Answer 24

• Pour the sodium halide into test tube
• Add nitric acid to remove impurities
• Add silver nitrate

Question 25

Colour of precipitate that chlorine ions give in the halide test

Answer 25

White

Question 26

Colour of precipitate that bromine ions give in the halide test

Answer 26

Cream

Question 27

Colour of precipitate that iodine ions give in the halide test

Answer 27

Yellow

Question 28

Test for sulphates

Answer 28

- Pour sulphate into test tube - Add HCl to remove impurities - Add barium chloride (if sulphates present, it will go white deu to white precipitate forming)

Question 29

What colour flame do magnesium ions cause

Answer 29

No colour

Question 30

Advantages of modern instrumental analysis methods

Answer 30

- Highly accurate and sensitive - Quicker - Only a small amount of sample needed

Question 31

Disadvantages of modern instrumental analysis methods

Answer 31

- Usually very expensive - Require special training to use - Specialist training needed to interpret results - Results can sometimes only be compared to a data book value

Question 32

What happens during flame emission spectroscopy

Answer 32

- Energy provided by flame excites electrons - They jump into higher energy levels - When they go back down, they release light energy

Question 33

Test for water vapour

Answer 33

* Use blue cobalt chloride paper * IF water vapour is present, it will turn pink (hydrated)

Question 34

Why is the start line drawn in pencil in chromatography

Answer 34

* If we drew it using ink, the ink would move up with the solvent * This would affect the results and make them unreadable

Question 35

How to use paper chromatography to identify an unknown substance

Answer 35

* Set up chromatography in standard way with the unknown chemical on the pencil start line * Draw a line where the solvent got to (solvent front) * Measure the distance moved by the unknown chemical * Measure the distance moved by the solvent * Use these values in the Rf value equation * Look this Rf value up in a database but it may not be on there if the substance has never been analysed * This may need to be repeated with different solvents

Question 36

Why does the Rf value not have any units

Answer 36

Because it is a ratio of the distance moved by the substance compared to the distance moved by the solvent