C12

Question 1

How do you calculate Rf values?

Answer 1

distance moved by chemical (the ink dot) / distance moved by solvent (water)

Question 2

What can Rf values be used for?

Answer 2

Each Rf (Retention factor) value is unique, so we can use it to identify a specific substance from a database

Question 3

Explain why different substances move different distances in chromatography

Answer 3

The different substances are attracted differently to the molecules in the paper and solvent, so they travel at different speeds/distances up the paper

Question 4

The more soluble a substance is in the solvent during chromatography, the less/more rapidly/slowly it will move along the paper

Answer 4

The more soluble a substance is in the solvent during chromatography, the more rapidly it will move along the paper

Question 5

Why is the lid closed during chromatography?

Answer 5

To reduce evaporation of the solvent

Question 6

Flame tests for positive ions (results table)

Answer 6

lithium. Li. crimson
sodium. NA+. yellow
potassium. K+. lilac
calcium. Ca2+. orange/red
copper. cu2+ green

Question 7

Explain why metal ions emit light when in a flame

Answer 7

The electrons become excited by the heat energy and jump to a higher energy level. When the electrons relax back down to a lower state, the excess energy gained is released in the form of emitted light.

Question 8

How to carry out a flame test?

Answer 8

• The wire should be dipped in hydrochloric acid and then heated to clean it
• Dip into the metal compound that is being tested
• Hold the loop in the roaring blue flame of a bunsen burner

Question 9

How can you test for a carbonate?

Answer 9

If you add a dilute acid to a carbonate, it fizzes and produces carbon dioxide gas.

Question 10

How can you test for halide ions?

Answer 10

First, add dilute nitric acid and then silver nitrate solution.
If a precipitate forms, there are halide ions present.

Question 11

Test for halide ions (results table for precipitates)

Answer 11

chloride. bromide. iodine
white. cream. yellow

Question 12

How can you test an unknown compound for sulfate ions?

Answer 12

By adding dilute hydrochloric acid, followed by barium chloride solution.
A white precipitate tells you sulfate ions are present.

Question 13

Advantages of instrumental analysis

Answer 13

• More accurate
• Can work with smaller samples
• Quicker

Question 14

Disadvantages of instrumental analysis

Answer 14

• Expensive
• Equipment requires training to use
• Requires a database to compare

Question 15

Why do you add 2 drops of nitric acid when testing for halides?

Answer 15

To remove any carbonate ions which will affect your result by reacting with the silver nitrate solution

Question 16

Explain why a flame test cannot be used to identify a mixture of metals

Answer 16

Flame tests can only identify a single ion

Question 17

Explain why flame emission spectroscopy is better than a flame test

Answer 17

• Faster
• Works with very small sample sizes
• More accurate
• Can measure multiple metal ions

Question 18

How can you identify a metal cation?

Answer 18

Reacting unknown compounds with sodium hydroxide solution can help you identify some positive ions.

Question 19

What positive ions form white precipitates with sodium hydroxide?

Answer 19

Aluminium, calcium and magnesium

Question 20

Explain what the test for ammonia tells you about the properties of ammonia.

Answer 20

Ammonia dissolves in water to form an alkaline solution, because it turns damp red litmus paper blue.

Question 21

Testing for positive ions (results)

Answer 21

copper blue
iron. green
iron3+. brown

Question 22

Name a metal ion that would produce a green flame

Answer 22

Copper (Cu2+)

Question 23

What color flame do sodium ions produce?

Answer 23

Yellow

Question 24

What ion produces a lilac color flame?

Answer 24

Potassium (K+)

Question 25

What ion produces a cream precipitate when added to silver nitrate solution and dilute nitric acid?

Answer 25

Bromide (Br-)

Question 26

What does the Rf value of a dye depends on?

Answer 26

- Solubility of the dye in the solvent - Attraction of the dye to the paper

Question 27

What will produce a smaller Rf value if the solvent and paper are both changed?

Answer 27

If the dye is less soluble in the new solvent and it is more attracted to the new paper

Question 28

What two ways can increase the distance between ink spots in chromotography?

Answer 28

- Use a different solvent - Allow the solvent front to travel further

Question 29

What is the compound needed to identify an aluminium ion?

Answer 29

Sodium hydroxide

Question 30

If a brown precipitate forms when sodium hydroxide is added to a mystery solution, which metal ion was present?

Answer 30

Iron (III) Fe3+

Question 31

What is the correct test for sulfate ions?

Answer 31

Add barium chloride White precipitate forms

Question 32

What happens to aluminium in excess sodium hydroxide?

Answer 32

The aluminium hydroxide precipitate dissolves to form a colorless solution

Question 33

What happens to aluminium in excess sodium hydroxide?

Answer 33

The aluminium hydroxide precipitate dissolves to form a colorless solution

Question 34

What gas is produced when a carbonate reacts with an acid?

Answer 34

Carbon dioxide

Question 35

Children often do not like taking medicine. Suggest a substance that could be added to Aquamed to increase the desire for children to take it. Give a reason for your suggestion.

Answer 35

Sugar/flavouring so it tastes better

Question 36

What do impurities do to a substance?

Answer 36

- Lower the shelf life of substances - Increase the boiling point

Question 37

What are the 2 phases in chromatography?

Answer 37

Mobile phase and stationary phase

Question 38

The less/more attracted to the chromatography paper the ink is the further/lesser it will move

Answer 38

The less attracted to the chromatography paper the ink is the further it will travel

Question 39

How many chromatography spots are produced by pure samples?

Answer 39

1

Question 40

What does chromatography produce?

Answer 40

A chromatogram

Question 41

What is the apparatus required for testing chlorine?

Answer 41

- Test tube - Litmus paper

Question 42

In flame tests, what piece of apparatus is used to transfer the sample to the Bunsen flame?

Answer 42

Nichrome (nickel and chromium) wire loop

Question 43

What charge do anions carry?

Answer 43

Negative

Question 44

What is the instrument used in flame emission spectroscopy?

Answer 44

A spectroscope

Question 45

What is generated by a spectroscope in a flame emission spectroscopy?

Answer 45

Line spectrum

Question 46

What does flame emission spectroscopy identify?

Answer 46

Metal ions

Question 47

What must be added to the solution before silver nitrate when testing for halides?

Answer 47

Dilute nitric acid

Question 48

Which chemical forms a white precipitate when you test for sulfate ions in solution?

Answer 48

Barium Sulfate

Question 49

What state must a splint be in when testing for hydrogen?

Answer 49

Lit

Question 50

What feature of a line spectrum provides information about the concentration of a metal ion?

Answer 50

Intensity

Question 51

What safety precautions should you take for flame tests?

Answer 51

Safety googles and hair tied back

Question 52

What is a pure substance to a chemist?

Answer 52

A pure substance is one that is made up of just one substance (one element or compound).

Question 53

Examples of pure substances

Answer 53

- gold - copper wire - sugar - salt - hydrogen gas - diamond

Question 54

What are fixed points?

Answer 54

The melting and boiling point of an element or a compound

Question 55

What is a mixture?

Answer 55

A mixture consists of 2 or more different substances that are not chemically joined together

Question 56

How does the boiling and melting points of pure/impure substances differ?

Answer 56

- Pure substances have a sharp melting/boiling point ; the temperature stays the same when a pure substance changes state - Impure substances like mixtures have melting and boiling points that vary ; the temperature changes slightly as an impure substance changes state

Question 57

What is a formulation?

Answer 57

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

Question 58

Examples of formulations

Answer 58

fuels, cleaning products, paints and medicines

Question 59

Why do we draw our starting line in pencil during paper chromatography?

Answer 59

If we drew the line in pen, the pen ink would move up the paper, with the solvent.

Question 60

Required Practical - Chromatography (6 marks)

Answer 60

- Use a ruler to draw a horizontal line (2cm from the bottom) on the chromatograhpy paper - Mark (number of samples) spots on the pencil line of equal distance/size - Use a capillary tube to put a small spot of each sample on the pencil spots - Pour water to a beaker of depth 1cm - Attach the paper to a glass rod using tape, and lower the paper into the beaker - The water should be below the pencil line/spots and the sides of the paper should not touch the sides of the beaker