Chemical analysis

Question 1

what is chemical analysis?

Answer 1

Also known as analytical chemistry, it is all about the instruments and methods used to separate, identify, and quantify different substances

Question 2

what is a pure substance?

Answer 2

A pure substance is a substance where nothing has been added to it, so it is in its natural state

Question 3

what is a formulation?

Answer 3

Mixtures prepared using a specific formula containing precise amounts of different components for a particular function

Question 4

describe the boiling and melting points of a pure substance:

Answer 4

Pure substances melt and boil at specific temperatures

Question 5

where are formulations used in the modern world?

Answer 5

• pharmaceutical industry
• cleaning products
• fuels
• cosmetics
• fertilisers
• metal alloys

Question 6

what is chromatography?

Answer 6

an analytical method used to separate the substances in a mixture. You can then use it to identify the substances.

Question 7

what are the two phases of chromatography?

Answer 7

mobile phase - where the molecules can move. This is always a liquid or gas
stationary phase - where the molecules cannot move. This can be a solid or really thick liquid

Question 8

what happens during paper chromatography?

Answer 8

• the stationary phase is the chromatography paper (filter paper) and the mobile phase is the solvent
• molecules with a higher solubility in the solvent, and are less attracted to the paper, will spend more time in the mobile phase - and they’ll be carried further up the paper

Question 9

the amount of time the molecules spend in each phase depends on two factors, what are they?

Answer 9

• how soluble they are in the solvent
• how attracted they are to the paper

Question 10

how to calculate the Rf of a sample?

Answer 10

distance travelled by substance / distance travelled by solvent

Question 11

what is an Rf value used for?

Answer 11

Rf values can be used to identify unknown chemicals if they can be compared to a range of reference substances.

Question 12

How do you test for chlorine?

Answer 12

chlorine bleaches damp litmus paper, turning it white. (it may be red for the first moment)

Question 13

How do you test for oxygen?

Answer 13

If you put a glowing splint inside a test tube, the oxygen will relight the glowing splint

Question 14

How do you test for carbon dioxide?

Answer 14

bubble carbon dioxide through an aqueous solution of limewater, the solution will turn cloudy

Question 15

How do you test for hydrogen?

Answer 15

If you hold a lit splint at the open end of a test tube containing hydrogen, you’ll hear a squeaky pop

Question 15

How to test for carbonates?

Answer 15

Put a sample of the solution into a test tube and adding a few drops of dilute acid. Then connect it to a test tube of limewater. If carbonate ions are present, carbon dioxide will be released, turning the limewater cloudy

Question 16

How to test for sulphates?

Answer 16

To identify sulphate ions, use a pipette to add a couple drops of dilute hydrochloric acid followed by a couple drops of barium chloride solution to a test tube containing the unknown solution. If sulphate ions are present, a white precipitate of barium sulphate will form

Question 16

How to test for halides? Give the individual result of each halide

Answer 16

Add a couple drops of dilute nitric acid followed by a couple drops of silver nitrate solution to the unknown solution:
chloride - white precipitate of silver chloride
bromide - cream precipitate of silver bromide
iodide - yellow precipitate of silver iodide

Question 17

how to test for metal ions using flame tests?

Answer 17

• place a small amount of the sample into the flame:
  lithium - crimson flame
  sodium - yellow flame
  potassium - lilac flame
  calcium - orange-red flame
  copper - green flame

Question 18

what two mnemonics can be used to remember the order of the metals with the order of the colours for a flame test?

Answer 18

order of elements tested:
Lively Snakes Paint Colourful Chickens
Lively - lithium
Snakes - sodium
Paint - potassium
Colourful - calcium
Chickens - copper

order of colours (linked to the elements):
Crazy Yellow Lions Offer Real Greatness
Crazy - crimson
Yellow - yellow
Lions - lilac
Offer Real - orange/red
Greatness - green

Question 19

How to test for metal ions when reacting with sodium hydroxide?

Answer 19

Add a few drops of sodium hydroxide to the unknown compound, if you get one of the following coloured compounds, you can see what was in the compound:
Calcium - white
Copper (II) - blue
Iron (II) - green
Iron (III) - brown
Aluminium - white but redissolves so colourless
Magnesium - white

Question 20

What happens during a flame emission spectroscopy?

Answer 20

1) a sample is placed in a flame. As the ions heat up, the electrons become excited. When the electrons drop back to their original energy level, they transfer energy as light
2) The light passes through a spectroscope, which can detect different wavelengths of light to produce a line spectrum
3) This can be used to identify ions and calculate their concentrations

Question 21

Why is flame emission spectroscopy more useful than flame tests?

Answer 21

They can detect the different components of a mixture, but flame tests can only detect a single metal ion

Question 22

What are the advantages of using instrumental analysis (tests using machines)?

Answer 22

• very sensitive
• very fast
• very accurate