C12: Chemical analysis

Question 1

What is a pure substance?

Answer 1

A single element or compound containing only one type of particle

Question 2

How can melting and boiling points be used to determine the purity of a substance?

Answer 2

A pure substance has a fixed m.p./b.p.
An impure substance melts and boils over a range of temperatures, and are overall higher

Question 3

What is a formulation?

Answer 3

A complex mixture that’s been designed as a useful product. The quantities of each component are carefully measured so that the product has the properties that we need.
e.g. fuels, cleaning products, paint, medicine, alloys, fertilisers and food.

Question 4

What is paper chromatography?

Answer 4

A physical separation technique that allow substances to be separated based on their solubilities.

Question 5

What is the stationary phase of chromatography?

Answer 5

The paper because it doesn’t move

Question 5

What is the mobile phase of chromatography?

Answer 5

The solvent because it moves

Question 6

What happens to more soluble substances during chromatography?

Answer 6

Travel further up the paper

Question 7

Why is the starting line drawn in pencil?

Answer 7

Pen ink is soluble so it’ll move up with the solvent

Question 8

What is the formula for calculating the rf value?

Answer 8

Rf (no units) = distance moved by substance / distance moved by solvent.
Look up rf value in a database to identify the substance

Question 9

What do you do if 2 substances have the same Rf value?

Answer 9

Use a different solvent

Question 10

Describe the test for hydrogen

Answer 10

Insert a burning splint - a pop sound is produced when near hydrogen

Question 11

Describe the test for oxygen

Answer 11

Insert a glowing/smouldering splint - it relights when near oxygen

Question 12

Describe the test for carbon dioxide

Answer 12

Use limewater (aqueous calcium hydroxide). Bubble co2 gas through limewater, if present, limewater turns cloudy

Question 13

Describe the test for chlorine

Answer 13

Insert damp litmus paper into test tube - presence of chlorine bleaches the paper

Question 14

What is the flame colour of lithium ions?

Answer 14

Crimson

Question 15

What is the flame colour of sodium ions?

Answer 15

Yellow

Question 16

What is the flame colour of potassium ions?

Answer 16

Lilac

Question 17

What is the flame colour of calcium ions?

Answer 17

Orange-red

Question 18

What are the flame colours for copper and beryllium?

Answer 18

Green

Question 19

What are problems with using flame tests?

Answer 19

Colour can be difficult to distinguish (esp. if there is a low concentration of the compound)
Sample may contain impurities/ a mixture of ions and mask the colour of the flame

Question 20

What else can scientists use, apart from flame tests?

Answer 20

Flame emission spectroscopy

Question 21

What is flame emission spectroscopy?

Answer 21

A sample of a metal ion in a solution is placed into a flame, the light given out is passed through a spectroscope, which converts the light into a line spectrum. The positions are specific to the metal ion. It can also tell the concentration of a metal ion - lines become more intense with higher conc.

Question 22

What is an instrumental method?

Answer 22

A method carried out by a machine

Question 23

Why are instrumental methods good?

Answer 23

They are rapid - more so than flame tests
Very sensitive - work on a tiny sample of a metal compound
Accurate

Question 24

What 3 ions do you need to know that react with NaOH?

Answer 24

Magnesium, Calcium, Aluminium, Iron, Copper

Question 25

What happens when you add NaOH to the 3 aforementioned ions?

Answer 25

White precipitate forms

Question 26

What happens when you add NaOH to iron?

Answer 26

Brown precipitate forms (3+) Green precipitate forms (2+)

Question 27

What happens when you add NaOH to copper?

Answer 27

Blue precipitate forms

Question 28

How do you distinguish between Mg, Al and Ca precipitates?

Answer 28

Flame tests for calcium

Question 29

Describe the test for the carbonate ion

Answer 29

Add dilute acid to sample. It will react to form a gas and we see fizzing. Bubble gas through lime water to test for CO2

Question 30

Describe the test for the halide ions

Answer 30

Add dilute nitric acid to the sample. Add dilute silver nitrate solution Halide ions produce a precipitate of the silver halide. Each one makes a different colour precipitate. Chloride - white Bromide - cream Iodide - yellow