C12 - Chemical analysis

Question 1

Define a pure substance:

Answer 1

a single element/compound not mixed with anything else, that melts/boils at specific temperatures

Question 2

Name 3 types of litmus indicator:

Answer 2

-litmus solution
-red litmus paper
-blue litmus paper

Question 3

Describe the colour changes of the 2 types of litmus paper:

Answer 3

2 types, red and blue paper

Question 4

Describe the colours of litmus solution for different pH’s:

Answer 4

acid - red
neutral - purple
alkali - blue

Question 5

Define formulation:

Answer 5

a mixture designed to be a useful product, with precise amounts of different components in definite proportions, where each component has a particular purpose

Question 6

Give 5 examples of some formulations:

Answer 6

medicines, fuels, paints, alloys, fertilisers

Question 7

What is the test for hydrogen gas?

Answer 7

a lighted splint held at the end of an open test tube makes a squeaky pop

Question 8

What is the test for oxygen gas?

Answer 8

a glowing splint held inside the test tube of the gas relights itself

Question 9

What is the test for CO₂?

Answer 9

limewater turns cloudy/milky

Limewater is just calcium hydroxide solution, and it reacts with CO₂ to form a solid calcium carbonate precipitate

Question 10

Give the test for chlorine gas:

Answer 10

damp blue litmus paper is bleached white in the presence of chlorine gas

Question 11

Name 2 ways of testing for cations:

Answer 11

-flame test
-NaOH test

Question 12

Describe how to perform the NaOH cation test:

Answer 12

-mix unknown solution with NaOH solution
-note precipitate colour

The precipitate formed is an insoluble metal hydroxide

Question 13

RP7 - Describe how you would perform a flame test:

Answer 13

-dip nichrome wire into dilute HCl to clean it
-dip the wire into unknown solution
-hold tip of wire in the centre of a blue Bunsen burner flame
-record the colour of the flame

Question 14

Name all 5 flame tests for the cations:

Answer 14

Li - crimson
Na - yellow
K - lilac
Ca - orange-red
Cu - green

C, Y, LOG

Question 15

In a flame test, what might happen if a sample contains multiple cations?

Answer 15

some flame colours can be masked by others

Question 16

Name the 6 NaOH test precipitate colours:

Answer 16

Al³⁺ -white (don’t add excess or it redissolves)
Ca²⁺ -white
Mg²⁺ -white (differentiate between Mg and Ca with flame test)

Cu²⁺ -blue
Fe²⁺ -green
Fe³⁺ -brown

(WWW, BGB)

Magnesium has an intensely white flame, and calcium has an orange-red flame

Question 17

RP7 - Give the tests for sulfates:

Answer 17

add HCl to remove other anions, add BaCl₂ solution, forms a white precipitate (BaSO₄) if sulfates present

The acid is added first to react with any carbonate ions in the solution and forms an insoluble white precipitate, as they can also react with barium chloride to form a white precipitate - this prevents a false positive result.

Question 18

RP7 - Give the tests for carbonates:

Answer 18

react unknown solution with dilute HCl, and if bubbles seen, transfer with delivery tube to a test tube with limewater in it, and it should go cloudy/milky

Question 19

RP7 - How can you test for the halides?

Answer 19

-add dilute nitric acid (HNO₃) and silver nitrate solution to the unknown solution, and record precipitate colour:

Cl - white
Br - cream
I - yellow

The precipitate is a silver halide

Question 20

Give the pros and cons of instrumental analysis methods (over chemical analysis):

Answer 20

-more accurate
-faster
-more sensitive (can identify small amounts of things in small sample)

-more expensive to run/maintain
-requires special training

Question 21

Name a method that could be used to identify cations in solution and their concentration:

Answer 21

flame emission spectroscopy

Question 22

What is flame emission spectroscopy? Describe the process:

Answer 22

-an instrumental method used to identify unknown cations in solution

-put sample into flame and pass the light emitted through a spectroscope
-the output is an emission spectrum that can be analysed to identify the metal ions and their concentration (based on strength of light)

Question 23

If a question can’t be answered by science alone, what are the usual reasons?

Answer 23

-may be other factors involved
-based on opinion/lifestyle choice
-the thing in mind has different effects on different people
-ethical/economic/social issue
-cannot be determined by experiment alone

Question 24

RP6 - Define chromatography:

Answer 24

a process used to separate a mixture and give information in order to help identify the substances in the mixture

Question 25

RP6 - What is an Rf value, and how do you calculate it? What is it dependent on?

Answer 25

-ratio between the distance travelled by the solute to the solvent, both measured from the pencil line

-distance travelled by solute/distance to solvent front

-this is dependent on the solvent - using ethanol instead of water will give another Rf value

Question 26

RP6 - How can you use paper chromatography to separate 2 substances?

Answer 26

-draw pencil line 2cm up on chromatography paper
-put the solutes evenly spaced across the line
-pour some water into a beaker, and suspend the paper above it using a pencil across the beaker and some clips, so the water level is just below the pencil line
-put lid on to stop evaporation
-wait until the solvent front is near the top of the paper
-hang up paper to dry, then calculate Rf values

Question 27

RP6 - How can you use paper chromatography to identify if a known substance is mixed in with an unknown substance?

Answer 27

-run pure sample of the substance alongside the unknown
-if the pure sample’s spot’s matches the unknowns’ spots at the same level, then it is likely that it is contained in the unknown mixture

Pure substances produce only one spot on chromatography paper

Question 28

RP6 - Explain how paper chromatography separates different substances in a mixture:

Answer 28

-solvent moves up through paper
-different solutes have different solubilities in mobile phase (water), and different attractions to stationary phase (paper)
-carried different distances due to this

Question 29

RP6 - When asked to see if 2 mixtures on a chromatogram have a common substance, what should you say?

Answer 29

spots matched at same level

Question 30

A mixture contains more than 2 compounds - why might there only be 2 spots on the chromatogram?

Answer 30

-some of the compounds might be colourless
-some compounds may have the same Rf value as others

Question 31

How could you test if a substance is pure water?

Answer 31

-determine boiling point
-should be fixed at 100°C