C4.2 - Identifying Products of Chemical Reactions

Question 1

How do you detect CO2 (and how does it work)?

Answer 1

• Bubble the gas through limewater (calcium hydroxide solution)
• if it turns cloudy, The gas is CO2
• it works by…limewater + CO2 reacting to product a white precipitate

Question 2

How do you detect chlorine (and why does it work)?

Answer 2

• hover a dampened piece of litmus paper above the substance
• if chlorine present, paper bleaches (in blue, turns red then white) , (in red, just turns white)
• this works because….chlorine bleaches dyes

Question 3

Why does the litmus paper have to be damp in the test for chlorine?

Answer 3

• As when chlorine reacts with water it creates an acidic solution (kinda like a bleach)
• therefore, the new solution reacts with the litmus paper
• and so the paper becomes red then bleached

Question 4

How do you detect for hydrogen?

Answer 4

• place a lit splint (you can see a flame) near the mouth of test tube
• it should extinguish with a ‘squeaky pop’ sound

Question 5

How do you detect for oxygen?

Answer 5

• place a glowing splint (so almost gone), into mouth of test tube
• flame should relight (becomes a roaring flame)

Question 6

Describe why a flame test works (the science behind it)

Answer 6

• when metal ions are heated, energy is transferred to electrons
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  Electrons move to higher electron shells (outwards) - now unstable
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  When electrons move back to normal energy shells, energy is transferred to surroundings as radiation (we see as light, or a flame)

Question 7

Why do different metals produce different colours of light?

Answer 7

As they have different chemical properties or different placed electrons, so the electrons jump different lengths/release different amounts of energy when they move back to their initial energy level/shell (different wavelength so different colour produced)

Question 8

What kind of ions do flame tests work on?

Answer 8

Metal ions (or cations - positive ions)

Question 9

How do you carry out a flame test?

Answer 9

1) clean nichrome wire loop (with handle), by dipping in HCl acid
2) then rinse it in distilled water
3) place into bunsen burner to see if the blue flame changes colour, if it does dip it in HCL acid again and rinse it
4) dip clean loop into test powder/solution
5) place loop into blue flame on Bunsen burner
6) record flame colour

Question 10

What colour is lithium’s flame?

Answer 10

Red

Question 11

What colour is sodium’s flame?

Answer 11

Yellow

Question 12

What colour is potassium’s flame?

Answer 12

Lilac

Question 13

What colour is calcium’s flame?

Answer 13

Orange-red

Question 14

What colour is copper’s flame?

Answer 14

Green-blue

Question 15

What would the colour of iron (II) precipitate be?

Answer 15

Green

Question 16

What would the colour of iron (III) precipitate be?

Answer 16

Orange-brown

Question 17

What would the colour of copper (II) precipitate be?

Answer 17

Blue

Question 18

What would the colour of calcium precipitate be?

Answer 18

White

Question 19

What would the colour of zinc precipitate be?

Answer 19

White, if you add excess of sodium hydroxide then it becomes colourless

Question 20

How do you perform a hydroxide precipitate test?

Answer 20

1) Add a few drops of sodium hydroxide solution to the solution containing metal ions
2) record the colour of the precipitate

Question 21

Why do you use sodium hydroxide solutions in a precipitate test?

Answer 21

As group 1 hydroxides are soluble in water (most other metal hydroxides are insoluble).

Question 22

why must you clean the wire loop between each test?

Answer 22

to ensure the substance being tested is not contaminated with the previous test substance

(not confuse the flame colours)

Question 23

how do you detect sulfate ions?

Answer 23

1) add a few drops of dilute hydrochloric acid
2) add a few drops of barium chloride solution

• a white precipitate should form

Question 24

what is the basis to detect sulfate ions?

Answer 24

barium ions and sulfate ions react to make white insoluble barium sulfate

Question 25

state one measure used in a sulfate test to ensure the accuracy

Answer 25

• dilute HCl acid is added
• to remove the carbonate ions
  (or else they can react to produce a white precipitate)

Question 26

how do you detect carbonate ions?

Answer 26

1) add any dilute acid to sample (usually HCl)

• would react to release CO2 gas
• to check if CO2 bubble through limewater

Question 27

what is the basis to detect carbonate ions?

Answer 27

H+ ions in acid react with CO3 (2-) ions in the carbonate, to make CO2 + H20

• so the CO2 gas is an indicator that carbonate ions are present

Question 28

how do you detect halide ions?

Answer 28

1) add a few drops of dilute nitric acid
2) add a few drops of silver nitrate solution

• a precipitate is formed if chloride/bromide/iodide ions are there

Question 29

difference between chlorine and chloride?

Answer 29

chloride is the ion of chlorine

Question 30

what are the 3 colours produced in a positive halide test?

and what is the corresponding ion

Answer 30

white, cream, yellow

chloride, bromide, iodide

Question 31

what are the advantages of instrumental methods of analysis?

Answer 31

• more sensitive (can analyse small amounts of substance)
• higher accuracy (can be calibrated)
• greater speed (analyse quickly + run all the time)

Question 32

give 3 features of a gas chromatogram and what they mean

Answer 32

each peak = a substance present in mixture

area under peak = relative amounts of substance

retention time = time taken for substance to travel through the chromatography column (diff. for diff substances)

Question 33

what does a mass spectrometer do?

Answer 33

• measure the masses of atoms and molecules

- analyse the amounts of different isotopes of an element + structure of molecules

Question 34

give 3 features of a mass spectrum

Answer 34

• each peak represents a fragment of the molecule
• peak on far right represents the molecular ion (entire molecule)
• mass to charge ratio of molecular ion = to Mr of substance (relative formula mass)
• on x axis