Flmae tests and the test for ammonium ions

Question 1

flame tests are used for…………..ions only

Answer 1

cations, positive ions

Question 2

how to carry out a flame test:

Answer 2

• wear safety glasses and a lab coat
• light a Bunsen burner in a fume cupboard
• using a dropper, add a few drops of concentrated hydrochloric acid to the solid metal and mix together so that the metal compound begins to dissolve
• dip a clean metal wire (platinum or nichrome) or silica rod into the mixture to obtain a sample of the compound
• hold the end of the wire loop or rod in the blue flame of the burner and observe the colour

Question 3

hydrochloric acid is used in flame tests because

Answer 3

HCl converts any metal to a chloride, which are more volatile than other salts so are more likely to give better results

Question 4

what kind of wire loop or rod is used in flame testing?

Answer 4

platinum or nichrome wire, or a silica rod

Question 5

the two main problems with a flame test are:

Answer 5

• many compounds contain small amounts of sodium compounds as impurities, so the intense colour of sodium can mask other colours
• describing colours with words is subjective- people have different levels of colour vision, and a word description of a colour may mean different colours to different people

Question 6

the flame test colour for the cation Li+ is

Answer 6

red

Question 7

the flame test colour for the cation Na+ is

Answer 7

yellow/orange

Question 8

the flame test colour for the cation K+ is

Answer 8

lilac

Question 9

the flame test colour for the cation Rb+ is

Answer 9

red/purple

Question 10

the flame test colour for the cation Cs+ is

Answer 10

blue/violet

Question 11

the flame test colour for the cation Be2+ is

Answer 11

no colour

Question 12

the flame test colour for the cation Mg2+ is

Answer 12

no colour

Question 13

the flame test colour for the cation Ca2+ is

Answer 13

(brick) red

Question 14

the flame test colour for the cation Sr2+ is

Answer 14

(crimson) red

Question 15

the flame test colour for the cation Ba2+ is

Answer 15

(apple) green

Question 16

what causes the colours in flame tests: (3 marks)

Answer 16

• electron transitions
• during a flame test, electrons in their ‘ground state’ absorb energy and become exited or ‘promoted’ so move to higher energy levels
• electrons then return to their ground state and so release the energy absorbed, which is released as radiation
• if this radiation is in the visible light spectrum, then we see it as colour
• the movement when electrons become promoted and then go back to their original orbital is instant, and even so quick that the electron ‘teleports’, but don’t say this in the exam

Question 17

the visible light spectrum covers the wavelength range…..to…………..nm

Answer 17

400-700 nm

Question 18

the usual test for ammonium ions is

Answer 18

• add sodium hydroxide solution to solid ammonium or aqueous ammonium
• warm the mixture
• hold damp red litmus paper where the gas is released and it will turn blue

Question 19

the ionic equation showing the ammonium ions and the hydroxide ions in the reaction for the test for ammonium ions is

Answer 19

NH4+(aq) + OH-(aq) = NH3(g) + H2O(l)

Question 20

the two possible tests for ammonia gas is

Answer 20

• holding damp red litmus paper in the gas and it will turn blue
• reacting hydrogen chloride gas from concentrated HCL acid with ammonia to form white fumes of ammonium chloride

Question 21

the formation of ammonium chloride from ammonia gas is shown in the equation:

Answer 21

NH3 + HCl = NH4Cl