RP7 Identifying ions

Question 1

how do you carry out a flame test to identify metal ions?

Answer 1

• clean a platinum wire loop by dipping it in HCl and then holding it in the blue flame until it burns without any colour
• dip the loop into the sample you want to test and put it into the blue flame of the bunsen burner
• record the colour of the flame

Question 2

list precautions to take when using a bunsen burner

Answer 2

• don’t leave unattended
• turn off gas or leave on orange safety flame when not in use
• tie back long hair
• keep flammable chemicals away from the flame

Question 2

what is the result of the flame test on lithium ions?

Answer 2

lithium ions (Li+): crimson flame

Question 3

what is the result of the flame test on sodium ions?

Answer 3

sodium ions (Na+): yellow flame

Question 3

what is the result of the flame test of potassium ions?

Answer 3

potassium ions (K+): lilac flame

Question 4

what is the result of the flame test on calcium ions?

Answer 4

calcium ions (Ca2+): orange-red flame

Question 5

how can you test for carbonate ions?

Answer 5

• add a few drops of HCl to the sample in a test tube
• connect this test tube to a test tube of limewater
• if carbonate ions are present, carbon dioxide will be produced. limewater will turn cloudly when CO2 is bubbled through.

Question 5

why must the wire be cleaned before carrying out a flame test?

Answer 5

to remove any unwanted ions that might obscure the colour of the flame

Question 5

what is the result of the flame test on copper ions?

Answer 5

copper ions (Cu2+): green flame

Question 6

why can a flame test not be used when a compound contains a mixture of metal ions?

Answer 6

the flame colours of some ions may be hidden by the colours of other metal ions

Question 7

write the chemical equations for the reaction between HCl and Na2CO3

Answer 7

Na2CO3 + 2 HCl –> CO2 + 2 NaCl + H2O

Question 8

write the chemical equation for the reaction between BaCl2 and MgSO4

Answer 8

BaCl2 + MgSO4 –> BaSO4 + MgCl2
BaSO4 is a white precipitate

Question 8

how can you test for sulfate ions?

Answer 8

• add HCl to remove any CO3 2- ions as these will obscure the results
• add a couple of drops of barium chloride
• if sulfate ions are present a white precipitate of barium sulfate will form

Question 8

how do you carry out a test for halide ions?

Answer 8

• add a couple of drops of nitric acid to react with any carbonate ions which might obscure the experiment
• add a couple of drops of silver nitrate
• observe the colour of the precipitate

Question 9

what colour precipitate is formed when silver nitrate is added to a chloride solution?

Answer 9

white precipitate of silver chloride

Question 10

what colour precipitate is formed when silver nitrate is added to a bromide solution?

Answer 10

cream precipitate of silver bromide

Question 10

what colour precipitate forms when sodium hydroxide reacts with calcium ions?

Answer 10

white precipitate

Question 10

what colour precipitate forms when sodium hydroxide reacts with aluminium ions?

Answer 10

white precipitate at first.
redissolves with excess NaOH to form a colourless solution.

Question 11

what colour precipitate is formed when silver nitrate is added to an iodide solution?

Answer 11

yellow precipitate of silver iodide

Question 11

what colour precipitate forms whe sodium hydroxide reacts with copper(II) ions?

Answer 11

blue precipitate

Question 12

what colour precipitate forms when sodium hydroxide reacts with iron(II) ions?

Answer 12

green precipitate

Question 13

what colour precipitate forms when sodium hydroxide reacts with iron(III) ions?

Answer 13

brown precipitate

Question 14

what colour precipitate forms when sodium hydroxide reacts with magnesium ions?

Answer 14

white precipitate

Question 15

given two solutions, how can you identify which contains aluminiun ions and which contains magnesium ions?

Answer 15

add excess sodium hydroxide.
both will form white precipitates but the one containing aluminiun ions will redissolve to form a colourless solution.