Test For Ions

Question 1

Describe the reagents used in testing for SO4 [2-] ions and state the observations that would be made? [3]

Answer 1

add hydrochloric acid                   1 
add BaCl2 (barium chloride)              1 

observe white solid / ppt 1

Question 2

Briefly describe a method of how you would identity a sample that contains Ca [2+] ions, and state the observations that would be made? [3]

Answer 2

use flame test 1
dip (nichrome) wire loop into hydrochloric acid 1

observe (brick red) flame 1

Question 3

A student tested for the presence of ammonium ions using damp red litmus paper.
Describe why the litmus paper changed colour when NaOH was added and state the colour change?[3]

Suggest why the litmus paper must be damp? [1]

Answer 3

OH- accept a proton form NH4 + 1
forms (NH3) ammonia gas (alkali) 1
change to blue 1

so the gas can dissolve through the paper 1

Question 4

when testing for sulfate ions, a student added strong acid to the sample.
Suggest why this was an important step? [2]

Answer 4

removes traces of carbonate ions ( CO3 [2-] ) 1

also, produce white ppt 1

Question 5

A student wanted to test for the presence of the halide ion ( I- ).
Describe a method they could have used to identify this halide, and state the observations? [3]

State the formula of the product formed? [1]

Answer 5

add (dilute) nitric acid 1
add silver nitrate 1
forms yellow ppt 1

AgI ( silver iodide ) 1

Question 6

When testing for two halides (Br- and I- )a student struggled to distinguish between the colour of the precipitates formed.
Give the name of a reagent that could be added to clarify her results? [1]
In one sample the ppt dissolved. Give the identity of the halide present, and suggest why it dissolved? [2]

Answer 6

add (concentrated) NH3, [ MUST be conc. ] 1

Br - (bromide) 1
more soluble 1

Question 7

When a student added hydrochloric acid to a sample of carbonate ions ( CO3 [2-] ) the sample began to fizz.

i) Describe why the sample fizzed and identity the product responsible? [2]
ii) Give a reagent that the student could use to test for the presence of the product in part i), and sate the observations made? [2]

Answer 7

i) carbonate ions react with H + ions in acid 1
forms CO2 1

ii) use limewater 1
turns cloudy 1