Rp 04- Testing For Cations And Anions

Question 1

How do you test for group 2 metal cations?

Answer 1

• place 10 drops of group 2 compound in test tube
• add 10 drops of 0.6moldm -3 NaOH to test tube, record any observations
• continue to add NaOH so is in excess, record any observations
• repeat with other group 2 compounds

Question 2

What are the results from these reactions?

Answer 2

Barium Chloride- initial - colourless sol - 10 drops NaOH- colourless sol- excess NaOH - colourless sol
Calcium bromide - initial- colourless sol - 10 drops NaOH- slight white ppt- excess NaOH- slight white ppt
Magnesium chloride- initial- colourless sol- 10 drops NaOH- slight white ppt- excess NaOH- white ppt
Strontium chloride- initial- colourless sol- 10 drops NaOH- slight white ppt- excess NaOH- slight white ppt

Question 3

How would you distinguish between calcium bromide and strontium chloride ?

Answer 3

-add 10 drops of each into separate test tubes
- add 10 drops of H2SO4, record observations
- continue to add H2SO4 until in excess, record observations l
- repeat for other group 2 compounds
- calcium bromide- initial- colourless sol.- 10 drops H2SO4- slight white ppt- excess- same
- strontium chloride- initial- colourless col.- 10 drops H2SO4- white ppt- excess- same

Question 4

What are the overall results for the group 2 reactions?

Answer 4

Ba2+ - excess NaOH, no change - excess H2SO4, white ppt
Ca2+ - excess NaOH, white ppt of Ca(OH)2 - excess H2SO4, slight white ppt
Mg2+- excess NaOH, white ppt of Mg(OH)2 - excess H2SO4, colourless solution
Sr2+ - excess NaOH, slight white ppt - excess H2SO4, white ppt

Question 5

How do you test for ammonium (NH4+)ions

Answer 5

• add 10 drops of ammonium chloride to a test tube
• add 10 drops NaOH solution and shake
• warm solution in test tube
• test gas released with damp red litmus paper
• if it goes blue, ammonium ions are present

Question 6

How do you test for group 7/halide ions?

Answer 6

• to the compound being tested, add nitric acid and silver nitrate. Record observations
• to samples of this solution, add dilute and then concentrated ammonia

Question 7

What are the results from these reactions?

Answer 7

• Cl- - silver nitrate, white ppt - dilute NH3, white ppt disappears solution goes colourless - conc. NH3 remains colourless
• Br- - silver nitrate, cream ppt - dilute NH3, cream ppt remains - conc. NH3, cream ppt disappears solution colourless
• I- - silver nitrate, yellow ppt - dilute NH3, yellow ppt remains - conc. NH3, yellow ppt remains

Question 8

How do you test for hydroxide (-OH) ions?

Answer 8

• test a 1cm depth of solution in a test tube with red litmus paper or universal indicator paper
• record observations
• sodium hydroxide will turn damp red litmus paper blue

Question 9

How do you test for carbonate (CO3 2-) ions?

Answer 9

• bubble sodium carbonate (sample) into test tube of calcium hydroxide
• if Ca(OH)2 goes cloudy, carbonate ions were present in the other test tube

Question 10

How do you test for (SO4 2-) ions?

Answer 10

• Add HCl and BaCl2 to the suspended sulphate solution.
• if sulfate ions are present a white ppt of BaSO4 will form.

Question 11

What is the order of testing ions? (Carbonate, halide and sulfate)

Answer 11

Carbonate —> sulfate —> halide
- prevents false positive results occurring