4.6 CHEMICAL TESTS Flashcards
How to carry out a test for hydrogen
- Collect a sample of gas in a test tube
- Attempt to ignite the gas with a lit splint
- You will hear a squeaky pop
How to carry out a test for hydrogen
- Collect a sample of gas in a test tube
- Attempt to ignite the gas with a lit splint
- You will hear a squeaky pop
What is the chemical reaction when testing hydrogen
Combustion - hydrogen forming water
How to test for oxygen
- Collect a sample of gas in a test tube
- Place a glowing splint in the test tube
- The splint will relight
What is the chemical reaction taking place when testing for oxygen
Combustion of the wood - Pure O2 is more concentrated that the O2 in normal air (splint combusts faster)
How to test for the presnece of water
- Add anhydrous copper (II) sulfate to water
- Will turn from white to blue
How to test the purity of water
- Measure the boiling point
- Closer to 100c = more pure
How to test for CO2
- Bubble a sample of gas through limewater
- A white precipitate forms (goes cloudy)
How to test for CO2
- Bubble a sample of gas through limewater
- A white precipitate forms (goes cloudy)
What is the chemical reaction taking place when testing for CO2
- Limewater is saturated calcium hydroxide solution
- Calcium Carbonate is the white precipitate formed with water when reacting
How to test for chlorine
- Place damp blue litmus paper in a sample of the gas
- It wil be bleached white
What is the chemical reaction occuring, when testing for chlorine
- Chlorine dissolves into the dampness (water) and removes colour from blue litmus
- It is a good bleaching agent
How to test for ammonia
- Place damp red litmus paper in sample of gas
- It will turn blue
What is the chemical reaction taking place when testing for ammonia
- Ammonia acts as a base accepiting H+ ions from water
- OH- ions remain
- Alkaline so turns litmus blue
- Ammonia + water -> Ammonium + hydroxide
Describe how to conduct a flame test
- Dip a piece of nichrome / platinum wire into hydrochloric acid to clean the wire
- Dip the wire into the sample of solid or solution
- Place the tip of the wire into a non-luminous bunsen flame
Red flame test result
Li+
Yellow flame test result
Na+
Lilac flame test result
K+
Orange-red flame test result
Ca 2+
Blue-green flame test result
Cu 2+
Flame test result of Li+
Red
Flame test result for Na+
Yellow
Flame test result for K+
Lilac
Flame test result for Ca 2+
Orange-red
Flame test result for Cu 2+
Blue-green
How to carry out a NaOH test
- Dissolve sample in water
- Add a few drops of NaOH (aq)
Cu 2+ result - NaOH test
Blue precipitate forms
Fe 3+ result - NaOH test
Brown precipitate forms
Fe 2+ result - NaOH test
Green precipitate forms
How to carry out ammonium test
- Dissolve sample in water
- Add NaOH (aq) and warm it
- Test the gas evolved with red litmus paper
- Red litmus turns blue
What is the chemical reaction taking place when carrying out the ammonium test
Ammonium ions (from sample), and OH - ions (from NaOH) form ammonia
How to test for halides
- Dissolve the sample in water
- Add nitric acid
- Add silver nitrate
What ion gives a cream precipitate in silver nitrate test
Br -
What ion gives a white precipitate in the silver nitrate test
Cl -
What ion gives a yellow precipitate in the silver nitrate test
I -
Why must you add nitric acid when carrying out a silver nitrate test
- Ag + ions form precipitates with other anions
- The HNO3 reacts with those anions preventing false positives
Why must you add nitric acid when carrying out a silver nitrate test
- Ag + ions form precipitates with other anions
- The HNO3 reacts with those anions preventing false positives
How to test for a carbonate
- Add hydrochloric acid and look for effervescence
- If there is effervescene bubble the gas through limewater
- If ions are present, the gas evolved (CO2) turns limewater cloudy
Why must you bubble the gas evolved from a carbonate test through limewater
Acids evolve gases that aren’t CO2, so it prevents false positives
How to test for a sulfate
- Add hydrochloric acid and look for effervescence
- If there’s no effercescence add barium chloride solution
- If the ions are present, they will form a white precipitate with the Ba 2+ ions from the BaCl2
Why must you still add HCl to a sample testing for a sulfate ion
- Carbonate ions also form a white precipitate with Ba 2+ ions
- It prevents a false positive
Why must you bubble the gas evolved from a carbonate test through limewater
Acids evolve gases that aren’t CO2, so it prevents false positives
What is the chemical reaction occuring, when testing for chlorine
- Chlorine dissolves into the dampness (water) and removes colour from blue litmus
- It is a good bleaching agent
