CHEM: What is this?

Describe a test for hydrogen gas.

Describe a test for hydrogen gas.

Place a lit splint into test tube

Hydrogen burns with a distinct squeaky pop sound

Describe a test for carbon dioxide gas.

Calcium carbonate in dilute hydrochloric acid will bubble

Bubble gas through limewater (calcium hydroxide)

Limewater should turn cloudy (calcium carbonate)

How would you test for a cation?

Using sodium hydroxide

Test and positive result for Cu2+

Put cation solution in test tube and add few drops of sodium hydroxide NaOH.

Blue precipitate forms.

Equation for Cu2+ and NaOH

CuSO4 (aq) + 2NaOH (aq) → Cu(OH)2 (s) + Na2SO4 (aq)

Copper sulphate + 2 sodium hydroxide → copper hydroxide + sodium sulphate

Test and positive result for Fe2+

Add sodium hydroxide to cation solution

Green precipitate forms (turns brown if left as Fe2+ is oxidised by air to Fe3+)

Equation for Fe2+ and NaOH

FeSO4 (aq) + 2NaOH (aq) → Fe(OH)2 + Na2SO4(s)

Test and positive result for Fe3+

Add sodium hydroxide to cation solution

Brown precipitate forms

Equation for Fe3+ and NaOH

FeCl3 (aq) + 3NaOH (aq) → Fe(OH)3 (s) + 3NaCl (aq)

Flame test description

1. Clean piece of nichrome wire by dipping end into hydrochloric acid

2. Dip wire into small sample of solid being tested

3. Place wire into hottest part of non-luminous Bunsen flame

4. Record colour observed

Test and positive result for (solid) Li+

Red

Positive result for (solid) Na+

Yellow

Positive result for (solid) K+

Lilac

Positive result for (solid) Ca2+

Orange-red

Positive result for (solid) Cu2+

Blue-green

Test and positive result of ammonium cation.

Ammonium NH4+

1. Solution being tested in test tube

2. Add few drops sodium hydroxide

3. Carefully heat the test tube

4. Gas is given off

5. Place small piece of damp red litmus paper at mouth of test tube

6. Damp red litmus paper turns blue.

What happens when the ammonia gas NH3 touches the damp red litmus paper?

It dissolves in the water to form NH4+ and OH-

OH- ions are alkaline so turns the litmus paper blue

Test for halide anions (chloride, bromide, iodide)

Add a few drops of nitric acid

Then a few drops of silver nitrate

Why do you add some nitric acid before testing for halide anions?

To remove any carbonate CO3 2- ions, because Ag2CO3 is also a white precipitate.

Positive result for Cl- after adding nitric acid and silver nitrate

White precipitate (AgCl)

Positive result for Br-

Cream precipitate (AgBr)

Positive result for I-

Yellow precipitate (AgI)

Describe a test and positive result for sulphate SO4 2- anions.

1. Add few drops of hydrochloric acid

2. Then add a few drops of barium chloride solution

3. White precipitate (BaSO4) means positive result.

Test for chlorine gas

If chlorine gas is present, damp blue litmus paper turns red initially (as acids are produced when chlorine reacts with the water) but then is bleached white.

Describe a test for oxygen gas.

Place a glowing splint in test tube

Splint should relight