Chemistry -Chemical Tests

Question

Answer

Test for ammonia gas

Warm with sodium hydroxide → damp red litmus turns blue

Test for carbon dioxide

Bubble through limewater → turns milky

Test for chlorine gas

Damp blue litmus turns red then bleached white

Test for hydrogen gas

Burning splint makes a squeaky pop

Test for oxygen gas

Glowing splint relights

Steps for flame test

Clean nichrome wire with HCl → dip in sample → place in blue Bunsen flame → observe colour

Flame test colour for potassium (K+)

Lilac

Flame test colour for sodium (Na+)

Yellow

Flame test colour for copper (Cu2+)

Blue-green

Flame test colour for lithium (Li+)

Red

Flame test colour for calcium (Ca2+)

Orange-red

Test for ammonium ion (NH4+)

Warm with NaOH → ammonia gas released → turns red litmus blue

Test for Cu2+ with NaOH

Blue precipitate of Cu(OH)2

Test for Fe2+ with NaOH

Green precipitate of Fe(OH)2 (turns brown on standing)

Test for Fe3+ with NaOH

Brown precipitate of Fe(OH)3

Test for Ca2+ with NaOH

White precipitate of Ca(OH)2

Test for carbonate (CO3 2-)

Add HCl → effervescence

Test for sulfate (SO4 2-)

Add HCl then BaCl2 → white precipitate of BaSO4

Test for chloride (Cl-)

Add HNO3 then AgNO3 → white precipitate

Test for bromide (Br-)

Add HNO3 then AgNO3 → cream precipitate

Test for iodide (I-)

Add HNO3 then AgNO3 → yellow precipitate

Solubility of nitrates

All soluble

Solubility of chlorides

All soluble except silver and lead(II)

Solubility of sulfates

All soluble except barium

Solubility of carbonates

Only sodium

Solubility of hydroxides

Only sodium

Group 7 halogens charge

Form -1 ions

Reactivity trend in halogens

Decreases down the group

Why halogens decrease in reactivity down the group

Outer shell further from nucleus

Properties of halogens

Non-conductors

Colour and state of fluorine at room temp

Pale yellow gas

Colour and state of chlorine at room temp

Green gas

Colour and state of bromine at room temp

Brown liquid

Colour and state of iodine at room temp

Grey solid

General reaction of metals with halogens

Metal + halogen → metal halide (salt)

Example equation of Fe and Cl2

2Fe + 3Cl2 → 2FeCl3

Which halogen is the strongest oxidising agent?

Chlorine (highest in the group)

Displacement rule for halogens

A more reactive halogen displaces a less reactive halogen from a salt

Displacement: chlorine into NaBr

Cl2 + 2NaBr → 2NaCl + Br2

Ionic equation for chlorine displacing bromide

Cl2 + 2Br- → 2Cl- + Br2

Reduction half equation for chlorine

Cl2 + 2e- → 2Cl-

Oxidation half equation for bromide

2Br- → Br2 + 2e-

Observation when chlorine added to KI solution

Solution turns reddish brown

Observation when bromine added to KI solution

Solution turns brown

Observation when iodine added to halide solution

No change