Colour and chemistry reactions

ZnO solid colour

Yellow when heated, turns white when cooled

FeO solid

Black solid with yellow sparks given during heating

PbO

Orange when heated, yellow when cooled

Fe2O3 solid

Look reddish brown when heated

CuO solid

Black

K2Cr2O7 solid

Orange

NO2 gas

Brown

Br2 in room temperature

reddish brown liquid

Br2 dissolved in organic solvents

Orange

Br2 aq

Yellowish-brown

Br2 at high temperature

Reddish brown gas

I2 at room temperature

Black solid

I2 dissolved in organic solvents

Purple

I2 aq

Brown

I2 at high temperature

Purple gas

With these three cations, everything is soluble

Na+, K+, NH4+

With these two anions, everything is soluble

NO3-, HCO3-

Halide ions EG Cl-, I-, Br- are MOSTLY SOLUBLE except for….

Halides paired with Ag+ or Pb2+ (EG AgCl2 PbI2) are insoluble

Cmpds paired with SO4 2- sulphate are MOSTLY SOLUBLE except for….

BaSO4, PbSO4, CaSO4 => white precipitate

Cmpds with CO3 2- carbonate are MOSTLY INSOLUBLE except for

K2CO3, Na2CO3, (NH4)2CO3

Cmpds with O2- oxide are MOSTLY INSOLUBLE except

Na2O, K2O, CaO (quicklime), MgO

When Iodine is heated

Black solid sublimes (s → g) and purple vapour evolves

When NH4Cl (s) Ammonium chloride is heated

White solid sublimes (Heated NH4Cl turns into NH3 and HCl, but the two gases when cooled decompose back into NH4Cl)

NH4+ identification method

Add NaOH (aq). Upon warming, it gives off a gas with a pungent smell which turns moist red litmus paper blue

K+ identification method

Conduct flame test. Lilac flame observed

Na+ identification method

Conduct flame test. Golden yellow flame observed

Ca2+ identification method (not chemical methods only physical)

Conduct flame test. Brick-red flame

Ca2+ CHEMICAL identification method

Add NaOH (aq). white precipitates form (Ca(OH)2 slacked lime)

Chemical test applicable for both Ca2+ or Ba2+

Add dilute sulphuric acid, white precipitates are formed (Ca(OH)2, Ba(OH)2)

When NH3 (aq) is added to Ca2+, no observable change. WHY?

NH3 is a weak alkali which only partially ionizes in water to produce a low concentration of OH- → Only small amount of Ca(OH)2 (s) formed → Ca(OH)2 slightly soluble in water → dissolve in water → no precipitates

Zn2+ 2 identification methods

Add NaOH (aq). white precipitates are formed, will dissolve in excess NaOH to give a colourless solution [Zn(OH)4]2-（圓裏星）

Add NH3(aq). white precipitates are formed, precipitates will dissolve in excess NH3 aq to give a colourless solution [Zn(NH3)4]2+ aq (星級銀）

Al3+ identification method

Add NaOH(aq). White precipitates are formed (Al(OH)3), precipitates will dissolve in excess NaOH to give a colourless solution [Al(OH)4]- aq 圓裏星

All cations except for Zn2+, Cu2+, Ag+ when reacting with NH3 aq

White precipitates are formed

Pb2+ identification method

Add NaOH(aq). White precipitates are formed, the precipitates dissolve in excess NaOH to give a colourless solution (圓裏星）

Add dilute hydrochloric acid. White precipitates are formed. (PbCl2)

Add dilute sulphuric acid. White precipitates are formed (PbSO4)

Ag+ identification method

Add NaOH (aq). Brown precipitates are formed (Ag2O + H2O) (BUT DOESNT DISSOLVE IN EXCESS NAOH TO FORM COLOURLESS SOL)

Add NH3(aq). Brown precipitates are formed, the precipitates will then dissolve in excess NH3 to give a colourless solution (2[Ag(NH3)2]+ aq) (星級銀）

Cl-, Br-, I- ion identification method for all 3 ions

Add acidified silver nitrate solution/ excess dilute nitric acid → silver nitrate solution

Cl- forms white precipitates (AgCl)

Br- forms pale yellow precipitates (AgBr)

I- forms yellow precipitates (AgI)

Why do we use excess dilute nitric acid?

To dissolve and remove any unwanted precipitates (which are carbonates, sulphites, oxides and turn them into solutions)

Br-, I- identification method (the answer isn’t acidified silver nitrate sol)

Add aqueous chlorine, followed by the addition of an organic solvent (eg heptane) with shaking
Br-: First, the bromide solution changes from colourless to yellow-brown. Then, the organic layer becomes orange.

I-: First, the iodide solution changes from colourless to brown. Then, the organic layer becomes purple.

CO3 2- Carbonate ion Identifying method

Add dilute acids (EG HCl). Colourless gas will be given out, which turns limewater milky.

Strong heating the solid (Decomposition method). Colourless gas will be given out, which turns limewater milky. (Exceptions: Na+ and K+)

HCO3- hydrogencarbonate ion identifying method

Add dilute acids (EG HCl). Colourless gas will be given out, which turns limewater milky.

Strong heating. Colourless gas will be given out, which turns limewater milky

Identifying Na2CO3 (aq) and NaHCO3 (aq)

Add CaCl2 (aq). Na2CO3: White precipitates are observed (CaCO3). NaHCO3: no observable change

Sulphate SO4 2- identification

Add acidified BaCl2/CaCl2 solution. White precipitates form (CaSO4)

OCl- identification

Add dilute HCl(aq). A greenish gas will evolve, which turns moist blue litmus paper red, then white (OCl is a bleaching agent, which turns coloured litmus paper white)

Sulphite SO3 2- identification

Add acidified dichromate solution. Solution turns from orange to green

NH3 (g) Ammonia obs + characteristics + tests

obs: pungenet smell

characteristics: soluble, toxic (→ fumehood)
test: Add onto a piece of moist red litmus paper. Turns the paper blue.

Place NH3 (g) near conc HCl (aq) → Dense white fumes (NH4Cl)

CO2 test

Bubble the gas into limewater. Limewater turns from colourless to milky.

Excess CO2 in limewater

the solution changes from milky to colourless again

Cl2 Chlorine obs + characteristics + test (rmb because Cl2 colour is very light, hard to see)

obs: greenish yellow gas, pungent and choking smell
characteristics: toxic (→ fumehood)
test: Add onto a piece of moist blue litmus paper. Paper turns red then white (OCl- bleaching action).

I2 Iodine obs + test

obs: purple gas, pungent smell

test: moist starch paper. paper turnsdark blue

O2 characteristics + test

char: slightly soluble
test: glowing splint test. oxygen relights the glowing splint

H2 characteristics + test

char: flammable, insoluble
test: burning splint test. The burning splint burns with a ‘pop’ sound

HCl (g) characteristics + test

char: pungent, corrosive, soluble
Test: Place near NH3(g). Dense white fumes form (NH4Cl)

SO2 (g) characteristics + test (photo needed)

char: choking smell of burning sulphur, toxic
test:

Add onto filter paper soaked with acidified potassium dichromate solution. The paper changes from orange (Cr2O7 2-) to green (Cr3+).

Add onto filter paper soaked with Br2 (aq). The paper turns from yellow-brown to colourless

Add onto filter paper soaked with acidified potassium permanganate solution. The paper changes from purple to colourless.

Bubble the gas into limewater. It turns limewater milky. (CaSO3(s), this eq used in scrubbers)

H2O(g)/(l) test

Test with anhydrous blue cobalt(II) chloride paper. The paper turns from blue to pink.

Test with anhydrous copper(II) sulphate powder. The powder turns from white to blue.

C=C alkenes identification method

Add bromine dissolved in organic solvents in the dark. Orange to colourless.
Use cold and dilute acidified potassium permanganate solution. Purple to colourless.
Use cold and dilute alkaline potassium permanganate solution. Purple to colourless with brown precipitates (MnO2).

-OH Alcohol identification method

For primary and secondary: Heating with acidified potassium dichromate solution. Solution changes from orange to green
Tertiary alcohols: no observable change

Heating with carboxylic acid under reflux in the presence of concentrated sulphuric acid. Gives a sweet fruity smell (ester) (tertiary alcohols form ester too)

Both ketone C=O and aldehyde’s CHO identification method

Add 2,4-dinitrophenylhydrazine. Yellow/red/orange precipitates formed (write either one)

identification method between ketone and aldehyde

Heating with acidified potassium dichromate solution under reflux. ketone: no observable change. aldehyde: orange to green

Warming with Tollens’ reagent (some silver nitrate + ammonia sol). Aldehyde: A silver mirror forms on the inner wall of the test tube. Ketone: no observable change

Aldehyde CHO

Warming with Tollens’ reagent (some silver nitrate + ammonia sol). A silver mirror forms on the inner wall of the test tube

COOH carboxylic acid identification method

Add sodium carbonate sol/ metal. Colourless gases given out (CO2)
Heating with alochols under reflux in the presence of concentrated sulphuric acid. Gives a sweet fruity smell (ester)

2 methods for Cu2+ chemical identification

Add dilute NaOH (aq). Pale blue precipitates formed

Add NH3(aq). Pale blue precipitates are formed. The precipitate will redissolve in excess NH3(aq) to give a dark blue solution. (Cu(NH3)4)2+) (aq)