Testing Inorganic Compounds - Practical Notes

Testing Inorganic Compounds FLAME TESTS To carry out the flame test, a clean nichrome wire is used to mix a sample of the solid with acid. The wire is held in progressively hotter parts of the Bunsen burner. (blue flame – cleaner) FLAME COLOUR

INFERENCE

Yellow

Sodium ion

Lilac

Potassium ion

Brick-red

Calcium ion

Red*

Lithium or Strontium ion

Pale-green

Barium ion

HEATING Gases or vapours may be evolved on heating the solid compound. GAS OR VAPOUR

POSSIBLE SOURCE

Carbon dioxide

Carbonates of metals other than Group (1)

Oxygen

Group (1) nitrates (other than 2)

Oxygen & Nitrogen dioxide

Nitrates (other than Na or K)

Water vapour

Hydrated salts

RECOGNITION & IDENTIFICATION OF COMMON GASES GAS

OBSERVATION

Oxygen

Colourless – lights a glowing splint

Carbon dioxide

Colourless – gives white ppt with limewater (Calcium hydroxide solution)

Ammonia

Colourless – turns moist red litmus paper blue & forms white smoke with HCl

Nitrogen dioxide

Brown gas

Hydrogen

Colourless – ignites with a pop sound

Hydrogen chloride

Steamy fumes on exposure to moist air, acidic & forms white smoke with NH3

Chloride

Pale green gas – bleaches moist litmus paper

Bromide

Brown gas

Iodide

Purple vapour

Water vapour

Turns blue Cobalt chloride paper into pink

ACTION OF DILUTE ACIDS When dilute Sulphuric acid or Hydrochloric acid is added to a substance, a gas may be evolved or there may be a colour change. ACTION OF ACID

POSSIBLE SOURCE

Carbon dioxide evolved

Carbonate

Hydrogen evolved

Metal

HYDROGEN PEROXIDE SOLUTION (H2O2) Aqueous H2O2 can act as both oxidizing and reducing agent often with an evolution of Oxygen although this may be unreliable. OBSERVATION (ON ADDING H2O2)

INFERENCE

Decolourized

Manganate (VII) in acid solution

Pale green solution turns yellow

Iron (II) is turning to Iron (III) in acid solution

ANIONS BARIUM CHLORIDE SOLUTION Aqueous BaCl2 forms precipitates of insoluble Barium salts with a number of anions but is usually a test for Sulfate ions. Aqueous BaCl2 is used with dilute HCl. ANION

PRECIPITATE

ADDITION OF DILUTE HCl

COLOUR

FORMULA

Sulfate

White

BaSO4

ppt is insoluble

Sulfite

White

BaSO3

ppt dissolves

Carbonate

white

BaCO3

ppt dissolves with effervescence (CO2)

If dilute HCl is added to the anion solution before aqueous BaCl2, then only Sulfate will form as ppt. SILVER NITRATE SOLUTION Aqueous AgNO3 is commonly used to test for the presence of halide ions in a solution. Anions which would interfere with the test (Carbonate) are removed by adding dilute HNO3 acid before the aqueous AgNO3. Silver halides dissolve in NH3 so as to form a colourless solution of the complex ion [Ag(NH3)2]+

ANION

PRECIPITATE

ADDITION OF AQ. NH3

COLOUR

FORMULA

DILUTE

CONCENTRATED

Chloride

White

AgCl

Soluble

Soluble

Bromide

Cream

AgBr

Soluble

Soluble

iodide

yellow

AgI

Insoluble

Insoluble

When a few drops of concentrated H2SO4 acid is added to a halide, the dissolved reaction can be used to identify the particular ion present. This is a potentially hazardous reaction.   

It must be carried out in a small scale & in a fume cupboard. The products in the brackets will not be observed since they are colourless gases. The halide ion may be identified without the need for these gases. No attempt should ever be made to detect these gases by smell. HALIDE Chloride

Bromide

iodide

OBSERVATION ON ADDING CON. H2SO4

OBSERVED REACTION PRODUCTS

Steamy fumes

HCl

Vigorous reaction Steamy fumes

HBr

Brown vapour

Br2

Vigorous reaction

(SO2)

Steamy fumes

HI

Black solid

I2

Purple vapour Yellow solid

Sulfur

Vigorous reaction

(H2S) – smell of rotten eggs