C4.2 Flashcards
Flame test colours
Sodium - yellow Lithium - red Copper - green blue Calcium - orange red Potassium - lilac
Hydroxide precipitate colours
Iron (II) - green Iron (III) - orange brown Copper - blue Calcium - white Zinc - white BUT if excess NaOH is added then turns colourless
How to detect sulfate ions?
Add a few drops of hydrochloric acid and then add a few drops of BaCl2 - if sulfate ions present then white barium sulfate precipitate formed
HCl is used BECAUSE CARBONATE IONS ALSO PRODUCE A WHITE PRECIPITATE WITH BARIUM CHLORIDE SO ACID REACTS WITH ANY CARBONATE IONS PRESENT TO REMOVE THEM AND STOP GIVING A FALSE POSITIVE RESULT
How to detect carbonate ions?
Add dilute Hydrochloric acid and bubble the gas into limewater - if it turns cloudy then carbonate ions are present
Halide ions
Add a few drops of nitric acid and then a few drops of silver nitrate solution (AgNO3) - if ions present then you get a halide precipitate
Chlorine - white
Bromine - cream
Iodine - yellow
Acidified with dilute nitric acid as carbonate ions also produce a white precipitate with silver nitrate solution - acid reacts with any carbonate ions removing them and stopping any incorrect positive result for chloride ions to be shown
Advantages of using instrumental methods of analysis
Sensitivity
Accuracy
Speed
Test for oxygen
Glowing splint should relight - combustion
Test for hydrogen
Ignites in air - if hydrogen present then a lighted splint near its mouth should ignite with a squeaky pop
Test for carbon dioxide
CO2 reacts with calcium hydroxide solution to produce a white precipitate of calcium carbonate - limewater is a solution of calcium hydroxide ; limewater turns milky or cloudy white if CO2 present
Test for chlorine
Damp blue litmus paper turns red then white (bleached)
How to carry out a flame test?
Clean in chrome wire loop several times by dipping it in HCl and rinsing with distilled water
Dip clean loop into test powder or solution
Use handle to hold loop in the edge of roaring blue flame
Record flame colour
Clean wire loop to prevent contamination
Science behind flame test
When heated electrons of ions move into higher electron shells and when they move back energy given off as radiation which is seen as light
Hydroxide precipitate science
Group 1 hydroxides are soluble - dilute sodium hydroxide reacts with metal ions to form insoluble metal hydroxides and many of these are insoluble forming precipitates - sodium hydroxide + solution containing metal ions and then note colour of precipitate
Zinc vs calcium hydroxide
Both form white precipitate but if excess sodium hydroxide added then calcium hydroxide remains unchanged but zinc hydroxide precipitate dissolves to form a colourless solution
