Chemical Analysis

Question 1

what do flame tests test

Answer 1

positive ions (cations) - metal ions

Question 2

steps for flame test practical

Answer 2

1. Dip the platinum wire into concentrated HCl
2. Place the wire in the hottest part of the flame
3. Repeat stages 1 & 2 until no colour is produced
4. Dip the wire in concentrated HCl
5. Place the wire into the chemical to be analysed
6. Place the wire in the hottest part of the flame of bunsen burner
7. Record colour produced

Question 3

Why do we use a platinum wire

Answer 3

unreactive, doesn’t colour flame, high mpt

Question 4

what type of flame do we use for the bunsen burner

Answer 4

a blue non-cone flame

Question 5

Flame Test - Lithium

Answer 5

crimson red

Question 6

Flame Test - Sodium

Answer 6

yellow/orange

Question 7

Flame Test - Potassium

Answer 7

lilac

Question 8

Flame Test - Calcium

Answer 8

brick red

Question 9

Flame Test - Copper

Answer 9

green-blue

Question 10

Flame Test - Barium

don’t have to know

Answer 10

apple green

Question 11

Flame Test - Strontium

don’t have to know

Answer 11

red

Question 12

what is the NaOH test used for

Answer 12

A coloured precipitate is produced when NaOH solution is added to a solution containing other positive ions

Question 13

Steps for NaOH test

Answer 13

1. Add no more than 1cm³ of chosen ion solution to test tube
2. Record colour of solution
3. Add no more than a few drops of NaOH solution to this test tube
4. Record observations

Question 14

how does the test differ for Ammonium

Answer 14

you will need to heat the resulting solution gently and insert a piece of damp red litmus paper

Question 15

how does the test differ for Al and Ca

Answer 15

add excess NaOH to resulting solution to distinguish between them

Question 16

NaOH test - Copper (II)

Answer 16

blue solution -> blue ppt

Question 17

NaOH test - Iron (II)

Answer 17

clear solution -> green ppt

Question 18

NaOH test - Iron (III)

Answer 18

orange solution -> orange/brown ppt

Question 19

NaOH test - Ammonium

Answer 19

NH3 gas produced - turns damp red litmus blue

Question 20

NaOH test - Aluminium

Answer 20

clear solution -> white ppt -> clear solution

turns back to clear solution when you add excess NaOH

Question 21

NaOH test - Calcium

Answer 21

clear solution -> white ppt

doesn’t turn back to clear solution even after excess NaOH is added

Question 22

Sulphate test steps

Answer 22

1. Add a few drops of HCl acid to solution of BaCl2
2. Add a few drops of acidified BaCl2 to unknown substance
3. Record observations

after step 1, BaCl2 is now acidified BaCl2

Question 23

what is the positive result for the Sulphate test

Answer 23

a white ppt is formed (BaSO4)

Question 24

Why is HCl added to BaCl2 solution

Answer 24

to prevent formation of other unwanted (white) ppts that give a false positive

e.g. BaCO3

Question 25

Halide ion test steps

Answer 25

1. Add a few drops of HNO3 to AgNO3 solution 2. Add no more than 1cm³ of the halide to a test tube 3. Carefully add a few drops of the acidified AgNO3 solution to your halide 4. Record observations | after step 1, AgNO3 is now acidified AgNO3

Question 26

Halide ion test - Chloride

Answer 26

white precipitate | AgCl

Question 27

Halide ion test - Bromide

Answer 27

cream precipitate | AgBr

Question 28

Halide ion test - Iodide

Answer 28

yellow precipitate | AgI

Question 29

why is nitric acid added to AgNO3 solution

Answer 29

to prevent any unwanted ppts that may form | e.g. Ag2CO3

Question 30

Carbonates test steps

Answer 30

1. Place 1cm³ of your "carbonate" (or 1 spatula if solid) into test tube 2. Add a delivery tube to it to another test tube 3. To the second test tube, fill it up with limewater up to where the delivery tube reaches 4. Add some acid to first test tube e.g. HCl 5. Place bung on top of first test tube 6. Record results

Question 31

positive result of Carbonate test

Answer 31

effervescence | if passed through limewater, it turns cloudy

Question 32

what is limewater

Answer 32

Ca(OH)2 (aq)

Question 33

reaction of carbonate and acid

Answer 33

produces salt, water, carbon dioxide

Question 34

what makes the limewater go cloudy

Answer 34

CaCO3 - precipitate | formed from reaction of limewater and CO2

Question 35

what would you see if CO2 was bubbled through limewater for a longer period of time

Answer 35

turns colourless again

Question 36

why does limewater turn colourless again if CO2 was bubbled through it for a longer period of time

Answer 36

CaCO3 + H2O + CO2 -> Ca(HCO3)2 (aq) | Ca(HCO3)2 is soluble so solution becomes colourless again

Question 37

why may effervescing stop after a few seconds when acid is added to carbonate

Answer 37

reaction forms precipitate which coats any remaining carbonate therefore preventing further reaction with acid meaning no more CO2 is produced so no more effervescence.