Chemical Analysis

Question 1

What is a pure substance?

Answer 1

A pure substance is a single element or compound, not mixed with any other substance

Question 2

What is a formulation and how is it made? Give examples

Answer 2

• A formulation is a mixture of compounds in measured quantities that has been designed as a useful product.
• Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
  -E.g. alloys are mixtures of metals; they are harder than pure metals, so have a particular purpose.

Question 3

Describe paper chromatography.

Answer 3

a) Astart line is drawn near the bottom of the paper. The mixture is spotted on the line.
b) A beaker is filled with small amount of solvent (it cannot touch or go above the start line when paper is placed in a beaker)
c) Paper is hung on a rod and placed in a beaker.
d) Solvent travels up the paper, thus separating the components.
e) Before solvent level reaches the end, the paper is taken out and the finish line is marked. The paper is dried.
f) The procedure works when different compounds have different affinities for the solvent/paper. Stronger attraction for the paper - travels slowly with the solvent etc.
g) Paper is called the stationary phase - it doesn’t move. Solvent is the mobile phase.

Question 4

How is R, calculated?

Answer 4

Distance moved by the spot / distance moved by solvent

Question 5

In a paper chromatography experiment, a compound A was found to have an R, value of
0.85 - what does it tell you about the compound?

Answer 5

It has a higher affinity for the solvent than for the paper.

Question 6

impure substances …

Answer 6

melt and boil over a range of temps

Question 7

Formulations

Answer 7

Formulations are mixtures that have been prepared using a specific formula.

Question 8

How do you test for chlorine gas?

Answer 8

Expose it to damp blue litmus paper
The damp litmus paper turns red, then bleaches white

Question 9

difference between pure and impure substance

Answer 9

pure melt and boil at specific temps
impure melt and boil over a range

Question 10

mobile phase in chromatography

Answer 10

molecules can move in it
- a liquid or gas
- solvent

Question 11

stationary phase in chromatography

Answer 11

molecules can’t move
- solid or thick liquid
- paper

Question 12

carbonat test

Answer 12

CARBONATE + ACID—>
SAlT + CARBON DIOXIDE + WATER

Question 13

how to test for carbonates

Answer 13

add dilute hydrochloric acid

any gas produced run through lime water

if carbonate present lime water will turn cloudy

Question 14

how to test for sufate ions

Answer 14

dilute hydrochloric acid, to remove impurities

add barium chloride solution

white precipitate = sulphate present

Question 15

how to test for halide ions

Answer 15

add dilute nitric acid to remove impurities

add silver nitrate

colour precipitate depends on halide

Question 16

halide ion colour tests

chloride ions
bromide
iodide

Answer 16

chloride - white precipitate
bromide- cream precipitate
iodide - yellow precipitate

Question 17

how to do flame tests

Answer 17

wire loop - clean it by dipping it in some dilute hydrochloric acid, rinsing it in distilled water, and then heating it over a Bunsen burner flame.

Dip the wire loop into the compound you want to test.

Hold the wire loop in the clear blue part of the Bunsen burner flame (this is the hottest part)

See what colour the flame turns as the compound burns.

Question 18

lithium

Answer 18

crimson

Question 19

sodium

Answer 19

yellow

Question 20

potassium

Answer 20

lilac

Question 21

calcium

Answer 21

orange red

Question 22

copper

Answer 22

green

Question 23

sodium hydroxide and Calcium

Answer 23

= white precipitate

Question 24

sodium hydroxide and Iron 11

Answer 24

= green precipitate

Question 25

sodium hydroxide and Iron 111

Answer 25

= brown precipitate

Question 26

sodium hydroxide and magnesium

Answer 26

= white precipitate

Question 27

sodium hydroxide and aluminium

Answer 27

white precipitate that redissolves to colourless solution

Question 29

Describe the tests for hydrogen, oxygen, carbon dioxide and chlorine

Answer 29

Hydrogen - pop with burning splint over gas
Oxygen - glowing splint relights
Carbon dioxide - turns limewater (Ca(OH),) milky
Chlorine - bleaches damp litmus paper and makes it white

Question 30

Describe the flame test results

Answer 30

Lithium compounds- crimson red flame
Sodium compounds - yellow flame
Potassium compounds - lilac flame
Calcium compounds - orange red flame
Copper compounds -green flame.

Question 31

Describe the test for carbonate anions

Answer 31

Add dilute acid, e.g. HCI
Fizzing observed, as CO, is released.
E.g. Na,CO, + 2 HCI -> 2 NaCI + H,° + CO,

Question 32

Describe sulfate tests and give an equation

Answer 32

Add a solution containing Ba?* cations, e.g. a solution of BaCl
White precipitate of BasO, forms
E.g. K,SO, + BaCl, -* 2 KCI + Baso,
(!!!) can also be thought of a test for barium (Il); add sulfates - white precipitate forms.

Question 33

Describe the test for halide anions

Answer 33

Add a solution of AgNO (acidified with HNOz)
Chlorides - white precipitate, silver chloride; Ag* + CI-› AgCI

Bromides - cream precipitate, silver bromide; Ag* + Br~ - AgBr

lodides - yellow precipitate, silver iodide; Ag* + I -* Agl

Question 34

What are instrumental methods?

Answer 34

They are accurate, sensitive and rapid methods which are useful when the amount of sample is very small

Question 35

Describe the flame emission spectroscopy

Answer 35

a) instrumental method used for identifying metal ions in solution or measuring their concentration
b) Spectroscope measures the exact wavelength of the light emitted by a metal ion
c) That allows for definite identification - sometimes colours are difficult to distinguish.
d) Concentrations are found by measuring the intensity of light emitted. The more intense light, the greater the concentration of the metal ion in a solution.
e) From the intensity vs concentration graph, you can read off a relevant concentration value at a given intensity.