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 have 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 start line is drawn near the bottom of the paper, the mixture is spotted on the line
• 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)
• paper is hung on a rod and placed in a beaker
• solvent travels up the paper, thus separating the components
• before solvent level reaches the end, the paper is taken out and the finish line is marked, the paper is dried.
• the procedure works when different compounds have different affinities for the solvent/paper, stronger attraction for the paper - solvent travels slowly with the solvent etc.
• paper is called the stationary phase - it doesn’t move
• solvent is the mobile phase

Question 4

How is Rf 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 Rf 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

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

Answer 6

hydrogen - pop with burning splint over gas
oxygen - glowing splint relights
carbon dioxide - turns limewater (Ca(OH)2) milky
chlorine - bleaches damp litmus paper and makes it white

Question 7

Describe the flame test results

Answer 7

lithium - crimson red flame
sodium - yellow
potassium - lilac
calcium - orange red
copper - green

Question 8

Describe the sodium hydroxide test results

Answer 8

copper(II) forms a blue precipitate
iron(II) forms a dirty green precipitate
iron(III) forms a brown precipitate
Aluminum, calcium and magnesium form white precipitates but only Al dissolves in excess NaOH to form a colourless solution

Question 9

Describe the test for carbonate anions

Answer 9

• add dilute acid, e.g. HCl
• fizzing observed, as CO2 is released
• e.g. Na2CO3 + 2HCl –> 2NaCl + H2O + CO2

Question 10

Describe sulfate tests and give an equation

Answer 10

add a solution containing Ba cations, e.g. a solution of BaCl2
white precipitate of BaSO4 forms
(!!!) can aso be thought of a test for barium (II); add sulfates - white precipitate forms

Question 11

Describe silver nitrate tests and give equations

Answer 11

add a solution of AgNO3 (acidified with HNO3)
chlorides - white precipitate, silver chloride; Ag + Cl –> AgCl
bromides - cream precipitate, silver bromide; Ag + Br –> AgBr
iodides - yellow precipitate, silver iodide; Ag + I –> AgI

Question 12

What are instrumental methods?

Answer 12

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

Question 13

Describe the flame emission spectroscopy

Answer 13

• instrumental method used for identifying metal ions in solution or measuring their concentration
• spectroscope measures the exact wavelength of the light emitted by a metal ion
• that allows for definite identification - sometimes colours are difficult to distinguish
• concentrations are found by measuring the intensity of light emitted, the more intense the light, the greater the concentration of the metal ion in a solution
• from the intensity vs concentration graph, you can read off a relevant concentration value at a given intensity

Question 14

what are cations and anions?

Answer 14

cations are the positive ions, anions are the negative ions