P2-Topic 8- 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

how to identify a pure substance 2

Answer 2

it has specific melting and boiling points at specific temperatures

this would be different to mixtures and formulations e.g. pure milk

Question 3

What is a formulation and how is it made?

Answer 3

• A formulation is a mixture of compounds in measured quantities that has been
  designed as a useful product.

Question 4

examples of formulations 2

Answer 4

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 5

how to identify a pure substance in chromatography

Answer 5

a pure compound will produce a single spot in all solvents

Question 6

Describe paper chromatography. 7

Answer 6

1. A start line is drawn 1 - 2 cm from the bottom
2. use a pipette or capillary tube to add small spots mixture is spotted on the line.
   b) A beaker is filled with small amount of solvent (not touching start line on beaker)
   c) Paper is hung on a rod and placed in a beaker.
   d) Solvent travels up the paper, 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.

Question 7

why does chromatography work 3

Answer 7

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 8

How is Rf calculated?

Answer 8

Distance moved by the spot / distance moved by solvent

Question 9

n 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 9

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

Question 10

Describe the tests for
hydrogen, oxygen, carbon
dioxide and chlorine 4

Answer 10

Hydrogen – squeaky 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 11

Describe the flame test results 5

Answer 11

Li- crimson red flame

Na - yellow flame

K - lilac flame

Ca – orange/red flame

Cu -green flame.

Question 12

Describe the sodium

hydroxide test results for copper(II), iron(II) and iron(III) ions is added 5

Answer 12

Copper(II)= blue precipitate
Cu2²+ + 2OH− →Cu(OH)₂

iron(II)=a green precipitate
Fe²+ + 2OH− → Fe(OH)₂

iron(III) a brown precipitate.

Question 13

Describe the sodium hydroxide test results and write the equations with aluminium, calcium and magnesium 4

Answer 13

Al3+, Ca2+, Mg2+ form white precipitates
but only the Al(OH)3
dissolves in excess
NaOH to form a colourless solution.

Ca² + 2OH−→ Ca(OH)₂

Mg²+ + 2OH−→ Mg(OH)₂

Al³+ + 3OH− → Al(OH)₃

Question 14

Describe the test for carbonate anions 4

Answer 14

-Add dilute acid, e.g. HCl
-Fizzing observed, as CO₂
is released.
-bubble the gas through lime-water
-if goes cloudy proves it has CO₂
-started with carbonate ion

E.g. Na₂CO₃ + 2 HCl → 2 NaCl + H₂O + CO₂

Question 15

Describe the test for sulfate

anions 3

Answer 15

• add dilute HCl to sample
• add barium chloride solution
• if sulfate ions present we will see a white precipitate

E.g. K₂SO₄ + BaCl₂ → 2KCl + BaSO₄

Question 16

Describe the test for halide

anions 3

Answer 16

• add dilute nitric acid to sample with silver nitrate in the solution
• halide ions produce a precipitate with silver halide

chloride=white
bromide = cream
iodide= yellow

Question 17

Describe the precipitate with halide anions3

Answer 17

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 18

What are instrumental methods?4

Answer 18

accurate,
sensitive
rapid methods

which are useful when the amount of sample is very small

Question 19

Describe the flame emission spectroscopy3

Answer 19

● Example of an instrumental method used to analyse metal ions in solutions
● Sample is put into a flame and the light given out is passed through a
spectroscope
● Output is a line spectrum that can be analysed to identify the metal ions in the
solution and measure their concentrations