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Chemistry > Qualitative Analysis > Flashcards

Qualitative Analysis Flashcards

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1
Q

What are the tests for cations?

A

flame test or precipitation with sodium hydroxide

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2
Q

Describe how you would carry out a

Flame Test.

A
  • wear safety goggles and a lab coat
  • work in a fume cupboard
  • dip nichrome wire in concentrated hydrochloric acid to clean
  • heat in a roaring Bunsen flame
  • add a few drops of concentrated hydrochloric acid to the solid salt
  • mix until the solid starts to dissolve
  • dip a clean nichrome wire into the mixture
  • hold the wire in a roaring Bunsen flame
  • observe the colour
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3
Q

What happens inside an atom during a

flame test?

A
  • the electrons gain energy as heat
  • they release energy straight away as light
  • each atom has electrons at different energy levels so will release a specific light wavelength
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4
Q

What is the flame colour of Lithium ion?

(Li+)

A

bright red

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5
Q

What is the flame colour of Sodium ion?

(Na+)

A

yellow

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6
Q

What is the flame colour of Potassium ion?

(K+)

A

lilac

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7
Q

What is the flame colour of Calcium ion?

(Ca2+)

A

brick red

/orange-red

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8
Q

What is the flame colour of Copper ion?

(Cu2+)

A

blue-green

(green with blue flecks)

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9
Q

What is the flame colour of Strontium ion?

(Sr2+)

A

deep red

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10
Q

What is the flame colour of Barium ion?

(Ba2+)

A

apple green

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11
Q

What is the flame colour of Magnesium ion?

(Mg2+)

A

bright white

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12
Q

What would be the expected observation if aqueous sodium hydroxide was added to a solution containing Aluminium ions?

(Al3+)

A

A white precipitate is formed

(it does redissolve in excess)

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13
Q

What would be the expected observation if aqueous sodium hydroxide was added to a solution containing Calcium ions?

(Ca2+)

A

A white precipitate is formed.

It does not redissolve in excess.

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14
Q

What would be the expected observation if aqueous sodium hydroxide was added to a solution containing Copper ions?

(Cu2+)

A

A blue precipitate is formed.

(similar to the colour of the solution)

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15
Q

What would be the expected observation if aqueous sodium hydroxide was added to a solution containing Iron(II) ions?

(Fe2+)

A

A green precipitate is formed.

The precipitate would go brown over time due to oxidation with the air.

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16
Q

What would be the expected observation if aqueous sodium hydroxide was added to a solution containing Iron(III) ions?

(Fe3+)

A

A red-brown precipitate is formed.

The precipitate goes more brown as it oxidises with the air.

17
Q

What would be the expected observation if aqueous sodium hydroxide was added to a solution containing ammonium.

(NH4+)

A

No precipitate is formed.

18
Q

Describe how you would test for

halide ions.

(Cl-, Br-, I-)

A
  • add a few drops of nitric acid (to remove any carbonate or hydroxide impurities which could produce false positives)
  • add aqueous silver nitrate solution
  • a precipitate indicates the presence of a silver halide, observe the colour of this precipitate to determine what anions are present
19
Q

What colour precipitates do halides Cl-, Br- and I- form with silver nitrate?

A

Chlorine forms silver chloride (AgCl) which is white
Bromine forms silver bromide (AGBr) which is cream
Iodine forrms silver iodide (AgI) which is pale yellow

The precipitates go darker (and more yellow) as you go down the column.
The precipitates go grey over time afer reacting with UV light.

20
Q

Describe how you would test for

carbonate ions.

(CO32-)

A
  • add dilute nitric acid
  • bubbles indicate the presence of carbonate
21
Q

Describe how you would test for

sulfate ions.

(SO42-)

A
  • add a few drops of hydrochloric acid
  • add aqueous barium chloride or barium nitrate (to remove carbonate impurities)
  • a white precipitate indicates the presence of sulfate ions
22
Q

Why are instrumental methods of analysis better than simple chemical tests?

A

They are faster, more accurate and more sensitive.

23
Q

What is a

flame photometer?

A

A scientific instrument based on flame testing.

24
Q

What does the flame photometer do?

A

Identifies metal ions in a sample and determines the concentration of these in solutions.

25
Q

How does the flame photometer work?

A

The substance you are testing is first put through a nebulizer. When the solution evaporates, the sample becomes ions. These ions excite after they absorb heat from the flame. When these ions return back to their normal state, they give off radiation in a specific wavelength. This coloured light can be split up to produce an emission spectrum. Each metal ion produces a unique emission spectrum. The metal present in a sample is identified by comparing its spectrum with reference spectra.

26
Q

Describe how you would test for

hydrogen.

(H2)

A
  • collect the gas in a test tube
  • test with a lit splint
  • hydrogen will burn with a squeaky pop
27
Q

Describe how you would test for

oxygen.

(O2)

A
  • collect the gas in a test tube
  • test with a glowing splint
  • oxygen will relight a glowing splint
28
Q

Describe how you would test for

carbon dioxide.

(CO2)

A
  • bubble the gas through limewater (Ca(OH)2)
  • if the gas is carbon dioxide the limewater will change from colourless to cloudy
29
Q

Describe how you would test for

ammonia.

(NH3)

A
  • hold a piece of damp red litmus above the reaction
  • if the gas being evolved is ammonia, it will turn blue
30
Q

Describe how you would test for

chlorine.

(Cl2)

A
  • hold a piece of damp blue litmus above the reaction
  • if the gas being evolved is chlorine, it will turn red
  • the litmus will then turn white (bleach)
31
Q

Describe how you would test for water in a substance.

A

Add anhydrous copper (II) sulfate. If there is water present, the solid will go from white to blue.

Chemistry (6 decks)

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