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EdExcel GCSE Chemistry > Qualitative Analysis: Tests for Ions - Topic 9 > Flashcards

Qualitative Analysis: Tests for Ions - Topic 9 Flashcards

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1
Q

Why must the test for any ion be unique?

A
  • so the test only detects that ion -> so no confusion with other ions
  • if same result is given by more than one ion -> so don’t know which ion is present
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2
Q

What are flame tests used to test for?

A

test for positive ions

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3
Q

How do you carry out a flame test?

A
  1. Dip platinum loop into concentrated HCl
  2. place wire in the hottest part of the flame
  3. Repeat steps 1 and 2 until no colour change is produced
  4. Dip the wire into concentrated HCl
  5. Dip the wire into the chemical to be tested
  6. Place into the hottest part of the flame and note the colour
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4
Q

Colour of lithium ions (Li+) in flame test:

A

crimson, red

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5
Q

Colour of sodium ions (Na+) in flame test:

A

yellow/orange

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6
Q

Colour of potassium ions (K+) in flame test:

A

pale lilac

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7
Q

Colour of calcium ions (Ca+) in flame test:

A

brick red, orange-red

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8
Q

Colour of barium ions (Ba2+) in flame test:

A

apple green

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9
Q

Colour of copper ions (Cu2+) in flame test:

A

blue-green

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10
Q

How do you test for positive ions with sodium hydroxide?

A
  1. Add 1cm3 of chosen ion solution to test tube
  2. Observe colour of solution carefully
  3. Add a few drops NaOH solution
  4. Write down observations
  5. To test for ammonium ions heat the solution gently after NaOH is added and insert a piece of damp red litmus paper
  6. To test for Al3+ and Ca2+ ions add excess NaOH solution to the resulting solution to distinguish between them
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11
Q

Tests for copper (II) (Cu2+) ions + observation when sodium hydroxide added:

A

blue precipitate

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12
Q

Test for iron (II) (Fe2+) ions + observation when sodium hydroxide added:

A

green precipitate which turns brown on standing

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13
Q

Test for iron (III) (Fe3+) ions + observation when sodium hydroxide added:

A

brown precipitate

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14
Q

Test for ammonium (NH4+) ions + observation when sodium hydroxide added:

A
  • to test for ammonium ions heat the solution gently after NaOH is added and insert a piece of damp red litmus paper
  • NH3 gas produced - pungent smelling gas produced + this gas turns damp red litmus paper blue
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15
Q

Test for aluminium (Al3+) ions + observation when sodium hydroxide added:

A
  • to test for Al3+ and Ca2+ ions add excess NaOH solution to the resulting solution to distinguish between them
  • Al3+ has white precipitate which redissolves in excess NaOH
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16
Q

Test for calcium (Ca2+) ions + observation when sodium hydroxide added:

A
  • to test for Al3+ and Ca2+ ions add excess NaOH solution to the resulting solution to distinguish between them
  • white precipitate which doesn’t re-dissolve in excess NaOH
17
Q

What is the chemical test for ammonia?

A
  • NH3 - pungent smelling gas
  • turns damp red litmus paper blue
  • NH3 also forms a white smoke of ammonium chloride when hydrogen chloride gas from concentrated HCl acid is held near it
18
Q

How do you test for carbonates?

A
  1. Take 1cm3 of carbonate of 1 spatula and add to test tube
  2. Add HCl acid (can use any other acids - sulphuric acid, nitric acid) to test tube
  3. Gas produced bubbled through limewater - if limewater goes cloudy, gas produced is CO2 (carbonates react with dilute acid to produce CO2)
19
Q

What happens when the carbonate is added to the HCl acid?

A
  • effervescence
  • CO2 gas produced
20
Q

What happens when the product is passed through limewater (testing for carbonates)?

A

cloudy white precipitate formed

21
Q

How do you test for sulphate ions?

A
  1. Add few drops of HCl acid to a solution of BaCl₂ - this is now acidified BaCl₂ - acid is added to ensure no carbonate ions are present and no unwanted precipitates form
  2. Add few drops of the “sulphate” to the acidified BaCl₂ - if sulphate present cloudy white precipitate formed
22
Q

Observations when testing for sulphate ions:

A

after “sulphate” added to acidified barium chloride sulphate if cloudy white precipitate formed then sulphate ions are present in the solution

23
Q

How do you test for halide ions?

A
  1. Take small amount of AgNO₃ in a test tube and add few drops of HNO₃ - this is now acidified silver nitrate
    1. nitric acid is added to silver nitrate solution to remove any carbonates so preventing any unwanted precipitates from forming
  2. Add 1cm3 of the “halide” to a test tube
  3. Add few drops of the acidified AgNO₃ solution to your halide
24
Q

Observations when testing for chloride ions (Cl⁻) using acidified silver nitrate:

A

white precipitate

25
Q

Observations when testing for bromide ions (Br⁻) using acidified silver nitrate:

A

cream precipitate

26
Q

Observations when testing for iodide ions (I⁻) using acidified silver nitrate:

A

yellow precipitate

27
Q

Positives of instrumental methods of analysis:

A
  • sensitive - can detect v. small amounts of substance
  • rapid, v. fast - many hundreds of samples in a day
  • v. accurate - can cope with mixtures, objective
28
Q

Negatives of instrumental methods of analysis:

A
  • expensive
  • require expertise to operate
  • can be larger machines
  • cannot cope with new chemicals - only report on things they have seen + require databases to work
29
Q

What does flame photometry allow?

A

this method of analysis - allows us to be able to identify ions in dilute solutions

30
Q

How does flame photometry work?

A
  • high-tech flame test
  • measures wavelength of light given out in flame
  • can identify each ion in the mixture
31
Q

Uses of flame photometry:

A
  • can be used to find concentration of ions in solution e.g. for kidney dialysis
  • measure the intensity of light for solutions of known concentrations to create graph
  • measure intensity of solution being tested and use graph to measure concentration

EdExcel GCSE Chemistry (29 decks)

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