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GCSE Chemistry > Chemical analysis > Flashcards

Chemical analysis Flashcards

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1
Q

What is meant by a pure substance in chemistry?

A

A single element or compound that has not been mixed with any other substance.

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2
Q

State how we can identify if a substance is pure or impure:

A

By using melting point and boiling point data.

  • A pure substance melts at a specific fixed temperature.
  • A pure substance has a specific fixed boiling point.
  • Impure substances melt and boil over a range of temperatures.
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3
Q

Describe the melting point and boiling point of pure and impure substances.

A
  • A pure substance melts at a specific fixed temperature.
  • A pure substance has a specific fixed boiling point.
  • Impure substances melt and boil over a range of temperatures.
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4
Q

Draw a graph for a pure substance’s melting and boiling point.

Draw a graph for an impure substance’s melting and boiling point.

A

Done :)

The temperature at which a pure substance melts and boils over is constant, fixed and specific.

The temperature at which an impure substance melts and boils over ranges over time.

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5
Q

What is meant by a formulation?

A

A mixture that has been designed as a useful product.

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6
Q

In a formulation how are the quantities of each component measured and why?

2 Marks

A

The quantities of each component is carefully measured (1) so the products have the properties we need (1)

^^Each chemical component of a formulation has a purpose.

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7
Q

State 7 formulations:

A
  • Fuels
  • Cleaning agents
  • Paints
  • Medicines
  • Alloys
  • Fertilisers
  • Foods.
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8
Q

An example of a pure substance:

A
  • Pure milk
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9
Q

What does paper chromatography allow us to separate substances based on?

A

Their different solubilities.

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10
Q

What is a solvent?

A

A liquid that will dissolve substances.

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11
Q

What type of phase is the paper in chromatography and why?

A
  • The stationary phase.
  • Bc it doesn’t move.
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12
Q

What type of phase is the solvent in chromatography and why?

A
  • The mobile phase.
  • Because it moves.
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13
Q

After chromatography is carried out on an ink, how do we know that the ink is a pure substance?

A

If a single spot has been formed.

A pure compound will produce a single spot in all solvents

^If a substance was a mixture it would have separated into one or more spots.

Compounds in a mixture may separate into different spots depending on the solvent

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14
Q

A more soluble substance travels _____ up the chromatography paper than a less soluble substance

A

Further - because it is less attracted to the chromatography paper.

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15
Q

Why do we draw the starting line in pencil during chromatography?

A

Because if we drew it in pen, the pen ink would move up the chromatography paper with the solvent.

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16
Q

How can we use paper chromatography to identify an unknown substance?

A
  • Place the dot of the unknown ink onto a pencil line drawn above the chromatography paper and place the end of the chromatography paper into the solvent without the pencil line touching it.
  • Measure the distance the chemical has move to from the starting line to the centre of the chemical’s spot (in mm).
  • Measure the distance the solvent has moved to from the pencil line to where the solvent stopped moving (in mm)
  • Calculate the Rf value of the unknown substance by dividing the distance moved by the substance by the distance moved by the solvent.
  • Input this value into a data base which will identify what the solvent is.
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17
Q

What is the issue with using chromatography to identify an unknown substance by its Rf value?

State 2.

A
  • Different substances may have the same Rf value, so we need to repeat the experiment using different solvents.
  • If the substance has never been analysed before there will not be an Rf value on the database, so we would need to carry out further analysis to identify the substance.
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18
Q

What is meant by the Rf value of a substance?

A

The ratio of the distance moved by the substance to the distance moved by the solvent.

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19
Q

Describe the test for hydrogen gas.

A
  • Insert a burning splint into the test tube that may contain hydrogen gas.
  • Because hydrogen gas burns rapidly a squeaky pop sound will be produced if the gas is hydrogen.
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20
Q

Describe the test for Oxygen gas.

A
  • Insert a glowing splint into a test tube that may contain oxygen.
  • If the splint relights then oxygen gas is present in the test tube.
21
Q

Describe the test for Carbon Dioxide.

A
  • Draw some of the gas into a plastic pipette and bubble it through a test tube containing limewater.
  • If the lime water turns cloudy then CO2 gas was present.
22
Q

Describe the test for Chlorine.

A
  • Insert damp litmus paper into the mouth of a test tube that may contain the gas chlorine.
  • If chlorine is present the litmus paper is bleached and turns white.
23
Q

What is limewater?

A

An aqueous solution of calcium hydroxide (calcium hydroxide dissolved in water).

24
Q

How is a flame test conducted?

A
  • Place a small amount of our chemical onto a wire mounted in a handle.
  • Place the end of this wire into a blue Bunsen burner flame.
  • The colour this flame turns can be used to determine the metal ion present in the chemical.
25
Q

Flame test result for Lithium ion

A

Crimson

26
Q

Flame test result for Sodium ion

A

Yellow

(NOT ORANGE-RED, THAT’S CALCIUM)

27
Q

Flame test result for Copper ion

A

Green

28
Q

Flame test result for Potassium ion

A

Lilac

29
Q

Flame test result for Calcium ion

A

Orange-red.

30
Q

Problems with determining metal ions using flame tests:

State 2

A
  • Colours are hard to distinguish especially if there is a low concentration of the metal ion in the chemical.
  • If a sample contains a mixture of metal ions some of the flame colours can be masked.
31
Q

What is a more accurate why of identifying a metal ion?

A
  • Flame emission spectroscopy.
32
Q

Describe how flame emission spectroscopy is carried out.

A
  • A sample of the metal ion IN SOLUTION is placed into a flame.
  • The light given out from this is passed into a machine called a spectroscope.
  • The spectroscope converts the light into a line spectrum.
  • The positions of the lines in a line spectrum is specific for a given metal ion, so this can be used to identify the metal ion.
33
Q

How can flame emission spectroscopy used to be determine the concentration of the metal ion in a sample?

A
  • We can use the line spectrum.
  • The lines on the line spectrum become more intense if the concentration of the metal ion in the solution is higher.

^The higher the conc. the more intense the lines.

34
Q

In flame emission spectroscopy the:

Position of the lines on the line spectrum tells us:
The intensity of the lines on the line spectrum tells us:

A
  • The metal ion present.
  • The concentration of the metal ion in the sample.
35
Q

True or false, flame emission spectroscopy is an example of an instrumental method and why?

A

True because it is carried out by a machine.

ADVANTAGES OF INSTRUMENTAL METHODS:

  • Their analysis is rapid.
  • They are sensitive (FES can work on a small sample of the metal compound).
  • They are accurate - not subjective.
36
Q

What is flame emission spectroscopy?

A

An example of an instrumental method used to analyse metal ions in solutions.

37
Q

Describe how we can use sodium hydroxide solution to test for:

  • Calcium
  • Magnesium
  • Aluminium
A
  • Add sodium hydroxide solution into test tubes containing the metal ions.
  • In each test tube white metal hydroxide precipitates are formed.
  • Continue to add excess sodium hydroxide until in one test tubes the metal ion precipitate redissolves. This metal ion is Aluminium.
  • Use flame tests to identify which test tube contains calcium ions. If the flame turns orange-red that test tube contained calcium ions.
38
Q

When we carry out the sodium hydroxide solution test for Calcium, Magnesium and Aluminium, which metal ion precipitate redissolves when excess sodium hydroxide solution is added?

A

Aluminium hydroxide precipitates (See ya Al)

39
Q

Students should be able to write balanced equations for the reactions to produce the insoluble hydroxides.

A

Calcium Nitrate + Sodium Hydroxide -> Sodium Nitrate + Calcium hydroxide

(This applies to any metal compound reacting with sodium hydroxide).

^Complete some questions

40
Q

How can we tell a precipitate is formed from a word equation?

A

If the state symbol is ‘s’ (solid).

41
Q

Aside from using sodium hydroxide solution to test for:
- Calcium
- Magnesium
- Aluminium

What other metal ions can we use sodium hydroxide solution to identify?

A
  • Copper (II)
  • Iron (II)
  • Iron (III)

^They form coloured precipitates.
^Transition metals

42
Q

Iron (II) in sodium hydroxide forms?

A

A green precipitate of Iron (II) Hydroxide

43
Q

Iron (III) in sodium hydroxide forms?

A

A brown precipitate of Iron (III) Hydroxide.

44
Q

Copper (II) in sodium hydroxide forms?

A

A blue precipitate of Copper (II) Hydroxide.

45
Q

How can we identify the non-metal ions:

  • Halide ions (Chromide, Bromide, Iodide)
  • Sulfate ions
  • Carbonate ions
A
46
Q

How do we test for carbonate ions?

A

Using Dilute acid:

  • Add dilute acid to our sample.
  • If carbonate ion is present, the carbonate ion will react with the acid to produce carbon dioxide gas and this is seen through effervescence.
  • To test to see if the gas is CO2, bubble the gas through lime water.
  • If the gas is Carbon dioxide the lime water will become cloudy meaning we started with a carbonate ion.
47
Q

How do we test for halide ions (chromite, bromide and iodide)?

A
  • Add dilute nitric acid into a sample.
  • Add dilute silver nitrate solution.
  • If a halide ion is present a coloured silver halide precipitate will be formed.

WHAT ARE THE COLOURS FOR EACH HALIDE ION?

  • Bromide ion - Cream silver bromide precipitate.
  • Iodide ion - Yellow silver iodide precipitate
  • Chloride ion - White silver chloride precipitate
48
Q

How do we test for sulfate ions?

A
  • Add dilute hydrochloric acid into our sample
  • Add barium chloride into the solution.
  • If sulfate ions are present a white precipitate will be formed

GCSE Chemistry (13 decks)

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