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Chemistry > Analysis > Flashcards

Analysis Flashcards

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1
Q

define pure substance

A

A substance that consists of only one element or only one compound.

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2
Q

how can you identify pure substances and mixtures from data about melting and boiling points?

A
  • Pure substances have a sharp
    melting point but mixtures melt over a range of temperatures.
  • This difference is most easily seen when the temperature of a liquid is measured as it cools and freezes
  • a horizontal part of the graph shows that the substance has a sharp melting point, so it is pure.
  • Impure substances would produce a gradual fall in temperature as it freezes (the graph goes from curved to a straight diagonal) (this is the melting range)
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3
Q

define formulation

A

A mixture that is always made with the same proportions of the same substances that has been designed as a useful product

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4
Q

what is chromatography?

A

used to separate mixtures (different substances dissolved in a liquid) of
soluble (able to dissolve in solvent) substances and to provide information on the possible identity of the substances present in the mixture

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5
Q

what 2 different phases does chromatography rely on?

A
  • mobile phase- phase in chromatography that moves, usually a solvent or mixture of solvents.
  • stationary phase- phase in chromatography that does not move, for instance, the paper in chromatography.
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6
Q

how can you identify pure substances from chromatography?

A
  • a pure substance produces one spot on the chromatogram
  • an impure substance produces two or more spots
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7
Q

what does Rf value of a compound represent?

A
  • the distance travelled by the substance divided by the distance travelled by the solvent (water)
  • it’s always the same for a particular substance if run in the same solvent system
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8
Q

what values does Rf vary between and what do they represent?

A
  • Rf values vary from 0 (the substance is not attracted to the mobile phase)
  • to 1 (the substance is not attracted to the stationary phase)
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9
Q

what is the test for oxygen?

A
  • Glowing splint held in a test tube
  • Splint relights if present
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10
Q

what is the test for hydrogen?

A
  • Lighted splint held in a test tube
  • Pop sound heard if present
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11
Q

what is the test for carbon dioxide?

A
  • Gas bubbled through limewater
  • Limewater turns milky or cloudy white if present
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12
Q

what is the test for chlorine?

A
  • Damp litmus paper held in a test tube
  • Paper turns white if present
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13
Q

how do you carry out a flame test?

A
  1. dip a clean wire loop into a solid sample of the compound being tested
  2. put the loop into the edge of the blue flame from a Bunsen burner
  3. observe and record the flame colour produced
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14
Q

what ion does a crimson flame represent?

A

Lithium, Li+

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15
Q

what ion does a yellow flame represent?

A

Sodium, Na+

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16
Q

what ion does a lilac flame represent?

A

Potassium, K+

17
Q

what ion does an orange-red flame represent?

A

Calcium, Ca2+

18
Q

what ion does a green flame represent?

A

Copper, Cu2+

19
Q

what can sodium hydroxide be used to identify and how?

A
  • Dilute sodium hydroxide solution is used in tests for some metal
    ions, which form metal hydroxides that are insoluble.
  • This means that the metal hydroxides appear as precipitates.
20
Q

what 3 metal ions reacted with sodium hydroxide solution does a white precipitate represent?

A
  • Aluminium, Al3+
  • Calcium, Ca2+
  • Magnesium, Mg2+
21
Q

what metal ion reacted with sodium hydroxide solution does a blue precipitate represent?

A

Copper(II), Cu2+

22
Q

what metal ion reacted with sodium hydroxide solution does a green precipitate represent?

A

Iron(II), Fe2+

23
Q

what metal ion reacted with sodium hydroxide solution does a brown precipitate represent?

A

Iron(III), Fe3+

24
Q

how do you write balanced equations for reactions between sodium hydroxide solution and and metal ions?

A
  • the general equation is: metal sulfate + sodium hydroxide → sodium sulfate + metal hydroxide
  • however the precipitation reactions can be represented using
    ionic equations
  • these only include the ions which are involved in the formation of the precipitate. They ignore the
    spectator ions, which are present but not involved.
  • these only represent the metal ion reacting with the hydroxide ion to make metal hydroxide
  • they are structured like this: Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)
25
Q

how do you distinguish between aluminium ions, calcium ions and magnesium ions being present when reacting with sodium hydroxide since the precipitate is white for all of them?

A
  • the aluminium hydroxide precipitate dissolves to form a colourless solution
  • the calcium hydroxide precipitate is unchanged
  • the magnesium hydroxide solution is unchanged
26
Q

how are do you test for negative carbonate ions?

A
  • Carbonate ions, CO32-, are detected using a dilute acid.
  • Bubbles are given off when an acid, usually dilute hydrochloric acid, is added to the test compound.
  • The bubbles are caused by carbon dioxide.
  • Limewater is used to confirm that the gas is carbon dioxide. It turns milky/cloudy when carbon dioxide is bubbled through it.
  • Unlike the tests for other negative ions, the test for carbonate ions works whatever acid is added.
27
Q

how do you test for negative sulfate ions?

A
  • Barium ions react with sulfate ions, SO42- to form insoluble white barium sulfate:
  • Ba2+(aq) + SO42-(aq) → BaSO4(s)
  • To test for sulfate ions:
  • add a few drops of dilute hydrochloric acid to the sample
  • add a few drop of dilute barium chloride solution
  • A white precipitate forms if sulfate ions are present.
28
Q

how do you test for negative halide ions?

A
  • Silver ions react with halide ions (Cl-, Br- or I- ions) to form insoluble precipitates.
  • the colours of these silver halide precipitates:
  • Chloride, Cl- : White
  • Bromide, Br- : Cream
  • Iodide, I- : Yellow
  • For example, when silver nitrate is added to a solution containing chloride ions:
  • Ag+(aq) + Cl-(aq) → AgCl(s)
  • To test for halide ions:
  • add a few drops of dilute nitric acid to the sample
  • add a few drop of dilute silver nitrate solution
  • Observe and record the colour of any precipitate that forms.
29
Q

state 3 advantages of using instrumental (machine) methods to identify elements and compounds compared to chemical tests

A

Compared to simple laboratory tests, instrumental methods of analysis may give improved:
* speed
* accuracy
* sensitivity (they can detect very small amounts of a substance in a small amount of sample)

30
Q

what is a flame emission spectroscope and what is it used for?

A
  • The flame emission
    spectroscope is a scientific instrument based on flame testing.
  • Data from a spectroscope can be used to:
  • identify metal ions in a sample.
  • determine the concentration of metal ions in dilute solutions
31
Q

how can metal ions be identified using flame emission spectroscopy?

A
  • In the flame emission spectroscope, the coloured light from a
    vaporised sample can be split to produce an emission spectrum.
  • The different lines in an emission spectrum look like a coloured barcode.
  • Each metal ion produces a unique emission spectrum.
  • The metal present in a sample is identified by comparing its
    spectrum with reference spectra.
  • These are emission spectra from known metal ions.
  • If two spectra match, they must be from the same metal ion.

Chemistry (18 decks)

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