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Chemistry > C8 - Chemical Analysis > Flashcards

C8 - Chemical Analysis Flashcards

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1
Q

What is a pure substance?

A

Consists only of one element or one compound.

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2
Q

What is a mixture?

A
  • Consists of two or more different substances
  • Not chemically joined together
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3
Q

What is the difference in melting and boiling points between mixtures and pure substances?

A
  • Pure substances have a sharp melting point
  • Mixtures melt over a range of temperatures
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4
Q

What is a formulation?

A
  • A mixture which has been designed as a useful product
  • Every chemical has been added in a carefully measured amount
  • Each chemical has a specific purpose in the formulation.
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5
Q

Give examples of a formulation.

A

Fuels, cleaning products, paints, medicines, alloys, fertilisers, foods

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6
Q

What is paper chromatography used for?

A
  • Separate mixtures of soluble substances
  • Provide information on the possible identity of the substances present in the mixture.
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7
Q

Describe a chromatography practical?

A
  1. Draw pencil line across the paper, 2 cm from the bottom.
  2. Add small spots of each ink to the line on the paper.
  3. Place paper into a container with solvent in the bottom
  4. Allow solvent to move through the paper, but remove the chromatogram before it reaches the top
  5. Allow chromatogram to dry
  6. Measure distance travelled by each spot and by the solvent.
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8
Q

What are the two phases involved in chromatography?

A
  • Mobile phase –> solvent
  • Stationary phase –> paper
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9
Q

What is the Rf value? What is it used for?

A

Used to identify unknown chemicals if they can be compared to a range of reference substances.

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10
Q

How do you find the Rf value of a substance?

A

Distance travelled by substance
÷
Distance travelled by solvent

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11
Q

How do you test for the presence of hydrogen, oxygen, carbon dioxide and chlorine?

A
  • Oxygen: Glowing splint held in a test tube –> splint relights
  • Hydrogen: Lighted splint held in a test tube –> Pop sound heard
  • Carbon dioxide: Gas bubbled through limewater –> Limewater turns milky/cloudy white
  • Chlorine: Damp litmus paper held in a test tube –> Paper turns white
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12
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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13
Q

What colours are shown when metals undergo a flame test?

A
  • Lithium: Crimson
  • Sodium: Yellow
  • Potassium: Lilac
  • Calcium: Orange-red
  • Copper: Green
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14
Q

What is a precipitate?

A
  • A suspension of particles in a liquid
  • Formed when a dissolved substance reacts to form an insoluble substance
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15
Q

What does dilute sodium hydroxide solution + metal form?

A

Metal hydroxides that are insoluble

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16
Q

What are the colour of the precipitates formed when the common metals are reacted with dilute sodium hydroxide?

A
  • Aluminium: White
  • Calcium: White
  • Magnesium: White
  • Copper(II): Blue
  • Iron(II): Green
  • Iron(III): Brown
17
Q

What happens if excess sodium hydroxide solution is added to aluminium, calcium and magnesium?

A
  • Aluminium hydroxide precipitate dissolves to form a colourless solution
  • Calcium hydroxide precipitate is unchanged and the magnesium hydroxide solution is unchanged so have to use flame test.
18
Q

Why are group 1 metals tested for using flame tests and not sodium hydroxide solution?

A

Group 1 metals form soluble hydroxides when sodium hydroxide solution is added.

19
Q

How do you test for carbonate ions?

A
  • Bubbles given off when dilute acid added to test compound.
  • Lime water is used to confirm that the gas is carbon dioxide.
  • It turns milky/cloudy when carbon dioxide is bubbled through it.
20
Q

How do you test for sulfate ions?

A
  • Barium ions react with sulfate ions, to form insoluble white barium sulfate
    1. Add a few drops of dilute hydrochloric acid to the sample
    add
    2. Add few drops of dilute barium chloride solution
    3. A white precipitate forms if sulfate ions are present.
21
Q

Explain why, in a test for sulfate ions, the sample is acidified with hydrochloric acid first?

A
  • Carbonate ions also produce a white precipitate with barium chloride solution.
  • The acid reacts with any carbonate ions present.
  • This removes them, so stopping them giving a false positive result.
22
Q

What is formed when silver ions react with halide ions?

A

Silver halide precipitates (insoluble)

23
Q

What are the colours of silver halide precipitates?

A
  • Chloride: White
  • Bromide: Cream
  • Iodide: Yellow
24
Q

How do you test for halide ions?

A
  1. Add few drops of dilute nitric acid to the sample
  2. Add few drops of dilute silver nitrate solution
  3. Observe and record the colour precipitate that forms
25
Q

Explain why, in a test for halide ions, the sample is acidified with dilute nitric acid first?

A
  • Carbonate ions produce white precipitate with silver nitrate solution.
  • Acid reacts with carbonate ions present. This removes them
  • Stopping them giving an incorrect positive result
25
Q

Explain why, in a test for halide ions, the sample is acidified with dilute nitric acid first?

A
  • Carbonate ions produce white precipitate with silver nitrate solution.
  • Acid reacts with carbonate ions present. This removes them
  • Stopping them giving an incorrect positive result
26
Q

What are instrumental methods of analysis?

A

Rely on machines

27
Q

Why might instrumental methods be better than simple laboratory tests?

A

May give improved:
- Speed
- Accuracy
- Sensitivity (can detect very small amounts of a substance in a small amount of sample)

28
Q

What can data from a spectroscope be used for?

A
  • Identify metal ions in a sample.
  • Determine the concentration of metal ions in dilute solutions.
29
Q

How do you identify metals using a flame emission spectroscope?

A
  1. Coloured light from a vaporised sample can be split to produce an emission spectrum.
  2. Different lines in an emission spectrum look like a coloured barcode.
  3. Each metal ion produces a unique emission spectrum.
  4. The metal present in a sample is identified by comparing its spectrum with reference spectra.
  5. If two spectra match, they must be from the same metal ion.
30
Q

What is a calibration curve?

A

Graph with readings from a machine plotted against the known amounts of a substance

Chemistry (8 decks)

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