C8 - Chemical Analysis Flashcards
What is a pure substance?
Consists only of one element or one compound.
What is a mixture?
- Consists of two or more different substances
- Not chemically joined together
What is the difference in melting and boiling points between mixtures and pure substances?
- Pure substances have a sharp melting point
- Mixtures melt over a range of temperatures
What is a formulation?
- 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.
Give examples of a formulation.
Fuels, cleaning products, paints, medicines, alloys, fertilisers, foods
What is paper chromatography used for?
- Separate mixtures of soluble substances
- Provide information on the possible identity of the substances present in the mixture.
Describe a chromatography practical?
- Draw pencil line across the paper, 2 cm from the bottom.
- Add small spots of each ink to the line on the paper.
- Place paper into a container with solvent in the bottom
- Allow solvent to move through the paper, but remove the chromatogram before it reaches the top
- Allow chromatogram to dry
- Measure distance travelled by each spot and by the solvent.
What are the two phases involved in chromatography?
- Mobile phase –> solvent
- Stationary phase –> paper
What is the Rf value? What is it used for?
Used to identify unknown chemicals if they can be compared to a range of reference substances.
How do you find the Rf value of a substance?
Distance travelled by substance
÷
Distance travelled by solvent
How do you test for the presence of hydrogen, oxygen, carbon dioxide and chlorine?
- 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
How do you carry out a flame test?
- Dip a clean wire loop into a solid sample of the compound being tested
- Put the loop into the edge of the blue flame from a Bunsen burner
- Observe and record the flame colour produced
What colours are shown when metals undergo a flame test?
- Lithium: Crimson
- Sodium: Yellow
- Potassium: Lilac
- Calcium: Orange-red
- Copper: Green
What is a precipitate?
- A suspension of particles in a liquid
- Formed when a dissolved substance reacts to form an insoluble substance
What does dilute sodium hydroxide solution + metal form?
Metal hydroxides that are insoluble
What are the colour of the precipitates formed when the common metals are reacted with dilute sodium hydroxide?
- Aluminium: White
- Calcium: White
- Magnesium: White
- Copper(II): Blue
- Iron(II): Green
- Iron(III): Brown
What happens if excess sodium hydroxide solution is added to aluminium, calcium and magnesium?
- 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.
Why are group 1 metals tested for using flame tests and not sodium hydroxide solution?
Group 1 metals form soluble hydroxides when sodium hydroxide solution is added.
How do you test for carbonate ions?
- 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.
How do you test for sulfate ions?
- 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.
Explain why, in a test for sulfate ions, the sample is acidified with hydrochloric acid first?
- 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.
What is formed when silver ions react with halide ions?
Silver halide precipitates (insoluble)
What are the colours of silver halide precipitates?
- Chloride: White
- Bromide: Cream
- Iodide: Yellow
How do you test for halide ions?
- Add few drops of dilute nitric acid to the sample
- Add few drops of dilute silver nitrate solution
- Observe and record the colour precipitate that forms
Explain why, in a test for halide ions, the sample is acidified with dilute nitric acid first?
- 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
Explain why, in a test for halide ions, the sample is acidified with dilute nitric acid first?
- 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
What are instrumental methods of analysis?
Rely on machines
Why might instrumental methods be better than simple laboratory tests?
May give improved:
- Speed
- Accuracy
- Sensitivity (can detect very small amounts of a substance in a small amount of sample)
What can data from a spectroscope be used for?
- Identify metal ions in a sample.
- Determine the concentration of metal ions in dilute solutions.
How do you identify metals using a flame emission spectroscope?
- Coloured light from a vaporised sample can be split to produce an emission spectrum.
- 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.
- If two spectra match, they must be from the same metal ion.
What is a calibration curve?
Graph with readings from a machine plotted against the known amounts of a substance
