Chemical analysis Flashcards
What is meant by a pure substance in chemistry?
A single element or compound that has not been mixed with any other substance.
State how we can identify if a substance is pure or impure:
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.
Describe the melting point and boiling point of pure and impure substances.
- 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.
Draw a graph for a pure substance’s melting and boiling point.
Draw a graph for an impure substance’s melting and boiling point.
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.
What is meant by a formulation?
A mixture that has been designed as a useful product.
In a formulation how are the quantities of each component measured and why?
2 Marks
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.
State 7 formulations:
- Fuels
- Cleaning agents
- Paints
- Medicines
- Alloys
- Fertilisers
- Foods.
An example of a pure substance:
- Pure milk
What does paper chromatography allow us to separate substances based on?
Their different solubilities.
What is a solvent?
A liquid that will dissolve substances.
What type of phase is the paper in chromatography and why?
- The stationary phase.
- Bc it doesn’t move.
What type of phase is the solvent in chromatography and why?
- The mobile phase.
- Because it moves.
After chromatography is carried out on an ink, how do we know that the ink is a pure substance?
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
A more soluble substance travels _____ up the chromatography paper than a less soluble substance
Further - because it is less attracted to the chromatography paper.
Why do we draw the starting line in pencil during chromatography?
Because if we drew it in pen, the pen ink would move up the chromatography paper with the solvent.
How can we use paper chromatography to identify an unknown substance?
- 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.
What is the issue with using chromatography to identify an unknown substance by its Rf value?
State 2.
- 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.
What is meant by the Rf value of a substance?
The ratio of the distance moved by the substance to the distance moved by the solvent.
Describe the test for hydrogen gas.
- 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.
Describe the test for Oxygen gas.
- 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.
Describe the test for Carbon Dioxide.
- 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.
Describe the test for Chlorine.
- 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.
What is limewater?
An aqueous solution of calcium hydroxide (calcium hydroxide dissolved in water).
How is a flame test conducted?
- 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.
Flame test result for Lithium ion
Crimson
Flame test result for Sodium ion
Yellow
(NOT ORANGE-RED, THAT’S CALCIUM)
Flame test result for Copper ion
Green
Flame test result for Potassium ion
Lilac
Flame test result for Calcium ion
Orange-red.
Problems with determining metal ions using flame tests:
State 2
- 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.
What is a more accurate why of identifying a metal ion?
- Flame emission spectroscopy.
Describe how flame emission spectroscopy is carried out.
- 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.
How can flame emission spectroscopy used to be determine the concentration of the metal ion in a sample?
- 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.
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:
- The metal ion present.
- The concentration of the metal ion in the sample.
True or false, flame emission spectroscopy is an example of an instrumental method and why?
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.
What is flame emission spectroscopy?
An example of an instrumental method used to analyse metal ions in solutions.
Describe how we can use sodium hydroxide solution to test for:
- Calcium
- Magnesium
- Aluminium
- 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.
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?
Aluminium hydroxide precipitates (See ya Al)
Students should be able to write balanced equations for the reactions to produce the insoluble hydroxides.
Calcium Nitrate + Sodium Hydroxide -> Sodium Nitrate + Calcium hydroxide
(This applies to any metal compound reacting with sodium hydroxide).
^Complete some questions
How can we tell a precipitate is formed from a word equation?
If the state symbol is ‘s’ (solid).
Aside from using sodium hydroxide solution to test for:
- Calcium
- Magnesium
- Aluminium
What other metal ions can we use sodium hydroxide solution to identify?
- Copper (II)
- Iron (II)
- Iron (III)
^They form coloured precipitates.
^Transition metals
Iron (II) in sodium hydroxide forms?
A green precipitate of Iron (II) Hydroxide
Iron (III) in sodium hydroxide forms?
A brown precipitate of Iron (III) Hydroxide.
Copper (II) in sodium hydroxide forms?
A blue precipitate of Copper (II) Hydroxide.
How can we identify the non-metal ions:
- Halide ions (Chromide, Bromide, Iodide)
- Sulfate ions
- Carbonate ions
How do we test for carbonate ions?
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.
How do we test for halide ions (chromite, bromide and iodide)?
- 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
How do we test for sulfate ions?
- Add dilute hydrochloric acid into our sample
- Add barium chloride into the solution.
- If sulfate ions are present a white precipitate will be formed
