Chemical Analysis Flashcards
What is a pure substance?
A pure substance is a single element or compound, not mixed with any other substance
How are melting and boiling points used to distinguish between a pure and impure substance?
-If a sample is impure, the melting and boiling point range will be greater than if the substance was pure.
-This is because a pure substance will have a single, fixed melting and boiling point rather than a range of temperatures.
What is the test and result for the presence of water?
Test:
-add anhydrous copper sulfate
-add anhydrous cobalt chloride
Result:
-anhydrous copper sulfate turns white to blue
-anhydrous cobalt chloride turns blue to pink
What is the test for pure water?
The test for pure water is that it melts at exactly 0ºC and boils at exactly 100ºC.
What is a formulation?
-A formulation is a mixture that has been designed as a useful product.
-They are made by mixing the components in carefully measured quantities to ensure the product has the required properties.
-Formulations include: paints, cleaning products and medicines
What is paper chromatography?
Paper chromatography separates the components of a mixture (often inks or dyes) by their attraction to the mobile and stationary phase (how well something dissolves in a solvent).
What is the mobile and stationary phase?
-The solvent is the mobile phase because it moves up the paper.
-The paper is the stationary phase because the paper does not move.
What does a chromatogram show?
-A pure substance shows one spot
-A mixture will show more than one spot (in a vertical line).
-The more soluble (attracted) the components is in the mobile phase, the further up the stationary phase it travels.
-The more attracted to the stationary phase (paper), the less far it travels.
Give a method for paper chromatography.
-Draw a horizontal line on the chromatography paper in pencil. The line should be around 2cm from the bottom of the paper.
-Along the baseline, add small spots of different coloured inks not too close together using a capillary tube. Pour the chosen solvent into a beaker to a depth of 1cm.
-Attach the chromatography paper to a glass rod. Lower the paper into the beaker, ensuring the solvent does not move above the baseline and the glass rod is safely balanced on the beaker.
-Stop the experiment when the solvent is around three quarters up the paper. At this point, draw a pencil line where the solvent reached and leave the paper to dry.
-Gather the results from the chromatogram by calculating the Rf value.
Why should the spots in paper chromatography be small and not too close together?
The spots should be small and not too close together to prevent the colours from merging together when they run up the paper.
Why should the paper in chromatography not touch the side of the beaker?
-Ensure the paper does not touch the side of the beaker as it may interfere with the way the solvent moves up the paper.
-For example, the solvent at the edges may move up faster than the solvent at the centre.
Why should the beaker not be moved during paper chromatography?
Do not move the beaker during the experiment as the solvent will wash the spots off the paper before they have a chance to move.
Common Mistakes Made During Paper Chromatography
- Drawing the baseline in pen rather than pencil. This is a problem as the baseline might dissolve in the solvent which could cause the baseline to move.
- Placing the solvent above the baseline. This is a problem as the solute will dissolve in the solvent rather than travelling up the paper.
- Using a solvent that the mixture is not soluble in. This is a problem as the sample will not move off the baseline.
What is the Rf value?
-The Rf value is the ratio of a distance moved by a compound to the distance moved by the solvent.
-The value will always be between 0 and 1.
How to calculate the Rf value?
Rf value= distance travelled by spot/ distance travelled by solvent
-Measure the distance from the middle of the spot
Why can Rf value not always be used to identify chemicals?
-Sometimes, a mix may contain chemical with the same (or very similar) Rf values. These two chemicals will run up the paper at very similar speeds.
-To solve this problem, we would need to repeat the experiment using a different solvent.
-The two chemicals may not run at the same speed in the new solvent, allowing us to identify them.
-The mix may contain a chemical which has never been analysed before. This means the chemical’s Rf value will not be on any database so we cannot look it up.
-In this case we would need to use another method to identify the chemical.
What is the test and positive result for Hydrogen?
-Insert a lit splint into a test tube containing a gas.
-If positive, the splint will go out and a squeaky pop will be heard.
What is the test and positive result for Oxygen?
-Insert a glowing splint into a test tube containing a gas.
-If positive, the splint will relight.
What is the test and positive result for Carbon Dioxide?
-Bubble the gas through limewater.
-If positive, the limewater will turn from colourless to cloudy.
What is the test and positive result for chlorine?
-Allow the gas to pass over damp, blue litmus paper.
-If positive, the paper will bleach white.
How to carry out a flame test.
-Dip a clean wire loop into a solution of the compound being tested.
-Put the loop into a blue Bunsen burner flame.
-Observe and record the flame colour produced. The colour can be used to identify the metal ion present.
What are the five flame test colours for metal ions?
-Lithium produces a crimson flame.
-Sodium produces a yellow flame.
-Potassium produces a lilac flame.
-Calcium produces an orange-red flame.
-Copper produces a green flame.
Describe the positives and negatives of flame tests.
-Flame tests are a rapid and easy way to identify metal ions.
-However, they have a low sensitivity.
-This means it can be difficult to determine the colour of the flame by eye, especially when the metal ion is present in a very low concentration.
-Secondly, they cannot be used to determine the metal ions present in a mixture.
What is instrumental analysis?
(Modern) instrumental analysis is using a machine instead of a chemical test as a method of analysing a product.
Give the advantages of modern instrumental analysis.
-Rapid
-Accurate
-Sensitive (can detect small amounts/ quantities)
-Quantitative (some methods)
Give the disadvantages of modern instrumental analysis.
-Expensive
-May need specialist training to do
-May need to be compared to a database of known results (although this is also true for Rf values in paper chromatography)
Describe flame emission spectroscopy.
-It is an example of an instrumental method.
-The sample is heated.
-Light is emitted and appears as a spectrum
-It can be compared to a database of known elements
-It can also indicate concentration (how faint or intense a line is).
What is a precipitate?
A precipitate is an insoluble solid formed in a reaction between solutions.
What colour changes happen when sodium hydroxide solution is added to certain metal ions?
-Copper (II) forms a blue precipitate
-Iron (II) forms a green precipitate
-Iron (III) forms a brown precipitate
-Aluminium forms a white precipitate which in excess sodium hydroxide solution dissolves
-Calcium forms a white precipitate
-Magnesium forms a white precipitate
What are the ionic equations for the six precipitate tests?
Copper (II)- Cu2+(aq) + 2OH-(aq) → Cu(OH)2(s)
Iron (II)- Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s)
Iron (III)- Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)
Aluminium- Al3+(aq) + 3OH- → Al(OH)3(s)
Calcium- Ca2+(aq) + 2OH-(aq) → Ca(OH)2(s)
Magnesium- Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s)
How to carry out a test for carbonate ions?
-Add any dilute acid to the sample (often nitric as it doesn’t interfere with other tests).
-If you see effervescence, collect the gas.
-Bubble the gas through limewater.
-If the limewater turns cloudy, then the gas is carbon dioxide.
Why is effervescence not enough to prove the presence of carbonate ions?
-The carbonate ion is not the only chemical which produces effervescence when reacted with an acid.
-For example, when a metal is reacted with an acid, effervescence is also seen (except hydrogen is produced).
-Therefore, in order to prove that we have carbonate ions, we have to prove that any gas produced is carbon dioxide.
What is the ionic equation for carbonate ions?
H+ + CO3 2+ → CO2 +H20
How to carry out a test for sulfate ions?
-Add dilute hydrochloric acid to the sample (the acid removes carbonate ions).
-Add barium chloride solution.
-If sulfate ions are present, a white precipitate will be formed.
Why can sulfuric acid not be used in the test for sulfate ions?
-We cannot use sulfuric acid as it already contains sulfate ions.
-This means if we used it, we would always get a positive result even if sulffate ions weren’t present in the sample.
What is the ionic equation for sulfate ions?
Ba2+(aq) + SO42-(aq) → BaSO4(s)
How to carry out a test for halide ions?
-Add dilute nitric acid to the sample (the acid removes carbonate ions).
-Add silver nitrate solution.
Why can hydrochloric acid not be used in the halide test?
-Hydrochloric acid contains chloride ions.
-This means if it is used in the test, the chloride ions present would react with the silver nitrate solution to form a white precipitate.
-Therefore, we would always get a positive result for the chloride ion.
What colour precipitates are formed from the halide test?
-Chloride ions form a white precipitate
-Bromide ions form a cream precipitate
-Iodide ions form a pale yellow precipitate
What is the general ionic equation for halide ions?
Ag+ + X- → AgX
