C8 - Chemical Analysis Flashcards
What is a pure substance?
Made up of just one substance either an element or a compound.
What is a mixture?
A combination of two or more substances.
What effect does an impurity have on melting and boiling points?
The presence of an impurity lowers the melting point and increases the boiling point.
What is a formulation?
A mixture designed as a useful product.
What can be said about substances that move far up chromatography paper?
They are soluble - attracted to the mobile phase.
What can be said about substances that don’t move far up chromatography paper?
They are less soluble - attracted to the stationary phase.
What is the mobile phase?
The water.
What is the stationary phase?
The chromatography paper.
How do you calculate retention factor?
Height of dot / height of solvent
Describe the test for hydrogen.
The test for hydrogen uses a burning splint held at the open end of a test tube of the gas. Hydrogen burns rapidly with a pop sound.
Describe the test for oxygen.
React MgO2 and H2O2.
Place a glowing splint in the test tube with the mixture.
If the test is + the splint relights.
Describe the test for carbon dioxide.
The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water). When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy).
Describe the test for chlorine.
Electrolysis of salt solution with damp blue litmus paper.
If +, should turn red then bleach white.
What is the speed of a solvent with a high Rf?
Fast
What is the speed of a solvent with a low Rf?
Slow
What is the only circumstance under which the Rf changes?
Solvent is changed.
Colour of sodium chloride in flame?
Orange/yellow
Colour of copper chloride in flame?
Turquoise blue / green
Colour of lithium chloride in flame?
Red
Colour of potassium chloride in flame?
Purple / lilac
Colour of calcium chloride in flame?
Orange / red
How do you test for halide anions?
Add dilute nitric acid, then add silver nitrate.
What colour precipitate forms from the test for negative chloride ions?
White - silver chloride.
What colour precipitate forms from the test for negative bromide ions?
Cream - silver bromide
What colour precipitate forms from the test for negative iodide ions?
Yellow - silver iodide
Describe the test for negative carbonate ions.
Carbonates react with dilute acids to form carbon dioxide gas.
Carbon dioxide can be identified with limewater.
3 benefits of instrumental methods.
They are accurate and sensitive.
They are quick.
They enable small samples to be analysed.
3 disadvantages of instrumental methods.
Expensive.
Takes special training to use.
Results can only be interpreted by comparison with data from know substances.
What do the flame tests test for?
Positive ions.
Describe the precipitate formed from the sodium hydroxide test for copper (II).
State the ionic equation.
Blue precipitate formed.
Cu2+ + 2OH- –> Cu(OH)2
Describe the precipitate formed from the sodium hydroxide test for iron (II).
State the ionic equation.
Forms dirty green precipitate.
Fe2+ + 2OH- –> Fe(OH)2
Describe the precipitate formed from the sodium hydroxide test for iron (III).
State the ionic equation.
Form brown precipitate.
Fe3+ + 3OH- –> Fe(OH)3
What 3 positive ions form a white precipitate (in the sodium hydroxide tests), however what ion becomes a colourless solution and why?
Al3+, Ca2+, Mg2+, Al3+ forms colourless solution as Al(OH)3 dissolves in excess NaOH, to form colourless solution.
What is the ionic equation for the test of halide ions with silver nitrate?
Ag+ + X- –> AgX
What is the ionic equation for the test of carbonate ions with dilute nitric acid?
CO3(2-) + 2H+ –> CO + H2O
What is the ionic equation for the test of sulfate ions with barium chloride?
Ba2+ + SO4(2-) –> BaSO
What is flame emission spectroscopy?
Instrumental method used for identifying metal ions in a solution.
What is a spectroscope?
Measures the exact wavelength of the light emitted by a metal ion.
Which allows for definite identification - sometimes colours are difficult to distinguish.
How is the concentration of metal ions in a solution determined?
Measuring the intensity of the light emitted. The higher the intensity, the greater the concentration.
What are cations?
Positive ions.
What are anions?
Negative ions.
How can you find the concentration of metal ions in a graph?
From a concentration-intensity graph, you can read a concentration from a given intensity.
What type of method is a flame emission spectroscopy?
Instrumental method.
How is flame emissions spectroscopy carried out?
Sample placed in a flame, light emitted is passed through a spectroscope to give a spectrum which can be compared to reference.
How can chromatography show the difference between pure and impure substances?
Pure ones will not separate into a number of spots.
How does a substance’s Rf value depend on?
How soluble it is in the solvent.
Why does carbon dioxide turn limewater cloudy?
Solid calcium carbonate forms.
Other than the flame tests, how can metal ions be identified?
Use of sodium hydroxide.
How can compounds of calcium and magnesium be distinguished from each other?
Using flame tests calcium compounds produce an orange - red flame.
What is the test for sulfate ions?
Add barium chloride and hydrochloric acid, forms white precipitate.
Why is it hard to identify the metal ions from the colour of the flame when there is a mixture of metal ions?
Flame colours will be masked.
What name is given to a useful product?
