chemical analysis (p2) Flashcards
What is a pure substance?
A single element or compound that is not mixed with any other substance
What type of substances melt and boil at specific temperatures?
Pure substances
If a substance boils between 55 and 58 ᵒC, will it be pure or impure?
impure
What type of substances melt and boil over a range of temperatures?
Impure substances
Is carbon dioxide pure or impure?
pure
Is oxygen pure or impure?
pure
What is a formulation?
A mixture that has been designed as a useful product
Is water from a tap pure or impure?
Impure as contains dissolved salts
Are formulations pure or impure?
Impure
What is chromatography used to separate?
Mixtures of soluble susbtances
Name the two phases in chromatography
Stationary phase - where molecules can’t move
Mobile phase - where molecules can move
What phase is the paper in chromatography?
Stationary phase
What phase is the water in chromatography?
Mobile phase
Why do substances separate in paper chromatography?
Substances move up the paper when they are dissolved in the solvent
The different substances have different solubilities in the solvent
Therefore they travel different distances up the paper
How could you tell if a substance was impure by looking at a chromatogram?
There would be more than one spot in a vertical column above the original spot
How could you tell if a substance was pure by looking at a chromatogram?
There would be only one spot in a vertical column above the original spot
Why should the baseline be drawn in pencil?
Pencil is insoluble
If it was drawn in pen the ink would dissolve and so the line would run
Why should the level of water be below the baseline?
If it was above the baseline the spots would wash off the paper
What is the solvent front?
The distance travelled by the solvent
When should you remove the chromatography paper?
When the solvent has nearly reached the top
Why is the chromatography paper often attached to a glass rod when carrying out chromatography?
So that it stands up in the water
Why should a lid be placed on the beaker during chromatography?
To saturated the atmosphere in the beaker with solvent vapour
prevent evaporation
The higher the Rf value the ………. the distance travelled by the substance
Greater
What equation is used to calculate Rf values?
Rf value = distance travelled by spot ÷ distance travelled by solvent
How many spots will a pure susbtances produce on a chromatogram?
one
Describe how you would set up a chromatogram
- Draw a baseline in pencil near the bottom of a piece of filter paper
- Add a spot of ink to this line and allow it to dry
- Place the paper in a beaker of solvent (usually water), make sure the level of solvent is below the baseline
- Add a lid to the beaker to stop the solvent evaporating
- Allow the solvent to run up the paper
- Remove the paper from the beaker just before the solvent reaches the top of the paper
What is the test for oxygen?
- Use a glowing splint
- If oxygen gas is present the glowing splint will relight
What is the test for hydrogen?
- Use a burning splint
- If hydrogen is present it will burn with a squeaky pop
What is the test for carbon dioxide?
- Bubble gas through limewater (calcium hydroxide solution)
- If the gas is carbon dioxide the limewater will turn from colourless to milky
What is the test for chlorine?
- Add damp litmus paper
- If chlorine gas is present the litmus paper is bleached and turns white
