C2.1 Purity And Seperating Mixtures Flashcards
What is relative atomic mass?
The mean mass of an atom of an element compared to 1/12th of the mass of a carbon 12 atom
Define relative formula mass
The mean mass of a unit of a sub trance compared to the mass of a carbon 12 atom
What is an empirical formula?
The simplest whole number ratio of the atoms of each element in a compound
How do you work out the empirical formula?
Divide by the HCF
How do you work out the empirical formula from percentages?
1) Divide each elements percentage by its mass number
2) Divide ALL the answers by the smallest answer
3) Write it out
Why do scientists use empirical formulas?
It makes little sense to write out formulas with huge numbers
What does pure mean?
A substance which consists of only one element or compound
What is a mixture
An impure substance made of more than one element or compound
Why are some mixtures useful?
Mixture can be useful for alloys which are a mixture of metals with more than one element. This adds strength and other beneficial qualities
How do you use melting points to distinguish between pure and impure?
Impure substances melt over a range of temperatures and the melting point is less than that of a pure substance, eg seawater has a lower melting point than pure water
The greater the difference, the more impure it is
Why should you heat a substance slowly and stir it?
It allows the temperature of the whole substance to increase evenly and mixing ensures that the whole thing is the same temperature
What is a solution?
a mixture formed when one substance dissolves in another
What is a solvent?
The substance a solute dissolves in
What is a solute?
The substance that dissolves in the solvent
What does soluble mean?
If a substance can dissolve in a solvent
What does insoluble mean?
If a substance cannot dissolve in a solvent
What is filtration used for?
Separating insoluble substances in solid state from a liquid.
How does filtration work?
1) fold some filter paper into a cone and put it in a funnel
2) put the funnel in a conical flask
3) pass the mixture through the filter paper
4) you are left with residue in the filter paper and the liquid in the flask because the filter paper has holes which are too small for solid to pass through
What is residue?
What is left behind after filtration
What is filtrate?
The substance that passes through the filter paper
What is solubility?
How well a substance can dissolve
What is crystallisation used for?
To separate a solute from a substance
Describe and explain how crystallisation works?
You gently heat the solution under a Bunsen burner until more than half has evaporated(saturated solution).
Then leave the remaining solution to cool so that the remaining solvent evaporates until you are left with crystals
What is simple distillation used for?
To separate a solvent from a solution which have different boiling points. It can be used to make distilled water by separating salts from water which have a higher boiling point.
What is fractional distillation used for?
For separating 2 or more substances in liquid state from a mixture with different boiling points
What is the stationary and mobile phase for thin layer chromatography?
Stationary - Silica or alumina powder on the plate
Mobile - Solvent
What is the mobile and stationary phase in paper chromatography?
Stationary - absorbent paper
Mobile - solvent
What is the mobile and stationary phase in gas chromatography?
Stationary - alumina or silica powder
Mobile - an unreactive carried gas such as nitrogen
Rf formula
Distance travelled by substance / distance travelled by solvent
Explain how you know which are the more soluble substances
and
less soluble substance
from TLC or PC
More soluble substances are more attracted to the mobile phase , therefore move further up the plate
Less soluble substances are more attracted to the stationary phase so they get dropped of on the plate sooner
What are the advantages of Thin layer chromatography compared to paper chromatography
It is quicker
It is more sensitive, so a smaller sample can be used
A wide variety of stationary and mobile phases can be used
You can extract the spots for further analysis
What does gas chromatography do additionally from separating the components of the mixture
Measures the amounts of the components of a mixture
What does the term (Retention time) mean in gas chromatography
Retention time is the time taken for the sample to pass through the metal column
Explain how simple distillation works
- Pour the solution into a flask and gently heat it
- The liquid solvent boils but the solute doesn’t, as the solvent has a lower boiling point
- The solvent evaporates and its vapours rise up and pass through the condenser
- The condenser is surrounded with cold water so the vapour molecules cool down and turn back into a liquid
- The liquid droplets are collected up in the beaker/test tube
Why should you draw the baseline with a pencil
Pencil marks are insoluble and won’t move with the solvent.
Pencil doesn’t contain ink, so it wont smudge
What is solvent front?
Where the solvent reached at the end of the analysis
What is a chromatogram?
Results of a chromatography analysis
Describe two main uses of chromatography in day to day life
1.- Checks if a sample is pure
2- Tells us the relative solubility of each substance in a sample
Explain fully how to carry out TLC (thin layer chromatography)
1* - Put solvent into a chromatography tank to the depth of 1 cm . If solvent is flammable make sure there are no naked flames
- 2* - Draw a baseline on the plate using a pencil
- 3* - add a small amount of sample to the baseline of the TLC plate, taking care not to damage the powder on the plate
- 4* - Place the plate inside the tank
- 5* - let the solvent travel through the powder and take the plate before it reaches the top
- 6* - analyse the pattern of coloured spots
What is a substance?
A collection of particles held together by im forces
How does gas chromatography work?
1* - Sample is turned into the gas state when it is injected into the column
- 2* - The carrier gas pushes the sample through the column and each substance of the sample separate
- 3* - The different substances take different times to travel through the column, depending on how strongly they bond to either phase
- 4* - A detector sends a signal to a computer as each substance leaves the column and the computer produces a chromatogram based on the results
