2 - Methods of purifying & separating substances Flashcards
What is a pure substance?
A single substance with a fixed composition that does not have anything mixed with it
Describe the composition of a pure substance:
- Cannot be changed
- Is the same in all parts of the substance
E.g Pure gold only contains gold atoms
What is an Element?
A simple substance made up of only one type of atom
What is a Compound?
A substance that can be split into simpler substances because it contains the atoms of two or more elements joined together
What is a Mixture?
A mixture containing two or more different substances that are not joined together
How would a heating curve look like for a mixture?
Melts over a range of temperatures
How would a heating curve look like for a pure substance?
Has a sharp melting point
What is an insoluble substance?
A substance that cannot be dissolved in a certain liquid
Give me some examples of filtration in day to day life?
- Cars
- Vacuum cleaners
- Air-conditioning systems
What is a Solvent?
Describes the liquid in which a substance dissolves to make a solution
What is a Solute?
Describes a substance that dissolves in a liquid to make a solution
What is a Solution?
A mixture formed by a solvent and a solute
What is Crystallisation?
Separating the solute from a solution by evaporating the solvent
What does crystallisation form?
Solid crystals of various sizes
How would you get the crystals to be larger & form an ordered pattern in crystallisation?
- Put the solution in an evaporating basin
- Warm the solution by placing the evaporating basin over a boiling water bath
- Stop heating before all the solvent has evaporated
After the remaining solution has cooled down, pour the excess liquid away (or filter it). Dry the crystals using a warm oven or in air.
What happens if the crystals in crystallisation form more slowly?
The particles have longer to form an ordered pattern & will make larger crystals
What is a Saturated solution?
Contains the maximum amount of solute that can dissolve in that amount of solvent at that temperature
How would you filter a solution?
- A filter funnel is lined with filter paper that has fine holes in it
—> The solvent & solute(s) pass through the fine holes to form filtrate - Some insoluble substances cannot fit through the holes and leave a residue in the filter paper
How would you do crystallisation after you’ve filtered your solution?
A Bunsen burner is used to evaporate the filtrate but don’t overheat the solution once it is saturated
Why must you take care to not overheat the solution once it’s saturated in crystallisation?
- Hot crystals may spit out
- Further heating may cause crystals to change chemically
What is a Hazard?
Something that could cause harm
What is a Risk?
The chance of a hazard causing harm
What is a risk assessment?
- The hazards of doing an experiment are identified
- Then, ways of reducing the risk of a hazard causing harm is considered
How can the risks from spitting be reduced during crystallisation?
- Wearing eye protection
- Removing the Bunsen burner before the solution is completely dry
- Using steam to heat the evaporating basin gently
What is Chromatography?
A technique of separating the components of a mixture
E.g. Different food colouring agents
What is the Stationary phase in chromatography?
The surface through which the solvent and dissolved substances move in chromatography
What is the Mobile phase in paper chromatography?
When the solvent moves along the paper carrying dissolved samples with it
What is a Chromatogram?
The piece of paper showing the results of chromatography
Explain what is paper chromatography:
The separation of mixtures
of soluble substances by running a solvent (mobile phase)
through the mixture on the paper (the paper contains the
stationary phase)
—> This causes the substances to move at different rates over the paper
What is the Rf Value?
And what do they range from?
Rf values range from 0 - 1
What is paper chromatography used for:
- Distinguishing pure & impure substances
- Identifying substances by calculating their Rf values
- Identifying substances by comparing the pattern on the chromatogram with the patterns formed by known substances
How can you ‘distinguishing pure & impure substances’ in a paper chromatogram?
Pure substance = one spot
Impure substance/mixture = two or more spots
*On the chromatogram
How can you ‘Identify substances by comparison with known substances’ in a paper chromatogram?
- They produce the same number of spots and match in colour. t
- The spots have the same R f value
How can you ‘Identify substances by calculation and use of Rf values’ in a paper chromatogram?
Substances which are the same will have the same Rf value
What is Distillation?
The process of separating a liquid from a mixture by evaporating the liquid and then condensing it (so it can be collected)
What is Simple distillation?
Separation method used to separate a solvent from a solution.
When does Simple distillation work?
When the dissolved solute has a much higher boiling point than the solvent
How does Simple distillation work?
When the solution is heated, solvent vapour evaporates from the solution
The gas moves away and is cooled and condensed.
The remaining solution becomes more concentrated in solute as the amount of solvent in it ⬇️.
Which way does water go in and out in the condenser?
What is a Still?
A piece of apparatus used to carry out distillation or fractional distillation
Why is this method of simple distillation NOT efficient?
Much of the vapour is lost
What is Fractional distillation?
A method of separating a mixture of liquids with different boilng points into individual components (fractions)
What are some of the uses of Fractional distillation?
- To separate the different products in crude oil
- To make alcoholic drinks (e.g. Whiskey, Vodka)
- To separate the gases in the air, after the air has been cooled & turned to a liquid at -200 degrees Celsius
Is the fractions in fractional distillation pure substances?
It can be pure or a mixture
Why is this method of fractional distillation efficient?
The condenser keeps the tube cool, so that almost all of the vapour condenses turn into a liquid
What is desalination?
A process that produces fresh drinking water by separating the water from the salts in salty water
What does potable mean?
Safe to drink
Using which process can sea water be made potable?
Simple distillation
When is purifying sea water by distillation carried out on a large scale?
- Where energy resources are cheap / plentiful
- An abundant supply of sea water
What is Chemical analysis?
Using chemical reactions or sensitive machines to identify & measure the substances in a sample
What will happen if the water used in Chemical analysis contains dissolved salts?
Incorrect results will be obtained
What will happen if you use tap water in Chemical analysis ?
Tap water contains small amounts of dissolved salts
—> May react to form unexpected cloudy precipitates
May hide the correct result of the analysis
What is an Aquifer?
Underground rocks containing groundwater
What does fresh water from reservoirs, lakes, rivers or aquifers contain?
And give an example of each point
- Objects (e.g. Leaves, Twigs)
- Small insoluble particles (e.g. Grit, Salt)
- Soluble substances (e.g. salts, pesticides & fertilisers)
- Bacteria / other microorganisms that may be harmful to health
What is Chlorination?
The process of adding Chlorine to a substance (often water)
What is Sedimentation?
The process in which rock grains & insoluble substances sink to the bottom of a liquid
What steps are needed to deal with impurities in fresh water so it can be made potable?
- Screening using a sieve
- Sedimentation
- Filtration (using tanks containing beds of sand & gravel)
- Chlorination (chlorine kills microorganisms in the treated water)
Core practical: Investigating inks
How would you use simple distillation to separate a sample of the solvent from the ink?
1) Set up apparatus
2) Heat the flask of ink using a Bunsen burner. Make sure ink simmers gently and does not boil over delivery tube
3) Continue heating until you have collected enough distilled solvent
4) Note max temp obtained
Core practical: Investigating inks
How would you use paper chromatography to separate the coloured substances in samples of ink?
1) draw a pencil line on a chromatography paper (around 2cm from bottom)
2) Add a small spot of ink to pencil line
3) Add water to a container to a depth of about 1cm
4) Place paper in container (make sure its supported). Allow water to travel through paper
5) Take paper out before it reaches the top
6) Calculate the Rf value for each substance
