Topic 2 - Methods of Separating and Purifying Substances Flashcards
2.5 - Explain the difference between a pure substance and a mixture
A pure substance cannot be changed and all the parts of the substance are the same. Pure substances don’t can’t be separated using physical methods
A mixture contains elements and compounds which aren’t chemically bonded together.
Mixtures don’t have a fixed composition like air where O2 may decrease when breathing.
2.6 - What is a melting point?
When a solid melts, its particles gain enough energy to overcome some of the weak forces of attraction. The temperature at which this happens is the melting point.
2.6 - How do melting points tell whether a substance is pure or is a mixture?
Pure substances will have the same composition so melting point is the same everywhere and melts a single temperature until it changes state. This can be interpreted on a graph as a sharp spike.
Mixtures have different elements and compounds so different parts of the substance melt at different temperatures, on a graph it would look curved and won’t have sharp, fixed melting points.
2.7 - Explain the types of mixtures that can be separated with filtration
Mixtures with big insoluble substances can be filtered out while smaller substances are let through.
Filtration is used in air conditioning, cars etc..
In a lab, a filter funnel in lined with filter paper, some of the solution passes through the filter forming the filtrate. Insoluble substances are trapped in the filter forming the residue.
2.7 - Explain the types of mixtures that can be separated with crystallisation
Crystallisation is used to separate a type of mixture called solution which is made of solutes ( dissolved substances ). These solutes are separated from the solution when evaporating the solvent. This forms solid crystals of the solute, slower the process takes, particles don’t form ordered patterns and make larger crystals.
Saturated solutions which have maximum amount of solute dissolved in a solution are also mixtures that can be separated via crystallisation.
Crystallisation is used in solution mining, where water is pumped into layers of salt underground. The solution is then heated to evaporate the water and produce salt crystals.
2.7 - Explain the types of mixtures that can be separated with paper chromatography
Mixtures of coloured compounds are separated into the colours the substance is made up of via paper chromatography.
2.7 - Explain the types of mixtures that can be separated with simple distillation
Mineral water is a mixture and is poured in a conical flask with a bung and a delivery tube pouring water into a test tube. When water is evaporated is turns into a gas, minerals which have higher boiling points are left behind. This leaves a pure water vapour which can be condensed to liquid again. Apparatus used in distillation is a still.
A more efficient apparatus instead of a delivery tube is a condenser which keeps the tube cool, condensing the evaporated water into a liquid. Anti bumping granules are also used in order to make liquid boil smoothly and prevent over boiling it.
2.7 - Explain the types of mixtures that can be separated with fractional distillation
Mixtures with different liquids can be separated by boiling the mixture. Liquids with lower boiling points will evaporate into fractionating column at a higher position while higher boiling points will turn into a gas later.
This can be effficiently done by fixing a fractionating column on a flask, then as the vapour rises up the column it moves into the condenser where it hits the cool glass and condenses into a conical flask. As heating continues substances with higher boiling points rise up the column as is collected from the column later.
Used to separate products in crude oil, produce alchohol and separate liquid air.
2.8 - Describe an experimental technique to separate a mixture knowing its properties
Simple distillation - Mixture with a solvent and a solute
Fractional distillation - Mixture which have substances with varying boiling points
Filtration - A mixture with a liquid containing insoluble substances
Crystallisation - Mixture called a solution with a solute and solvent
Paper Chromatography - Mixtures with different coloured compounds
2.9 - Describe the process of paper chromatography
A solvent moves along a strip of paper, carrying the mixture which has different substances at different speeds, separating them. The solvent ( what carries the mixture ) is the mobile phase ( molecules can move ) . The paper contains the stationary phase ( molecules can’t move ) where the solvent and dissolved substances from the mixture move. When the substances have finished separating, the final product is called a chromatogram.
2.10 - What can a chromatogram be used for?
Can distinguish between pure and impure substances ( impure has multiple colours )
Identify substances by comparing chromatogram pattern with known chromatogram of substances.
Identify substances by calculating their Rf values ( distance moved by spot/distance moved by solvent which is the water )
2.11 - Core practical : Separating inks through distillation and chromatography
Simple distillation - Set up an apparatus with ink and anti bumping granules in a conical flask over a bunsen burner, connected to a thermometer and delivery tube delivering the distillate to a test tube in a cold water bath to aid condensation.
Heat the flask with a bunsen burner making sure it doesn’t over boil.
Continue heating until there is enough distillate.
Note the maximum temperature obtained.
Paper chromatography - Draw a pencil line on a chromatography paper, 2cm from the bottom.
Add a small spot of ink to the line.
Add water to a container of 1cm depth.
Place paper in the container allowing the water to travel through the paper.
Take paper out and mark position of the solvent front ( how far solvent has travelled ) then let it dry.
Measure distance travelled by water from pencil line and distance of each coloured substance.
Calculate Rf value for each substance.
2.12 - Describe how ground and sea water can be made potable
Water from lakes and aquifers contain fresh water, which has impurities like twigs, grit, fertiliser and microorganisms. Different steps are used to make water potable : sedimentation is used to settle out small particles, filtration is used to get rid of particles by using sand and gravel through which water must move to get to the next stage, chlorination is the process of adding chlorine to kill microorganisms and treat the water making the water potable.
Sea water is made potable through simple distillation. The water is heated so water vapour leaves, then it is cooled and condensed to form water without salts. Takes lots of energy so only done in areas of abundant sea water and cheap energy.
2.12 - Describe how water is used in chemical analysis
Chemical analysis is using chemical reactions and machines to identify and measure amounts of substance in a sample. Water can’t have any dissolved salts as this would lead to incorrect results as the machine could detect these salts, it can also form cloudy precipitates which could hide correct results of the analysis.
