States of matter and mixtures Flashcards
How can you determine whether a substance is pure or impure using a graph showing the temperature when a substance melts?
- A pure substance will be horizontal during a state change as it has a sharp melting point
- And impure substance will have a slope during that state change as it melts over a range of temperatures
What do you use filtration for?
To separate an insoluble substance from a liquid or solution
How would you carry out a filtration?
- Put the mixture through a filter funnel lined with filter paper
- This will leave a residue in the filter paper and filtrate in the beaker
What do you use crystallisation for?
To produce solid crystals from a solution
How do you carry out a crystallisation?
- Place a solution in an evaporating dish and heat with a Bunsen burner
- Some of the solvent will evaporate and will leave a saturated solution
- Crystals will form in the solution
- The crystals are separated from the liquid and dried
What is simple distillation used for?
To separate a solvent from the solution
How do you carry out simple distillation for salt water?
- Salt water is heated
- The water vapour cools in the condenser and drips into a beaker
- The salt is left behind
What is fractional distillation used for?
To separate a liquid from a mixture of miscible liquids (liquids that mix completely with each other) 
How do you carry out fractional distillation to separate ethanol and water?
- Water and ethanol solution is heated
- The ethanol evaporates first, cools, then condenses
- Then the water evaporates cools and condenses
How does simple distillation work?
- The solute in the solution has a much higher boiling point than the solvent
- See the solute evaporates first
- The remaining solution becomes more concentrated
How does fractional distillation work?
- The liquids in the next year have different boiling points
- Vapours rise up a column which is hot at the bottom and cold at the top
- Vapours condense when they reach a part of the column that is below the temperature of their boiling point
- The liquid then flows out of the column
How does filtration work?
- The filter paper has tiny pours
- They are large enough to let water molecules and dissolved substances through
- But small enough to stop insoluble solid particles go through
How does crystallisation work?
- The solubility of the solute decreases as the saturated solution cools
- Crystals form from the excess solute
What is paper chromatography used for?
To separate mixtures of soluble substances
How do you investigate the composition of inks using chromatography?
- Draw a line at the bottom of the filter paper (baseline)
- Place small dots of ink on the line using a pipette
- Place the paper in a shallow amount of solvent but don’t submerge the sample of ink
- Cover with a lid to prevent evaporation
- Wait for the solvent to seep up the paper but remove before it reaches the top
- Allow the chromatograms to dry then measure the distance travelled by each spot and by the solvent
- Calculate the Rf values of each spot and compare
What are the two phases of chromatography?
Stationary - A substance that does not move (the paper)
Mobile - A substance that moves through the stationary phrase (the solvent)
How do you explain chromatography?
- Each soluble substance in the mixture forms bonds with the two phases
- Substances that from strong attractive forces with the stationary phase stay near the bottom
- Substances that form stronger attractive forces with the mobile phrase move towards the top
What can a chromatogram be used for?
- To distinguish between pure and impure substances (a pure substance will produce only one spot)
- To identify a substance by comparing its pattern of spots with those of a known substance
- To identify a substance using Rf values
How do you calculate the Rf value of a spot?
Distance travelled by substance / Distance travelled by solvent
How do you investigate the composition of inks using simple distillation?
- Add a small volume of ink to a flask and connect to a fractionating column
- Attach a condenser to the top of the fractionating column and connect it to cold water
- Heat the flask using a Bunsen burner
- Collect a small sample of distilled solvent
- Note down the difference in appearance of the distilled solvent and the original ink
- If the temperature was measured, compare this to the boiling points of possible solvents
What are the stages of water treatment?
- Screening, the water is passed through a mesh to remove solids
- Sedimentation, large insoluble particles sink to the bottom of the tank as a sludge
- Filtration, small insoluble particles are removed by filtering through beds of sand
- Chlorination, chlorine gas is bubbled from the water to kill microbes
How do you treat seawater?
Simple distillation
What are the advantages and disadvantages of simple distillation of seawater?
Advantages:
- Uses a plentiful raw material
- Produces pure water
- Kills microbes in seawater
Disadvantages:
- Needs a lot of energy to heat the water
Why must water used for chemical analysis not contain any dissolved salts?
- They could react with the substances used in the analysis
