States of matter and mixtures

Question 1

How can you determine whether a substance is pure or impure using a graph showing the temperature when a substance melts?

Answer 1

• 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

Question 2

What do you use filtration for?

Answer 2

To separate an insoluble substance from a liquid or solution

Question 3

How would you carry out a filtration?

Answer 3

• 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

Question 4

What do you use crystallisation for?

Answer 4

To produce solid crystals from a solution

Question 5

How do you carry out a crystallisation?

Answer 5

• 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

Question 6

What is simple distillation used for?

Answer 6

To separate a solvent from the solution

Question 7

How do you carry out simple distillation for salt water?

Answer 7

• Salt water is heated
• The water vapour cools in the condenser and drips into a beaker
• The salt is left behind

Question 8

What is fractional distillation used for?

Answer 8

To separate a liquid from a mixture of miscible liquids (liquids that mix completely with each other) 

Question 9

How do you carry out fractional distillation to separate ethanol and water?

Answer 9

• Water and ethanol solution is heated
• The ethanol evaporates first, cools, then condenses
• Then the water evaporates cools and condenses

Question 10

How does simple distillation work?

Answer 10

• 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

Question 11

How does fractional distillation work?

Answer 11

• 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

Question 12

How does filtration work?

Answer 12

• 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

Question 13

How does crystallisation work?

Answer 13

• The solubility of the solute decreases as the saturated solution cools
• Crystals form from the excess solute

Question 14

What is paper chromatography used for?

Answer 14

To separate mixtures of soluble substances

Question 15

How do you investigate the composition of inks using chromatography?

Answer 15

• 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

Question 16

What are the two phases of chromatography?

Answer 16

Stationary - A substance that does not move (the paper)

Mobile - A substance that moves through the stationary phrase (the solvent)

Question 17

How do you explain chromatography?

Answer 17

• 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

Question 18

What can a chromatogram be used for?

Answer 18

• 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

Question 19

How do you calculate the Rf value of a spot?

Answer 19

Distance travelled by substance / Distance travelled by solvent

Question 20

How do you investigate the composition of inks using simple distillation?

Answer 20

• 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

Question 21

What are the stages of water treatment?

Answer 21

• 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

Question 22

How do you treat seawater?

Answer 22

Simple distillation

Question 23

What are the advantages and disadvantages of simple distillation of seawater?

Answer 23

Advantages:
- Uses a plentiful raw material
- Produces pure water
- Kills microbes in seawater

Disadvantages:
- Needs a lot of energy to heat the water

Question 24

Why must water used for chemical analysis not contain any dissolved salts?

Answer 24

• They could react with the substances used in the analysis