C2.1- Purity And Separating Mixtures

Question 1

What is relative atomic mass?

Answer 1

• the mean mass of an atom of an element compared to 1/12 the mass of a carbon atom

Question 2

What is relative formula mass?

Answer 2

• the mean mass of a unit of a substance compared to 1/12 the mass of a carbon atom

Question 3

What is a chemical formula?

Answer 3

• how many atoms of each element there are in a unit of substance

Question 4

What is an empirical formula?

Answer 4

• shows the simplest whole-number ratio of the atoms of each element in a compound

Question 5

What does pure mean?

Answer 5

• pure describes natural substances that haven’t been changed or processed
• a pure substance consists of just one element or compound
• mixtures are impure substances since they contain more than one element or compound

Question 6

Can mixtures be useful?

Answer 6

• difficult to obtain pure substances, just one atom or molecule of something else makes a substance impure
• many useful materials are mixtures of different substances
• an alloy is a mixture of a metal with one or more other elements

Question 7

How do you use melting points to determine purity?

Answer 7

• the melting point of a substance is the temperature at which it changes from the solid state to the liquid state
• the melting point of a pure substance is a single temperature
• If a substance is impure:
  its melting point is less than that of a pure substance
  it often melts over a range of temperatures, not just one

Question 8

How do you determine melting point?

Answer 8

• by heating it
• then either measure the temperature at which it melts, or measure its temperature at regular time intervals and plot a graph - it’s important to:
  heat the substance slowly
  stir the substance as it melts
• heating a substance slowly allows the temperature of the whole sample to increase
• mixing ensures that the entire sample is at the same temperature
• these two actions improve the accuracy of a measurement of the melting point of a sample.

Question 9

What happens when a substance dissolves?

Answer 9

• a solution forms when one substance dissolves in another
• the solute is the substance that dissolves, and the solvent is the substance it dissolves in
• when a substance dissolves, it’s particles separate and become completely mixed with the particles of the solvent
• if a substance can dissolve in a particular solvent, it’s soluble in that solvent, if it can’t dissolve, it’s insoluble in that solvent
• substances can be soluble in one solvent but insoluble in another

Question 10

How does filtration work?

Answer 10

• filtration separates an insoluble substance in the solid state from substances in the liquid state
• it works because filter paper has tiny, microscopic holes
• when you filter a mixture of insoluble substance and a liquid substance, the liquid molecules are small enough to pass through the filter paper, but the after grains of insoluble substance cannot
• the insoluble substance stays behind in the filter paper as the residue while the water passes through as the filtrate
• particles of a dissolved solute are also small enough to pass through filter paper.

Question 11

How does crystallisation work?

Answer 11

• if you heat a solution, the solvent evaporates leaving the solute behind
• if you allow the solvent to evaporate slowly, you get regularly shaped crystals
• you need to heat the solution gently until it becomes a saturated solution
• a solution is saturated when no more solute can be dissolved at that temperature
• crystals will start forming at this point, so you let the solution cool slowly
• as the solution cools, the solubility of the solute decreases, so more crystals form
• you can separate them from the remaining solution by filtration, and dry them in a warm oven or by patting them with filter paper

Question 12

How does simple distillation work?

Answer 12

• separates a solvent from a solution
• it relies on the solvent having a much lower boiling point than the solute
• when the solution is heated, the solvent boils but the solute does not
• the solvent escapes from the solution in its gas state
• it’s then cooled and condensed back to its liquid state by a condenser, a piece of apparatus that is kept cold using a flow of cold water.

Question 13

What is fractional distillation?

Answer 13

• separates two or more substances from a mixture in the liquid state
• it relies on each substance having a different boiling point and uses a special piece of equipment called a fractionating column.

Question 14

How does chromatography work?

Answer 14

• relies on two different chemical phases:
  a stationary phase that doesn’t move
  a mobile phase that does move.
• a phase is a substance in the solid, liquid or gas state
• paper chromatography is sometimes used to separate the coloured substances in ink or sweets
• in paper chromatography:
  the stationary phase is absorbent paper
  the mobile phase is a solvent in the liquid state, such as water or propane

Question 15

How does thin-layer chromatography work?

Answer 15

• works in the same way as paper chromatography but the stationary phase is a thin layer of silica or alumina powder spread over a plate or glass of plastic
• you can use TLC to separate a sample into its components for identification or analysis.
• the pattern produced depends on how each component is distributed between the two phases
• a component travels further up the plate if it forms stronger bonds with the mobile phase than with the stationary phase
• a component will not travel very far if it forms stronger bonds with the stationary phase than with the mobile phase

Question 16

R^f values

Answer 16

R^f = distance travelled by substance
——————————————
distance travelled by solvent

Question 17

How does gas chromatography work?

Answer 17

• it’s different to paper/thin-layer chromatography:
  the stationary phase is silica or alumina powder packed into a metal column
  the mobile phase is an unreactive carrier gas such as nitrogen, which doesn’t react with the sample
• gas chromatography separates the components of a mixture and also measures their amounts

Question 18

How can you tell if a substance is pure

Answer 18

• you can use paper chromatography or thin-layer chromatography, but TLC has some advantages:
  it’s quicker
  it’s more sensitive, so a smaller sample can be used
  there is a larger range of stationary phases and solvents to choose from

Question 19

How to choose a separation method

Answer 19

• INSOLUBLE AND SOLUBLE SUBSTANCES: dissolving followed by filtration
• A SOLUTE DISSOLVED IN A SOLVENT (A SOLUTION): crystallisation to obtain the solute, simple distillation to obtain the solvent
• TWO OR MORE SUBSTANCES IN THE LIQUID STATE: fractional distillation
• COLOURED SOLUBLE SUBSTANCES: paper chromatography or thin-layer chromatography