purity and seperation

• a pure substance

   consists of a single element or compound which contains no other substances
• • A mixture consists of two or more elements or compounds that are physically mixed together, they are not chemically combined
• what is air?

  a mixture
• • Pure substances melt and boil at specific and sharp temperatures e.g. pure water has a boiling point of 100°C and a melting point of 0°C
• • Mixtures have a range of melting and boiling points as they consist of different substance that tend to lower the melting point and broaden the melting point range
• how can you use melting/boiling points to distinguish pure substances from mixtures?

  if the substance has a range of melting/boiling points = impure
• does this cooling graph show a pure or impure substance?

  pure because the horizontal part of the graph shows that the compound has a sharp melting point

  
• does this cooling graph show a pure or impure substance?

  impure because there is a gradual decrease in temperature due to the range of melting points.

  
• percentage range = (final value - original value) / original value x 100
• symbol for relative atomic mass?

  Ar.
• what is the relative atomic mass?

  the average mass of the atoms of an element measured relative to the mass of carbon-12
• what is the relative molecular mass?

  the average mass of the atoms of one molecule or compound measured relative to the mass of one atom of carbon-12
• symbol for relative formula mass?

  Mr and it refers to the total mass of the molecule
• • Empirical formula is the simplest whole number ratio of atoms of each element in a molecule 
  • The molecular formula tells you the actual number of atoms of each element in a molecule
• how to find percentage composition?

  % mass of an element = (Ar x no. of atoms of the element) / Mr of the compound x 100
• • An alloy is a mixture of metals
• • Alloys are also an example of a formulation
  • A formulation is a mixture that has been designed as a useful product 
• • Most metals are too soft to use on their own and are mixed with other elements to make them stronger and harder
• • Alloys contain atoms of different sizes, which distorts the regular arrangements of atoms
  • This makes it more difficult for the layers to slide over each other, so they are usually much harder than the pure metal
• filtration

  • Used to separate an undissolved solid from a mixture of the solid and a liquid / solution ( e.g. sand from a mixture of sand and water)
• fill in the gaps!

  A) filter funnel

  B) filter paper

  C) liquid only (filtrate)

  D) insoluble solid and liquid

  
• Crystallisation

  • Used to separate a dissolved solid from a solution, when the solid is much more soluble in hot solvent than in cold
• fill in the blanks

  A) crystallised solute

  B) the solute will slowly crystallise

  
• Simple Distillation

  • This is used to separate a liquid and soluble solid from a solution (e.g., water from a solution of salt water) or a pure liquid from a mixture of liquids
• fill in the blanks!

  A) pure water vapour

  B) salty water

  C) pure water

  D) vapur condenses in condenser

  E) 100 degrees

  
• Fractional Distillation

  • This is used to separate two or more liquids that are miscible with one another (e.g., ethanol and water from a mixture of the two)
• chromatography is used to separate substances and provide information to help identify them.
• the components of chromatography have different what?

  solubilities in a given solvent
• why is a pencil used in chromatography?

  the ink from a pen would run into the chromatogram along with the sample
• if a substance has a higher solubility what will it do?

  travel further than others
• what are the 2 phases in all types of chromatography?

  mobile and stationary
• what is the mobile phase in paper chromatography?

  the solvent in which the sample molecules move - the liquid
• what is the stationary phase in paper chromatography?

  chromatography paper
• what is the stationary phase in thin-layer chromatography?

  a thin layer of an intert substance (like silica) supported on a flat unreactive surface
• what is the mobile phase in thin layer chromatography?

  the solvent, the liquid
• how to calculate the retention factor?

  distance moved by compound / distance moved by solvent
• when is gas chromatography used?

  to seperate a mixture of gases
• what is the mobile phase in gas chromatography?

  unreactive carrier gas (such as nitrogen)
• what is the stationary phase in gas chromatography?

  a thin layer of an unreactive liquid e.g. silica
• what does the number of peaks tell you?

  number of compounds in mixture