States of Matter + Method of Separating/Purifying Substances

• Changes to particles during changes of state

  1. Arrangement changes

  2. Movement changes

  3. Energy changes
• Deciding state of a substance

  Based on temperature compared to melting point and boiling point
• The particle model explains state changes in terms of arrangement, movement and energy of particles
• Particle arrangement in different states

  • Solid: Regular, close together, vibrate
  • Liquid: Random, close together, move around
  • Gas: Random, far apart, fast movement in all directions
• Particles in solids contain the smallest amount of stored energy, particles in gases contain the most
• Sublimation


  Direct transition from solid to gas state without going through liquid state
• Deposition


  Direct transition from gas to solid state without going through liquid state
• As a substance is heated

  Its temperature stays constant during state changes (melting, boiling)
• Particles are attracted to one another by weak forces of attraction
• During melting


  Some attractive forces between particles are overcome
• During evaporation and boiling

  Remaining attractive forces between particles are overcome
• A pure substance has the same fixed composition in all its parts
• A mixture contains elements and/or compounds that are not chemically joined together
• Mixtures do not have a fixed composition, their composition can change
• When a solid melts


  Its particles gain enough energy to overcome the weak forces of attraction and move further apart
• Pure substances have the same melting point throughout, mixtures melt over a range of temperatures
• Filtration allows smaller pieces to pass through but traps larger pieces or insoluble substances
• Crystallisation


  Separating solutes from a solution by evaporating the solvent to leave the solutes behind
• Slow crystal formation allows particles longer to form an ordered pattern, making larger crystals
• Salt production by solution mining

  1. Water pumped into salt layers underground

  2. Salt solution heated to evaporate solvent

  3. Saturated solution forms salt crystals
• Saturated solution


  Contains the maximum amount of solute that can dissolve in that amount of solvent at that temperature
• Crystals form during crystallisation when the solution becomes supersaturated as the solvent evaporates
• Hazards must be identified and risks reduced when doing experiments with filtration and crystallisation
• Hazards in filtration and crystallisation

  • Spitting of hot crystals
• A hazard is something that could cause harm, a risk is the chance of a hazard causing harm
• Chromatography can be used to separate and identify the coloured compounds in mixtures like inks, paints and foods
• Chromatography separates mixtures based on the different ways their components move through a stationary phase
• Rf values can be calculated to identify components in a chromatogram
• Separating sand and salt from a mixture

  1. Grind up plants with methanol

  2. Methanol dissolves many plant compounds

  3. Make large crystals to help scientists work out what the compounds are made of
• Paper chromatography


  Technique that works because some compounds dissolve better in a solvent than others, so they are separated as the solvent moves along the paper
• Paper chromatography can be used to distinguish between pure and impure substances, identify substances by comparing the pattern on the result with known substances, and identify substances by calculating their Rf values
• The Rf value is the distance the compound has risen divided by the distance the solvent has risen, both measured from the starting positions
• The Rf value of a particular compound does not change if the chromatography conditions used remain the same
• A compound never rises as fast as the solvent, so Rf values are always less than 1
• If you calculate an Rf value bigger than 1, you've made a mistake
• Distillation


  Combination of evaporation followed by condensation to separate a liquid from dissolved solids
• Tap water contains dissolved minerals, especially in hard water areas, so it is a mixture
• To make pure water, we need to separate it from the dissolved solids, which is done by distillation
• Simple distillation


  Water is heated in a flask and the vapour travels along a tube where it condenses
• Fractional distillation


  Separates a mixture of liquids by taking advantage of their different boiling points