C8

• Pure substance


  Consists of only one element or only one compound
• Mixture


  Two or more different substances that are not joined together
• The components of a mixture can usually be separated without chemical reactions
• Types of chemical substance


  • Element (contains just one type of atom)
  • Compound (contains two or more types of atom joined together)
  • Mixture (contains two or more different substances not joined together)
• Pure substances


  • Have a sharp melting point
  • Melt at a single temperature
• Mixtures


  • Melt over a range of temperatures
  • Do not have a sharp melting point
• Impure substances produce a gradual fall in temperature as they freeze
• Formulation


  A mixture that is always made with the same proportions of the same substances
• The student's sample of sulfur is impure because it melts over a range of temperatures and the melting point is not the same as the 'standard' reference melting point for sulfur
• In a formulation, every chemical has been added in a carefully measured amount
• Each chemical in a formulation has a specific purpose
• Chromatography


  • Can be used to separate a mixture of solutes
• Lab tests


  • Can be used to identify common gases
• Formulations


  Mixtures designed with a specific purpose
• Formulations


  • Mixtures designed with a specific purpose
• Chromatography


  Used to separate a mixture of solutes
• Chromatography


  Can be used to separate a mixture of solutes
• Lab tests can be used to identify common gases
• Chromatography can be used to separate different substances dissolved in a liquid
• Oxygen test


  Glowing splint relights when held inside a test tube
• Soluble


  Able to dissolve in solvent
• Hydrogen test


  Lighted splint held near the mouth of a test tube ignites with a squeaky pop
• Chromatography


  • Provides information on the possible identity of the substances present in the mixture
  • Often used to separate coloured substances such as food colourings, inks, dyes or plant pigments
• Carbon dioxide test


  Bubbling the gas through limewater turns it milky or cloudy white
• Paper chromatography


  1. Spots of ink or plant dye are placed on a pencil line

  2. Paper is held above a basin containing solvent

  3. Solvent moves through the paper, carrying different substances with it
• Chlorine test


  Damp litmus paper is bleached white when placed in chlorine
• Mobile phase


  Phase in chromatography that moves, usually a solvent or mixture of solvents
• Tests for gases


  • Oxygen
  • Hydrogen
  • Carbon dioxide
  • Chlorine
• Stationary phase


  Phase in chromatography that does not move, for instance, the paper in chromatography
• Dissolved


  A substance is said to be dissolved when it breaks up and mixes completely with a solvent to produce a solution
• Chromatogram


  The results of separating mixtures by chromatography
• Pure


  A substance that consists of only one element or only one compound
• Impure


  A substance that consists of more than one element or compound
• A pure substance produces one spot on the chromatogram, an impure substance produces two or more spots
• R f value


  The ratio of the distance travelled by a substance to the distance travelled by the solvent
• Calculating R f value


  R f = distance travelled by substance / distance travelled by solvent
• R f values vary from 0 (the substance is not attracted to the mobile phase) to 1 (the substance is not attracted to the stationary phase)
• R f values can be used to identify unknown chemicals if they can be compared to a range of reference substances
• The R f value is always the same for a particular substance if run in the same solvent system