8: Chemical Analysis

• How is the word "pure" defined in chemistry?

  It means one element or one compound
• Why is 'pure' orange juice not pure in the chemical sense?

  It contains different substances mixed together
• What is a pure substance?

  One element or one compound only
• What is a mixture?

  Two or more different substances not joined
• How can the components of a mixture be separated?

  Without chemical reactions
• What does an element contain?

  Just one type of atom
• What does a compound contain?

  Two or more types of atoms joined
• What can the different substances in a mixture be?

  Elements or compounds
• What is the melting point characteristic of pure substances?

  They have a sharp melting point
• How do mixtures behave when melting?

  They melt over a range of temperatures
• What does a cooling curve for a pure substance show?

  The temperature stays the same during state change
• What does the horizontal part of a cooling curve indicate?

  A sharp melting point for a pure substance
• What happens to the temperature of impure substances as they freeze?

  It produces a gradual fall in temperature
• If a sample of sulfur melts between 95-101°C and completely melts at 113°C, is it pure?

  No, it is impure
• What indicates that a substance is impure based on melting point data?

  Melting point range differs from the known value
• What is a formulation?

  A mixture designed as a useful product
• Why are chemicals added in a formulation?

  Each chemical has a specific purpose
• What is paper chromatography used for?

  To separate mixtures of soluble substances
• What types of substances are often separated by chromatography?

  Coloured substances like inks and dyes
• What are the two phases in chromatography?

  • Mobile phase: the solvent that moves
  • Stationary phase: the paper that does not move
• How do different substances move in chromatography?

  They move at different rates through the paper
• How can a chromatogram distinguish between pure and impure substances?

  • Pure substance: one spot on chromatogram
  • Impure substance: two or more spots
• What indicates that two substances are likely the same in chromatography?

  Same number of spots and matching colors
• What does the Rf value indicate in chromatography?

  It helps identify unknown chemicals
• How is the Rf value calculated?

  Rf = distance travelled by substance / distance travelled by solvent
• What is the range of Rf values?

  From 0 to 1
• What does an Rf value of 0 indicate?

  The substance is not attracted to the mobile phase
• What does an Rf value of 1 indicate?

  The substance is not attracted to the stationary phase
• What is the purpose of tests for gases?

  To detect and identify gases and ions
• Why must the test for any gas or ion be unique?

  To determine which gas or ion is present
• What does oxygen support?

  Combustion
• What happens to a glowing splint in the presence of oxygen?

  It relights when held inside
• How does hydrogen behave in air?

  It ignites
• What sound indicates the presence of hydrogen in a test tube?

  A squeaky pop
• What reaction occurs when carbon dioxide is bubbled through limewater?

  It produces a white precipitate of calcium carbonate
• What is limewater a solution of?

  Calcium hydroxide
• What observation indicates the presence of carbon dioxide in limewater?

  Limewater turns milky or cloudy white
• What type of gas is chlorine?

  An acidic gas
• What happens to damp litmus paper in chlorine?

  It is bleached white
• What occurs when damp blue litmus paper is placed in chlorine?

  It turns red then white