C2

• What is meant when a substance is described as pure?

  A pure substance is made up of a single element or compound.
• How can melting points be used to identify a pure substance?

  A pure substance will have an exact sharp melting point. If a substance is not pure it will melt across a range of temperatures due to the fact that it contains a mixture of elements/compounds.
• What apparatus could be used to measure temperature? Which is the most precise?

  Thermometer or temperature probe.

  Temperature probe is the most precise as it can record temperature to 2 decimal places.
• What is the meaning of relative atomic mass?

  The average mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom.
• What is the meaning of relative formula mass?

  The weighted mean average masses of the formula units compared to 1/12th the mass of a carbon-12 atom.
• What is the meaning of relative molecular mass?

  The mean average mass of one molecule of an element or compound compared to 1/12th the mass of one atom of carbon-12.
• How is relative formula mass calculated?

  Add together the relative atomic masses of each of the elements in the chemical formula.
• What is the empirical formula?

  The smallest whole number ratio of atoms of each element in a compound.
• What is the molecular formula?

  The formula that shows the actual number of atoms of each element in the compound.
• What is an alloy?

  A mixture of two or more metals.
• Why are alloys commonly used instead of pure metals?

  Alloys can be engineered to have more desirable properties than pure metals. Alloys are often harder and stronger than pure metals because the different sized atoms within an alloy distort the layers so they can't slide over each other as easily.
• Many useful materials are formulations of mixtures. Explain what this means.

  A formulation is a mixture containing exact quantities of different substances. The quantities have been refined and tested to ensure the material has optimum properties for its purpose.
• Give examples of materials which are formulations of mixtures.

  • Sunscreen
  • Medicine
  • Perfume
  • Drinks
• State which process can used to separate an insoluble salt from a solution. How does it work?

  Filtration:

  • Put filter paper in a funnel and place it over an empty conical flask.
  • Pour the mixture of the insoluble salt and solution through the funnel. Use distilled water to wash any of the salt left in the beaker through the funnel.
  • Remove the filter paper and evaporate the water from the residue if the salt is being collected. The solution will collect in the conical flask.
• What process can be used to separate a soluble salt from a solution? How does it work?

  Crystallisation:

  • Gently heat the solution in an evaporating basin to increase the concentration of the solution.
  • Remove from the heat and allow the solution to cool.
  • Slowly the salt crystals will form as the rest of the water evaporates.
• When is simple distillation used as a separation technique?

  To separate one liquid from a mixture of liquids that have different boiling points.
• Describe the process of separating a mixture using simple distillation.

  • Place mixture in a round bottomed flask. Connect to a condenser with a beaker at the end. Cold water should enter the condenser at the bottom and leave at the top.
  • Heat the flask.
  • The liquid with the lower boiling point will evaporate first and enter the condenser.
  • The vapours cool in the condenser and drip into the beaker.
• What substance is the process of fractional distillation commonly used to separate? Why?

  Crude oil.

  Fractional distillation can separate several substance in a mixture if they have different boiling points. This is useful for crude oil as it allows the separation of the hydrocarbons it contains.
• How does the process of fractional distillation work to separate crude oil?

  1. The oil is heated until it evaporates.
  2. The vapours enter the fractioning column. The column has a temperature gradient with temperature decreasing up the column.
  3. Different compounds have different boiling points. The vapours slowly rise up the column and condense at different fractions depending on their boiling point.
  4. The separated hydrocarbons are converted into products for the petrochemical industry.
• What is chromatography? What does it involve?

  Chromatography is a process used to separate a mixture of soluble substances.

  It involves a stationary and a mobile phase. Separation depends on the distribution of substances between the two phases.
• What are the specific phases used in paper chromatography?

  Stationary phase: Paper

  Mobile phase: Solvent
• What are the specific phases used in thin layer chromatography (TLC)?

  Stationary phase: Thin layer of an inert substance supported on an unreactive surface.

  Mobile phase: Solvent.
• Describe how to carry out paper or thin layer chromatography.

  A pencil line is drawn 2cm up from the base of the stationary phase. A dot of the mixture being tested is placed on this line. The stationary phase is then placed in a beaker with 1cm of solvent. The solvent travels up the stationary phase and the mixture is dissolved into the mobile phase. The substances separate depending on how soluble they are in the solvent.
• What is meant by mobile phase?

  A substance in the liquid or gas state that moves during chromatography.
• What is meant by stationary phase?


  A substance in the solid or liquid state that does not move during chromatography.
• Why must the solvent level be below the pencil line in paper chromatography and TLC?

  To prevent the solvent submerging the substance being tested and washing it away.
• Why is pencil used to draw the baseline in paper chromatography?

  Pencil is insoluble so will not affect the results of the experiment. Ink is soluble so it would travel through the solvent and alter the results.
• What is an Rf value?

  Rf - Retention factor

  The Rf value is a ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent.
• How do you calculate the Rf value from a chromatogram?

  Rf = Distance travelled by substance/Distance travelled by solvent
• When is gas chromatography used?

  Gas chromatography is used to separate mixtures of volatile liquids.
• What are the specific phases used in gas chromatography?

  Stationary phase: Solid/liquid on solid support.

  Mobile phase: Inert carrier gas.
• How does gas chromatography separate a mixture of compounds?

  A gas (mobile phase) is used to carry substances through a column packed with a solid (stationary phase). The substances travel through the column at different speeds so are separated. The time they take to reach the detector is called the retention time. This can be used to identify each substance.
• How can chromatography be used to distinguish between pure and impure substances?


  Pure - one spot on chromatogram (paper/TLC) or one peak on one peak on graph (gas chromatography).

  Impure - multiple spots/peaks.
• What are metals and where are they found on the periodic table?

  Metals are elements which react to form positive ions.

  They are found on the left side of the periodic table.
• What are non-metals and where are they found in the periodic table?

  Non-metals are elements which react to form negative ions.

  They are found towards the top right of the periodic table.
• What are the general properties of metals?

  • Shiny
  • Good conductors of electricity
  • Dense
  • Malleable and ductile
  • High melting and boiling points
• What are the general properties of non-metals?

  • Dull appearance
  • Poor conductors of electricity
  • Lower density than metals
  • Low melting and boiling points
  • Brittle
• What is formed when a metal reacts with oxygen?

  Metal oxide.
• How are positive and negative ions formed?

  Positive ions are formed when a metal loses an electron.

  Negative ions are formed when a non-metal gains an electron.
• How are elements arranged in the periodic table?

  Elements are arranged in order of increasing atomic number so that elements in the same group (column) have similar properties.