chemical analysis

• Using melting and boiling point data to determine if a substance is pure


  Measure the melting point and boiling point of a substance to determine if it is pure
• Pure substance

  A single element or a single compound not mixed with any other substance
• Examples of pure substances

  • Pure silicon
  • Pure sodium chloride
• Experiment to determine purity using melting and boiling points

  Heat the substance, measure the melting point where the temperature stops rising, measure the boiling point where the temperature stops rising again
• Both the melting point and boiling point of a pure substance are fixed at specific temperatures
• Melting point of pure water is fixed at 0 degrees Celsius
• Boiling point of pure water is fixed at 100 degrees Celsius
• For any pure substance, the melting and boiling points are fixed at specific temperatures
• Mixtures have melting and boiling points over a range of temperatures, indicating impurity
• Formulation is a complex mixture designed as a useful product with carefully measured quantities of each component
• Properties of a formulation

  • Designed as a useful product
  • Carefully measured quantities of each component
  • Has the properties needed
• There are many examples of formulations
• Revision workbook with questions on pure and impure substances and formulations is available by clicking on the provided link
• Paper chromatography allows us to separate substances based on their different solubilities
• Using paper chromatography

  We take a piece of special paper called chromatography paper and draw a pencil line near the bottom. Then, we put dots of different colors on the paper and place it into a solvent. The solvent moves up the paper, carrying the ink with it. A pure compound will produce a single spot in all solvents, whereas compounds in a mixture may separate into different spots depending on the solvent
• Stationary phase in paper chromatography


  • The paper itself, which does not move
• Mobile phase in paper chromatography

  • The solvent, which moves up the paper carrying the substances
• More soluble substances travel further up the paper than less soluble substances in paper chromatography
• Reason for drawing the starting line in pencil in paper chromatography
• Identifying an unknown substance using paper chromatography

  We measure the distance moved by the unknown substance and the solvent, then calculate the RF value by dividing the distance moved by the substance by the distance moved by the solvent. This RF value can be used to identify the substance by looking it up in a database. If the substance has never been analyzed before, further analysis is needed
• RF values do not have a unit
• Method for carrying out paper chromatography

  Draw a horizontal pencil line on chromatography paper, mark five pencil spots across the line, use a capillary tube to put small spots of known and unknown food colorings, pour water into a beaker as the solvent, attach paper to a glass rod and lower it into the beaker, ensure the spots of ink are above the water surface, paper does not touch sidewalls, put a lid on the beaker, remove paper when water has traveled around 3/4, mark the point where the water reached, hang the paper up to dry
• Known food colorings

  • A
  • B
  • C
  • D
• Unknown color is a mixture of three colors
• Colors A, C, and D line up with the spots in the unknown color

  Unknown color is a mixture of colors A, C, and D
• Rf value calculation
• Rf value is calculated by dividing the distance moved by the chemical by the distance moved by the solvent
• Rf value does not have a unit
• RF value can be looked up in a database to identify the chemical
• Several different chemicals could have the same RF value, so repeating the experiment with a different solvent may be necessary to narrow it down further
• If the chemical has not been analyzed before, there may not be an RF value in the database, requiring further analysis to identify it
• Test for hydrogen gas


  Remove the stopper and insert a burning splint. Hydrogen gas burns rapidly and produces a pop sound
• Test for oxygen gas

  Use a glowing splint. If placed into a test tube of oxygen, the splint will light in other words it bursts into flames
• Test for carbon dioxide gas

  Use lime water, which is an aqueous solution of calcium hydroxide. Draw some of the gas into a plastic pipette and bubble it through the lime water. If the lime water turns cloudy, it indicates the presence of carbon dioxide
• Test for chlorine gas


  Insert damp litmus paper into the mouth of the test tube. Chlorine bleaches the litmus paper and turns it white
• It is important to learn the tests for hydrogen, oxygen, carbon dioxide, and chlorine as they could be in your exam
• Hydrogen gas burns rapidly and produces a pop sound when tested
• Oxygen gas causes a glowing splint to burst into flames when tested
• If lime water turns cloudy, it indicates the presence of carbon dioxide
• Chlorine bleaches litmus paper and turns it white when tested