IGCSE CHEMISTRY 0620 LESSON 5 EXPLANATION

Lesson 5

2.2 Purity
2.2.1 Criteria of purity




PART 1

Core

• Demonstrate knowledge and understanding of paper chromatography 
• 
  
   

  
  

  • This technique is used to separate substances that have different solubilities in a given solvent (e.g. different coloured inks that have been mixed to make black ink).
  • A pencil line is drawn on chromatography paper and spots of the sample are placed on it. Pencil is used for this as ink would run into the chromatogram along with the samples.
  • The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent so the samples don´t wash into the solvent container.
  • The solvent travels up the paper by capillary action, taking some of the coloured substances with it.
  • Different substances have different solubilities so will travel at different rates, causing the substances to spread apart. Those substances with higher solubility will travel further than the others.
  • This will show the different components of the ink / dye.
  •  
• Interpret simple chromatograms

• If two or more substances are the same, they will produce identical chromatograms.
• If the substance is a mixture, it will separate on the paper to show all the different components as separate spots.
• An impure substance will show up with more than one spot, a pure substance should only show up with one spot.

 

Paper chromatography is the name given to the overall separation technique while a chromatogram is the name given to the visual output of a chromatography run, namely the piece of chromatography paper with the separated components visible after the run has finished.
The verb run is used in this technique as the samples essentially “run” up the chromatography paper.

• Identify substances and assess their purity from
  melting point and boiling point information

• Pure substances melt and boil at specific and sharp temperatures.
• Melting and boiling points data can be used to distinguish pure substances from mixtures.
• An unknown pure substance can be identified by comparing its m.p and b.p and comparing to data tables.
• Mixtures melt over a range of temperatures as they contain two or more substances.

 

• Understand the importance of purity in substances in everyday life, e.g. foodstuffs and drugs

• A pure substance consists of only one substance and contains nothing else.
• To have a pure substance for food and drugs is very important as impurities could be dangerous even in small amounts.
• Melting and boiling point analysis is routinely used to assess the purity of food and drugs.

 

• Interpret simple chromatograms, including the use of Rf values

• These values are used to identify the components of mixtures.
• The R_f value of a particular compound is always the same.
• Calculating the R_f value allows chemists to identify unknown substances because it can be compared with R_f values of known substances under the same conditions.

Calculation

• Retention factor = distance moved by compound  ÷ distance moved by solvent.
• The R_f value is a ratio and therefore has no units.

 

• Outline how chromatography techniques can be applied to colourless substances by exposing chromatograms to substances called locating agents. (Knowledge of specific locating agents is not required.)

• For chromatography to be useful the chemist needs to be able to see the components move up the paper, which is not the case for invisible samples such as proteins.
• In such cases, locating agents can be used to react with the sample and produce a coloured product which is then visible.
• The chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye.

PART 2 

11.1 Water

• Describe chemical tests for water using cobalt(II) chloride and copper(II) sulfate

Chemical Tests for Water

• Cobalt (II) chloride turns blue to pink on the addition of water. This test is usually done using cobalt chloride paper.
• The equation is:
  • CoCl₂(s) + 6H₂O(l) → CoCl₂.6H₂O(s)

• Anhydrous copper (II) sulfate turns white to blue on the addition of water.
• The equation is:
  • CuSO₄(s) + 5H₂O(l) → CuSO₄.5H₂O(s)

 

• Describe, in outline, the treatment of the water supply in terms of filtration and chlorination

Water Treatment

• Untreated water contains soluble and insoluble impurities.
• Insoluble impurities include soil, pieces of plants and other organic matter.
• Soluble impurities include dissolved calcium, metallic compounds and inorganic pollutants.

• Filtration is the process used to remove large insoluble particles by passing the water through layers of sand and gravel filters that trap larger particles.
• But bacteria and other microorganisms are too small to be trapped by the filters so chlorination is used.

• This involves the careful addition of chlorine to the water supply which kills bacteria and other unwanted microorganisms.
• Cholera and typhoid are examples of bacterial diseases which can arise by the consumption of untreated water.

 

• Name some of the uses of water in industry and in the home

 

Water in industry

• As a coolant to reduce the temperature of some industrial processes e.g: in nuclear power plants.
• Watering crops.
• As a solvent in many chemical production processes.
• Hydroelectric power stations to generate electricity.
• As a first raw material for many processes e.g: the production of ethanol from ethene and steam (water).

Water in homes

• Drinking, cooking and washing.
• General sanitation.
• In car radiators, for gardens and plants.

• 
  
  • Exam questions on water treatment always focus on either filtration or chlorination.
  • You should be able to explain how each process works and the reason for each one.
  • Discuss the implications of an inadequate supply of water, limited to safe water for drinking and water for irrigating crops

• Clean and safe water supply is very important to mankind.
• Many problems arise in the event of an inadequate water supply, including:

• • Food shortages and famine due to a lack of crops which cannot grow without a clean water supply.
  • Poor sanitation leads to spread of bacteria and disease as drinking water becomes infected.

END OF PART 1