EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.2.4 Interpreting Chromatograms

EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.2.4 Interpreting Chromatograms

Identifying Mixtures

 

  • Pure substances will produce only one spot on the chromatogram
  • 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 therefore will produce a chromatogram with more than one spot

 

 

Chromatography-%E2%80%93-Pure-Impure

 

Diagram showing the analysis of a mixture and pure substances using chromatography

 

 

Rf Values

 

  • These values are used to identify the components of mixtures
  • The Rf value of a particular compound is always the same but it is dependent, however, on the solvent used
  • If the solvent is changed then the value changes
  • Calculating the Rvalue allows chemists to identify unknown substances because it can be compared with Rvalues of known substances under the same conditions
  • These values are known as reference values

Calculation

  • The Retention factor is found using the following calculation:

 

Rf = distance travelled by substance ÷ distance travelled by solvent

 

  • The Rf value will always lie between 0 and 1; the closer it is to 1, the more soluble is that component in the solvent
  • The Rf value is a ratio and therefore has no units

 

Using-Rf-values-to-identify-mixture-components

 

Using Rf values to identify components of a mixture

 

 

Exam Tip

For the Rf calculations, both distances are measured from the baseline.

转载自savemyexams

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