Critieria of Purity | Chemistry for EmSAT Achieve PDF Download

CIE IGCSE Chemistry

Revision Notes

• Experimental Techniques
• Measurement & Purity
• Criteria of Purity

Chromatography Technique Overview

• This method is utilized to separate substances with varying solubilities in a specific solvent, such as different colored inks blended to create black ink.
• Initially, a pencil line is drawn on chromatography paper, and sample spots are deposited on it. The reason for using a pencil is to prevent the ink from mixing with the samples.
• The paper is then immersed into the solvent container ensuring that the pencil line stays above the solvent level to prevent samples from washing into the container.
• Through capillary action, the solvent ascends the paper, carrying colored substances along with it.
• Distinct substances, due to their different solubilities, move at distinct rates, causing them to separate. Substances with higher solubility travel farther than others.
• This process reveals the various components present in the ink or dye being analyzed.

Analysis with Paper Chromatography

• One practical application of this technique is the analysis of ink composition using paper chromatography.

Paper Chromatography Overview

• 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, while a pure substance should only show up with one spot.

Key Terminology:

• Overall: Paper chromatography is the name given to the overall separation technique.
• Chromatogram: A chromatogram is the visual output of a chromatography run, showing the visibly separated components on chromatography paper after the process is complete.
• Visual Output: Refers to the separated components on the chromatography paper.

Pure Substances vs. Mixtures: Melting and Boiling Points

• Pure substances exhibit specific and distinct melting and boiling points, such as water with a boiling point of 100°C and a melting point of 0°C.
• Mixtures, on the other hand, comprise different substances that melt or boil at varying temperatures, giving them a range of melting and boiling points.
• Utilizing melting and boiling point data is a valuable method to differentiate between pure substances and mixtures.
• An unidentified pure substance can be recognized by experimentally determining its melting point and boiling point and then comparing these values with established data tables.
• Mixtures melt over a spectrum of temperatures since they contain two or more distinct substances.

Importance of Purity in Pure Substances

• A pure substance comprises only one type of substance with no impurities.
• Ensuring purity in substances used for food and drugs is crucial due to the potential dangers even small impurities can pose.
• Analysis of melting and boiling points serves as a common practice to evaluate the purity of food and pharmaceuticals.
• For instance, pure water will precisely melt at 0°C and boil at 100°C. Any deviation from these values indicates impurities, transforming it into a mixture.

• Identifying Components of Mixtures:
  • The Rf value is a crucial metric used to distinguish components within mixtures.
  • Each compound has a consistent Rf value, aiding in its identification.
  • Comparing the Rf value of an unknown substance to known substances helps chemists in identification.
• Calculation of Rf Value:
  • The Rf value is computed as the distance traveled by the compound divided by the distance traveled by the solvent.
  • This value is dimensionless as it represents a ratio.
• Utilizing Rf Values for Identification:
  • Visible movement of components up the paper is essential for effective chromatography.
  • Locating agents interact with samples, producing visible colored products.
  • Application of locating agents post chromatography reveals sample runs visibly.

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Downloadable Notes

• Notes available for download on the topic "Criteria of Purity" (2.1.2).