EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.2 2 Pure Substance vs Mixture

EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.2 2 Pure Substance vs Mixture

Pure Substance vs Mixture

 

  • In everyday language we use the word pure to describe when something is natural or clean and to which nothing else has been added
  • In chemistry a pure substance may consist of a single element or compound which contains no other substances
  • For example a beaker of a sample of pure water contains only H2O molecules and nothing else
  • If salt were added to the beaker then a mixture is produced
  • A mixture consists of two or more elements or compounds that are physically mixed together, they are not chemically combined
  • The chemical properties of the substances in a mixture remain unchanged
  • Substances in mixtures can be separated by physical means
  • Air for example is a mixture of nitrogen, oxygen and some other gases such as carbon dioxide and argon

 

 

Elements-compounds-mixtures

 

Diagram showing how to represent elements, compounds and mixtures using particle diagrams

 

 

 

Distinguishing Purity

  • Pure substances melt and boil at specific and sharp temperatures e.g. pure water has a boiling point of 100 °C and a melting point of 0 °C
  • Mixtures have a range of melting and boiling points as they consist of different substances that tend to lower the melting point and broaden the melting point range
  • Melting and boiling points data can therefore be used to distinguish pure substances from mixtures
  • Melting point analysis is routinely used to assess the purity of drugs
  • This is done using a melting point apparatus which allows you to slowly heat up a small amount of the sample, making it easier to observe the exact melting point
  • This is then compared to data tables
  • The closer the measured value is to the actual melting or boiling point then the purer the sample is

 

 

 

Cooling Curves

  • The influence of impurities can be more clearly seen on a heating / cooling curve
  • If the temperature of a liquid is measured as it cools and freezes the data can be used to produce a graph
  • The following graph shows the cooling curve for a sample of a compound
  • The horizontal part of the graph shows that the compound has a sharp melting point, so the compound is pure

 

Purity-Pure-Substance-Cooling-Curve

 

Cooling curve for a pure substance

 

 

 

  • An impure sample of the compound would produce a gradual decrease in temperature as it freezes as shown in the graph below

 

Purity-ImPure-Substance-Cooling-Curve

 

Cooling curve for an impure substance

 

转载自savemyexams

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