Purity and separating mixtures

• What is meant by the purity of a substance in scientific terms?

  A single element or compound, not mixed with any other substance
• How does the scientific use of the term 'pure' differ from everyday language?

  Scientific use refers to a single element or compound, while everyday language refers to a substance that has not been adulterated
• What is the characteristic of a pure substance in terms of its melting and boiling points?

  It melts and boils at specific temperatures
• How can melting point data be used to distinguish between pure and impure substances?

  By comparing the melting points of the substances, pure substances have a sharp melting point, while impure substances have a range of melting points
• What is the definition of relative formula mass (Mr) of a species?

  Sum of the relative atomic masses of the atoms in the numbers shown in the formula
• What is the definition of empirical formula of a compound?

  Simplest whole number ratio of atoms of each element in a molecule
• How is the relative formula mass (Mr) of a species calculated in a balanced chemical equation?

  By summing the Mr of the reactants in the quantities shown and equating it to the sum of the Mr of the products in the quantities shown
• How can the empirical formula of a compound be deduced from the relative numbers of atoms present or from a model or diagram?

  By dividing the number of each element by any common factors to give the simplest whole number ratio
• What is a formulation in the context of mixtures?

  A mixture that has been designed as a useful product
• Why are many useful materials formulations of mixtures?

  Because each chemical in the mixture has a particular purpose and the product has the required properties
• What is the process of filtration used for?

  To separate a precipitate from a solution
• How does the process of filtration work?

  By passing the solution through a filter, leaving the precipitate behind
• What is the process of crystallisation used for?

  To separate a soluble salt from a solution
• How does the process of crystallisation work?

  By warming the solution, allowing the solvent to evaporate, and then cooling the solution to form crystals
• What is the process of simple distillation used for?

  To separate a solvent from a solution
• How does the process of simple distillation work?

  By heating the solution, allowing the solvent to evaporate, and then cooling and condensing the vapor
• What is the process of fractional distillation used for?

  To separate a pure liquid from a mixture of liquids
• How does the process of fractional distillation work?

  By heating the mixture, allowing the liquids to evaporate and condense at different temperatures, and then collecting the fractions
• What is chromatography used for?

  To separate mixtures and give information to help identify substances
• How does chromatography work?

  By using a stationary phase and a mobile phase, and separating substances based on their distribution between the phases
• What is paper chromatography used for?

  To separate different pigments in a colored substance
• What is thin layer chromatography (TLC) used for?

  To separate mixtures and give information to help identify substances
• How does paper chromatography work?

  By using a stationary phase of paper and a mobile phase of solvent, and separating substances based on their relative speeds in the solvent
• How does TLC work?

  By using a stationary phase of a thin layer of an inert substance and a mobile phase of solvent, and separating substances based on their distribution between the phases
• What is the Rf value in chromatography?

  Distance moved by substance ÷ distance moved by solvent
• How can Rf values be used to help identify substances in chromatography?

  By comparing the Rf values of different substances in different solvents, which can be used to identify the substances
• What is gas chromatography used for?

  To separate and identify substances based on their distribution between a stationary phase and a mobile phase of an inert carrier gas
• How does gas chromatography work?

  By using a stationary phase of a liquid or solid on a solid support and a mobile phase of an inert carrier gas, and separating substances based on their solubility in the gas
• What is the purpose of suggesting suitable purification techniques given information about the substances involved?

  To select the appropriate technique to purify a substance based on its properties
• How can suitable purification techniques be suggested given information about the substances involved?

  By considering the properties of the substance, such as its solubility, melting point, and boiling point, and selecting a technique that is appropriate for those properties
• What is the purpose of suggesting chromatographic methods for distinguishing pure from impure substances?

  To select a method that can separate and identify the substances based on their distribution between a stationary phase and a mobile phase
• How can chromatographic methods be suggested for distinguishing pure from impure substances?

  By considering the properties of the substances, such as their solubility, melting point, and boiling point, and selecting a method that is appropriate for those properties