The nature of substances and chemical reactions

• What is an element

  A substance made up of a single atom. These substances cannot be broken down into any simpler substances by chemical means.
• What is a compound

  Substances made of two or more elements that are chemically joined and having completely different properties to its elements.
• How to calculate relative atomic mass

  Total number of all atoms ÷ total number of atoms
• How to calculate relative molecular mass

  Add up all atoms in the formualr
• Percentage composition

  . Work out Mr (add all atoms in formular)

  . Work out the Ar only the atom your interested in

  . Divide the Ar by Mr and × 100
• How can mixtures be separated

  Atoms/molecules in mixtures not being chemically joined and mixtures being easily separated by processes such as filtration, evaporation, chromatography and distillation
• What does chromatography show

  The more soluble a substance is the more it will travel up the paper and separates different pigments in a coloured substance.
• Rf value

  Distance moved by substance ÷ distance moved by solvent
• Observation

  Colour changes

  Temperature changes- exothermic reactions give off heat enegy t surroundings causing an increase in temperature whereas endothermic reactions take in energy from surroundings causing a decrease in temp

  Effervescence - as evidence that a chemical reaction has taken place
• Percentage yield

  Actual ÷ theoretical × 100
• Number of moles

  Mass (g) ÷ molar mass (g/mol)
• What does distillation do

  Used to separate liquid from the mixture
• What does filtration do

  separate insoluble solids from liquid
• What does evaporation do

  separates a soluble solid from a solution
• Distilling a salt solution

  Salt solution is heated

  Water evaporates and its vapour rises. The water vapour passes into the condenser, where it cools and condenses. Liquid water drips into a beaker

  All the water has evaporated from the salt solution, leaving the salt behind
• Process of evaporation

  A solution is placed in an evaporating basin and heated with a Bunsen burner.

  The volume of the solution has decreased because some of the water has evaporated. Solid particles begin to form in the basin.

  All the water has evaporated, leaving solid crystals behind.
• Process of filtration

  One beaker contains a mixture of solid and liquid, the other contains a funnel with filter paper.

  The solid and liquid mixture is poured into the filter funnel.

  The liquid drips through the filter paper but the solid particles are caught in the filter paper.