Anions

• Tests for Anions


  Procedures to detect the presence of certain negative ions (anions) dissolved in water
• Some evidence that a chemical reaction is taking place

  • Bubbles of gas (effervescence)
  • Colour change
  • Noise (bang)
  • Light given off by burning magnesium
  • Temperature change
  • Methylated spirits burning - heat and light given off
• These tests will then be explained in terms of the chemical reactions that have taken place.
• Chemical Equation


  Representation of a chemical reaction using symbols for elements and formulas for compounds, with reactants on the left and products on the right, separated by an arrow
• The Law of Conservation of Mass states that the total mass of the products of a chemical reaction is the same as the total mass of the reactants.
• Balancing a chemical equation

  1. Count the number of atoms of each element on the left and right sides

  2. Adjust the numbers in front of the formulas to balance the atoms

  3. Re-check the balancing to ensure the number of atoms on each side is equal
• Anions investigated in this course

  • Chloride ion (Cl-)
  • Sulfate ion (SO4^2-) and sulfite ion (SO3^2-)
  • Carbonate ion (CO3^2-) and hydrogencarbonate ion (HCO3-)
  • Nitrate ion (NO3-)
  • Phosphate ion (PO4^3-)
• Ionic equation


  A chemical equation involving ions rather than full balanced equations
• Anions


  • Chloride
  • Sulfate
  • Sulfite
  • Carbonate
  • Hydrogen-carbonate
  • Nitrate
  • Phosphate
• Test for Chloride

  1. Add a few drops of AgNO3 solution to a solution of the solid

  2. A white precipitate is formed which is soluble in dilute ammonia solution
• Test for Sulfate


  1. Add BaCl2 solution to a solution of the solid

  2. A white precipitate is formed

  3. If precipitate remains → sulfate

  4. If precipitate dissolves → sulfite
• Test for Carbonate

  1. Add dilute HCl to the solid

  2. Fizzing. A gas is given off that turns limewater milky

  3. MgSO4 is added - White precipitate forms → carbonate

  4. No precipitate forms → hydrogencarbonate
• Test for Nitrate


  1. To a solution of the solid add a freshly prepared FeSO4 solution

  2. Add concentrated sulfuric acid

  3. A brown ring is formed at the junction of the two liquids
• Test for Phosphate


  1. Ammonium molybdate is added to a solution of the solid

  2. Concentrated nitric acid is added

  3. Solution is warmed

  4. A yellow precipitate is formed
• Boiling a hydrogencarbonate solution gives a precipitate
• The brown ring is due to the presence of the nitrate ion
• The yellow precipitate indicates the presence of phosphate ions
• Chemical Reactions & Balancing Equations

  1. Compounds come together and react to form new substances in order to become more stable

  2. Redistribution of the chemical energy within the reacting system

  3. Bond breaking

  4. Bond making
• Reactants


  Compounds that react together in a chemical reaction
• Products


  New compounds that are formed in a chemical reaction
• Signs of a chemical reaction

  • Change in colour
  • Change in temperature
  • Bubbles of gas are produced (effervescence)
  • Production of light
  • Emission of sound
  • Distinct smell
  • Precipitate forms
• Types of chemical reactions

  • Combustion
  • Oxidation
  • Neutralisation
  • Decomposition
  • Electrolysis
• Salts


  Formed from acids by replacing the hydrogen of the acid with a metal ion
• Acids


  • Hydrochloric acid
  • Sulphuric acid
  • Nitric acid
  • Carbonic acid
• Reactions of acids


  1. Acid + Base -> Salt + Water

  2. Acid + Metal Carbonate -> Salt + Water + CO2

  3. Acid + Metal -> Salt + Hydrogen
• Salts are soluble or insoluble depending on the metal and anion</b>
• Insoluble salts can be formed by ion exchange reactions
• Insoluble salts


  • AgCl
  • CaCO3
  • MgCO3
  • AlPO4
  • FePO4
• Hardness in water


  Caused by the presence of Ca2+ and Mg2+ ions
• Tertiary treatment of sewage

  Removal of heavy metal ions (Pb, Hg, Ca) from water
• Ion Exchange Reactions


  1. AX + BY -> AY + BX

  2. Where A and B are metal atoms and X and Y are acid radicals
• Tests for Anions in Aqueous Solution


  1. Chloride: Add AgNO3

  2. Sulphate: Add BaCl2

  3. Carbonate: Add HCl, CO2 gas given off

  4. Hydrogen Carbonate: Add HCl, no precipitate

  5. Nitrate: Brown ring test with FeSO4 and H2SO4

  6. Phosphate: Add dilute NaOH, green-brown precipitate