Acids bases and salts

• Indicators


  Substances that change colour when they are added to acids and alkalis
• Litmus


  • The most well-known indicator
  • Turns red in acid and blue in alkalis
• Universal indicator


  • Most commonly used in the laboratory
  • Changes colour to show the pH of the solution
• pH scale


  • Acidic = pH < 7
  • Neutral = pH 7
  • Alkaline = pH > 7
• Acids and alkalis are commonly used both in industry and at home
• Acids


  Produce hydrogen ions, H+, when they dissolve in water
• Acid dissolving in water

  HCl(aq) → H+(aq) + Cl–(aq)
• Bases


  • Chemically opposite to an acid
  • A base that dissolves in water is called an alkali
• Alkali dissolving in water

  NaOH(aq) → Na+(aq) + OH–
• Some acids and alkalis are dangerous, others such as those in vinegar or lemon juice can be sprinkled on our food
• Dangerous acids and alkalis have the corrosive hazard warning symbol on their containers
• Neutralisation


  • Reaction when an acid and an alkali 'cancel each other out'
  • Always produces a salt and water
• Neutralisation reaction


  H+(aq) + OH–(aq) → H2O(l)
• The name of the salt produced depends upon the acid and alkali that react
• Strong acids


  • Fully dissociate (ionise) in water
  • Have high numbers of H+ ions in their solutions, so low pH values
  • React more quickly than weak acids
• Weak acids


  Only partially dissociate in water
• Temperature rise during reaction between magnesium and a strong acid

  Higher than that with a weak acid
• Acid strength and concentration are not the same thing
• Acid strength


  Measure of the degree of its dissociation
• Acid concentration


  Measure of the number of moles of acid in 1 dm3 of solution
• Reactions of acids


  • Metals
  • Bases
  • Carbonates
• Acid reacting with metal

  acid + metal → salt + hydrogen
• The hydrogen causes bubbling during the acid-metal reaction, which is exothermic
• The more reactive the metal, the faster the reaction, resulting in more bubbling and a bigger temperature rise
• Acid reacting with alkali/base


  acid + alkali → salt + water
• The reactions of acids with alkalis and bases are exothermic
• Acid reacting with carbonate

  acid + carbonate → salt + water + carbon dioxide
• The carbon dioxide causes bubbling during the acid-carbonate reaction, which is exothermic