Acid 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


  1. Produce hydrogen ions, H+, when they dissolve in water

  2. e.g. for hydrochloric acid: HCl(aq) → H+(aq) + Cl–(aq)
• Bases


  1. A base that dissolves in water is called an alkali

  2. Alkalis produce hydroxide ions, OH–, when they dissolve in water

  3. e.g. for sodium hydroxide: 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


  1. Happens when an acid and an alkali 'cancel each other out'

  2. Reaction always produces a salt and water

  3. H+ ions from the acid react with OH– ions from the alkali to form water
• Salt


  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
• 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
• Acids react with


  1. Metals

  2. Bases

  3. Carbonates
• Salt


  Name has two parts: first part from the metal, base or carbonate, second part from the acid
• Acids and metals


  1. Acids will react with metals to make a salt and hydrogen gas

  2. e.g. hydrochloric acid + zinc → zinc chloride + hydrogen
• The hydrogen causes bubbling during the reaction, and the reaction is exothermic
• The more reactive the metal, the faster the reaction is, resulting in more bubbling and a bigger temperature rise
• Acids and alkalis/bases


  1. Acids react with alkalis and bases to make a salt and water

  2. Alkalis are commonly metal hydroxides

  3. Bases are commonly metal oxides
• The reactions of acids with alkalis and bases are exothermic
• Acids and carbonates


  1. Acids will react with carbonates to make a salt, water and carbon dioxide gas

  2. The carbon dioxide causes bubbling during the reaction, and the reaction is exothermic