Acids, Bases and Salts

• INDICATOR: Litmus

  In acid: RED

  In neutral: PURPLE

  In alkali: BLUE
• INDICATOR: Thymolphthalein

  In acid: COLOURLESS

  In neutral: COLOURLESS

  In alkali: BLUE
• INDICATOR: Methyl Orange

  In acid: RED

  In neutral: ORANGE

  In alkali: YELLOW
• Rules for the pH Scale:

  1. ACIDS have a pH less than 7.
  2. The more acidic a solution, the lower the pH.
  3. NEUTRAL substances, such as pure water, have a pH of 7.
  4. ALKALIS have a pH greater than 7.
• ACID: a substance that dissolves in water to produce a solution that:

  1. Turns litmus red
  2. Has a pH lower than 7
  3. Contains an excess of H+ ions
• ACIDS are proton donors.
• ALKALI: a substance that dissolves in water to produce a solution that:

  1. Turns litmus blue
  2. Has a pH higher than 7
  3. Contains an excess of OH⁻ ions
• ALKALIS are proton acceptors.
• BASE: a substance that reacts with an acid to form a salt and water only.
• ALKALI: a base that is soluble in water. Alkalis are generally used in the laboratory as aqueous solutions.
• Examples of ACIDS:

  • lactic acid in yoghurt
  • citric acid in citrus fruits
  • acetic acid in vinegar
  • sulfuric acid in batteries
• Examples of ALKALIS:

  • sodium bicarbonate in baking soda
  • calcium carbonate in chalk
  • ammonia
• ACIDIC OXIDES: non-metals that dissolve in water to form acidic solutions.

  NON-METALS generally form acidic oxides.
• Examples of ACIDIC OXIDES:

  • SO₂
  • CO₂
  • NO₂
  • P₂O₅
• BASIC OXIDES: metals dissolve in water to form alkaline solutions.

  METALS generally form basic oxides.
• Examples of BASIC OXIDES:

  • CaO
  • MgO
  • CuO
  • K₂O
  • Na₂O
  • Fe₂O₃
• AMPHOTERIC METAL OXIDES OR AMPHOTERIC HYDROXIDE: a hydroxide or metal oxide that reacts with both an acid and an alkali to give salt and water.
• Examples of AMPHOTERIC METAL OXIDES or AMPHOTERIC HYDROXIDE:

  • ZnO
  • Al₂O₃
  • Zn(OH)₂
  • Al(OH)₃

  (just remember: Zinc and Aluminium)
• NEUTRAL OXIDES: do not react with either acids or alkalis.
• Examples of NEUTRAL OXIDES:

  • CO 
  • NO 
  • H₂O
• SOIL pH and PLANT GROWTH

  • if the soil is too acidic, it is usually treated by “liming” 
  • "lime" has the effect of neutralising the acidity of the soil
• “Lime” refers to calcium oxide, calcium hydroxide or powdered chalk or limestone (calcium carbonate).
• EFFLUENT AND WASTE WATER TREATMENT

  • liquid waste from factories is often acidic
  • slaked lime is often added to the waste to neutralise it
  • it is also used to treat streams, rivers and lakes affected by acid rain
• SCRUBBERS

  • to reduce emissions of sulfur dioxide, many modern factories and power stations now spray acidic waste gases with jets of slaked lime in scrubbers
  • this neutralises the gases before they leave the chimneys
• AMPHOTERIC - substances that can react with both acids and alkalis.