Chemical changes

• Physical change

  1. No new product formed
  2. Often easily reversible
  3. No changes in properties
• Chemical change

  1. New product(s) formed
  2. Seldom reversible
  3. Products formed have different properties as reactants
• Thermal decomposition: A single substance breaks up into 2 or more simpler substances when its heated
• Combustion: A substance reacts with oxygen when heated to form one or more new substances. Heat and light are given out.
• Oxidation: The addition of oxygen to a substance is called oxidation. Also occurs when other elements, metals and non-metals, burn in air.
• How do chemical changes occur?

  1. Mixing: 2 or more substances are mixed, they may combine irreversibly to form a product
  2. Heating: Thermal decomposition and heating involve heating its reactants
  3. Exposure to light
  4. Using electric current: Electrolysis
• Indicator: A substance which changes colour depending on whether the solution being tested is acidic or alkaline
• Types of indicators

  • Only show whether a solution is acidic or alkaline: Litmus paper, methyl orange, screened methyl orange, thymolphthalein
  • Can also show how acidic or alkaline a solution is: Universal indicator.
• Methyl Orange: Orange.

  pH at which colour changes (equivalent point): 4

  Red in acid

  Yellow in alkali
• Litmus paper:

  pH at which colour changes (equivalent point): 7

  Blue litmus paper turns red in an acid

  Red litmus paper turns blue in an alkali
• Screen Methyl Orange: Green.

  pH at which colour changes (equivalent point): 4

  Pink in acid

  Green in alkali
• Thymolphthalein: Colourless.

  pH at which colour changes (equivalent point): 10

  Colourless in acid

  Blue in alkali
• Universal indicator pH colours:

  0,1 2,3 4,5 6 7 8,9,10 11,12 13,14

  Red Orange Yellow Yellow-green Green Blue-green Blue Violet
• Acid: A substance that turns damp blue litmus paper red
• Acids and uses

  1. Hydrochloric acid: Removes impurities such as rust and scale from metals and aluminum alloys
  2. Nitric acid: Used in making explosives and fertilisers
  3. Sulfuric acid: Used in car batteries/manufacturing detergents and fertilisers
  4. Phosphoric: Added to food and beverages to give them a sour taste
  5. Ethanoic: Used in vinegar as food preservative and flavour enhancer
• Acid properties

  1. Corrosive
  2. Sour
  3. Turns damp blue litmus paper red
  4. Conductor of electricity
• 1)Acids reaction with reactive metal

  • Dilute acid + *Reactive metal* --> *Salt* + Hydrogen gas

  Example: Dilute hydrochloric acid + zinc --> zinc chloride + hydrogen gas


  Test for hydrogen gas

  • Place a burning splint into the mouth of a test tube. Gas evolved extinguishes the burning splint with a ‘POP’ sound. The gas is carbon dioxide gas. Effervescence is observed when the ‘metal’ is placed into the acid.
• Acid reaction with metal carbonate

  • Dilute acid + *Metal* carbonate --> *Salt* + Water + Carbon dioxide gas

  Example: Dilute nitric acid + Magnesium carbonate --> Magnesium nitrate + Water + Carbon dioxide gas


  Test for carbon dioxide gas:

  • Bubble the gas produced into limewater. Gas evolved causes white precipitate to form in limewater. The gas is carbon dioxide gas.
• Acid reaction with alkali:

  • Dilute acid + *Alkali* --> *Salt* + Water

  Example: Dilute sulfuric acid + Aqueous sodium hydroxide --> Sodium sulfate + Water
• Alkali: A substance that turns damp red litmus paper blue
• Alkali and uses

  • Sodium/potassium hydroxide: Used to make soaps
  • Calcium hydroxide: Used to neutralize acidic soil
  • Ammonia solution: Used to make fertilisers
• Properties of alkali

  1. Bitter taste
  2. Soapy feeling
  3. Turn damp red litmus paper blue
  4. Conductor of electrictiy
• Alkali reaction with ammonium salt:

  • Aqueuous alkali + Ammonium *salt* --> *Salt* + Water + Ammonia gas

  Example: Aqueous sodium hydroxide + Ammonium chloride --> sodium chloride + Water + Ammonia gas


  Test for ammonia gas:

  Place damp red litmus paper at the mouth of a test tube. (test tube is heating above bunsen burner) Colourless and pungent gas evolved turns damp red litmus paper blue. The gas is ammonia gas