11: chemical changes

• What Is a physical change

  Matter changes form but not identity (no new substances are formed
• What is a chemical change

  A chemical reaction forms new products (one or more new substances are formed)
• a chemical equation consist of 


  chemical formulas of the reactants and products
• Combustion


  A chemical reaction in which a substance is

  heated in the presence of oxygen gas to form

  one or more new substances.
• Thermal decomposition 


  A chemical reaction in which a substance, upon heating, breaks down into two or more simpler substances.
• Oxidation 


  Oxidation is a chemical process in which a

  substance gains oxygen
• Neutralisation 


  A chemical reaction between an acid and an

  alkali.
• Properties of acid


  Sour, corrosive, turns blue on litmus paper, can react with alkalis, metal and carbonate , pH level below 7
• Properties of alkali

  Bitter, soapy, turns red on litmus paper, can react with acids in neutralisation reactions, pH level above 7
• General word equation for neutralisation

  Acid + alkali -> salt ( compound contains a metal element by and non- metal element(s)+ water
• Acids react with metals to form salt and hydrogen gas
• Test for hydrogen gas


  Place a lighted splint at the mouth of the test tube. the lighted splint will be extinguished with a ‘pop’ sound. This would confirm that the gas produced is hydrogen
• Acids react with carbonates (compounds

  containing a metal and a carbonate ion*) to form

  salt, carbon dioxide and water.

  General word equations : acid + carbonate compound -> salt + carbon dioxide + water
• Test for carbon dioxide gas


  To test for carbon dioxide, bubble the gas through limewater. Carbon dioxide gas forms a white precipitate with limewater
• Mixing


  When two or more reactants are mixed together, they may combine irreversibly to form new products
• Heating


  some reactants do not begin spontaneously even if the reactants are mixed together. The reactants need enough thermal energy before they will start a chemical reaction
• Exposure to light

  Some reactants need to absorb enough light energy before they will begin a chemical reaction
• Interaction with oxygen

  Interaction with oxygen in the atmosphere may cause chemical changes to occur
• How do chem changes occur 


  With an electric current

  interaction with oxygen

  mixing

  heating

  exposure to light
• Chem changes in our daily lives

  Cooking

  respiration

  decomposition

  rusting

  combustion
• Cooking

  Causes chem reactions to occur in food. This may make food softer, more flavourful and easier for the body to digest and absorb
• Respiration

  In plants and animal cells, respiration takes place in the cyptoplasm and mitochondria of the cells

  Glucose is broken down in the presence of oxygen to produce water and carbon dioxide

  the CPE in the glucose molecules is converted to other forms of energy to an able the cells to do work
• Decomposition

  Organic matter, such as fresh food products, will undergo decomposition to be broken down into simpler substances

  to slow down this, we could vacuum pack food to reduce the amount of oxygen gas available to bacteria that decompose the food
• Rusting

  To prevent or slow down rusting, bare metal surfaces are coated with a substance to reduce their exposure to oxygen and water
• Combustion

  Combustion of fuels helps us convert CPE into other forms of energy to do work

  it also releases harmful gases like carbon monoxide, sulfur dioxide and oxides of nitrogen