CH11: Chemical Changes

• Properties of Physical Changes

  No new substances are formed

  Usually reversible

  Properties of constituents remain the same before and after the change
• Properties of Chemical Changes

  One or more new substances are formed

  Usually not easily reversed

  Products have different properties from that of the reactants
• Chemical changes - reactants and products

  Reactants are substances that react with each other

  Products are the new substances formed
• Two types of chemical reactions:

  1. reactions in which two or more substances chemically combine to form one or more new substances

  E.g. carbon + oxygen→carbon dioxide

  2. reactions in which a substance into two or more new substances

  E.g. calcium carbonate→calcium oxide + carbon dioxide
• • The mass of the reactants is the same as the mass of the products.
  • Mass is conserved during a chemical reaction.
• Types of Chemical Changes

  1. Combustion
  2. Thermal decomposition
  3. Oxidation
  4. Neutralisation
  5. Other reactions
• What is combustion?

  Combustion is a chemical reaction in which a substance is heated in the presence of oxygen to form one or more new substances.

  (Heat or light are usually given out during combustion)
• What is thermal decomposition?

  Thermal decomposition is the process in which a substance breaks down into two or more simpler substances upon heating.
• What is oxidation?

  Oxidation is a chemical change that occurs when a substance gains oxygen.

  
  

  E.g. Rusting and cellular respiration
• Rusting is a process that occurs when iron reacts with oxygen and water to form rust.
• Cellular respiration is the process in which glucose is broken down in the presence of oxygen to provide cells with energy.
• What is Neutralisation?

  Neutralisation is a chemical reaction which occurs when an acid reacts with an alkali.
• Properties of Acids

  • sour taste
  • Corrosive (can burn skin)
  • pH less than 7
  • Turns blue litmus paper red
  • React with alkalis, some metals and carbonates
• Properties of Alkalis

  • bitter taste
  • soapy feel
  • Corrosive (can burn skin)
  • pH more than 7
  • Turns red litmus paper blue
  • React with acids
• Test for hydrogen

  In the presence of hydrogen gas, a lighted splint extinguishes with a ‘pop’ sound.
• Test for CO2

  • When carbon dioxide gas is bubbled through limewater, a white precipitate is formed in limewater.

  • A precipitate is a solid formed in a solution.
• ph more than 7 is alkaline and less than 7 is acidic
• Chemical changes can occur by:

  • mixing
  • heating
  • interaction with oxygen
  • exposure to light
  • using an electric current
• Chemical changes by using an electric current

  A chemical change can occur when matter interacts with electricity.

  • E.g. electroplating (coating a metal with another metal)
• How Does the Knowledge of Chemical Reactions Help Us in Our Daily Lives?

  • Cooking
  • Respiration
  • Decay
  • Rusting
  • Combustion
  • Acidification of Oceans
• Cooking

  • Heat from cooking softens food and improves its

  flavour.

  • Cooking makes it easier for the body to absorb

  nutrients from food.

  • Some nutrients may be destroyed during cooking.
• Respiration

  • During respiration, glucose molecules react with oxygen to produce carbon dioxide and water.

  • The energy released allows the cells to survive and reproduce.
• Decay

  • Decomposition is the process of breaking down organic matter into simpler substances.
  • This allows nutrients to be returned to the environment.
  • Decay can cause food to spoil and harm our health. 
  • Vacuum packaging slows down food decay by removing air, thus limiting bacteria growth
• Rusting

  • Rusting occurs when iron or steel reacts with oxygen and water.

  • Rusting occurs rapidly in humid climates (e.g. Singapore).
• • Some ways to prevent rusting: painting, oiling, galvanising(coating iron with zinc), drying
  • These methods prevent iron or steel from coming into contact with water or oxygen.
• Combustion

  Combustion of fuels can produce air pollutants such as carbon monoxide, sulfur dioxide and oxides of nitrogen.