chemical changes

• Physical change


  A process in which a substance undergoes changes without changing the composition or identity of the substance
• Chemical change


  A process in which a substance undergoes changes by changing the composition and identity of the substance
• A chemical reaction follows the law of conservation of mass
• The total mass before and after a chemical change remain the same
• Types of chemical changes

  • Substances chemically combine to form one or more new substances
  • Substances break down into two or more new substances
• Chemical changes


  • Hydrogen reacts with oxygen to form water
  • Carbon reacts with oxygen to form carbon dioxide and water
  • Water is broken down into hydrogen and oxygen by an electric current
  • Silver bromide in photographic film is broken down into silver and bromine by light
• Chemical changes are usually not easily reversed because large amounts of energy are required to reverse them
• After hydrogen combines with oxygen chemically to form water molecules, we will not be able to recover the original elements, hydrogen and oxygen, easily from the water molecules
• However, by passing large amounts of electricity to break down the water molecules chemically, it is possible to obtain hydrogen and oxygen again
• Physical change


  No new substances are formed, the properties of substances remain the same after the change, a change could be temporary and can be reversed easily
• Chemical change


  New substances are always formed, the new substances have their own sets of properties that are different from the original substances, a permanent change that cannot be reversed easily
• Word equation


  An equation in which the reactants and products in a chemical reaction are represented by words
• Reactant


  A substance that is present at the start of the reaction
• Product


  A new substance that is produced by the reaction
• Produces/forms/yields/gives


  Terms used to represent the relationship between reactants and products
• A chemical reaction can be 'summarised' by using a word equation
• Chemical changes can be triggered by heat or an increase in temperature, using an electric current, exposure to light, or mixing substances
• Combustion


  A chemical reaction in which a substance is heated in the presence of oxygen to form one or more new substances, heat and/or light are usually given out during combustion, can be classified as complete or incomplete
• Combustion reactions


  • The burning of hydrogen as a fuel for rockets
  • Burning methane for cooking
  • Burning charcoal for grilling food
• Thermal decomposition


  A single compound breaks into two or more simpler substances when it is heated
• Thermal decomposition reactions


  • Decomposition of metal carbonates
  • Decomposition of metal hydroxides
• Photochemical degradation


  A chemical process in which a substance undergoes degradation or decomposition when exposed to light, particularly ultraviolet (UV) rays or visible light
• Electroplating


  A process used to coat the surface of an object with a layer of metal through a chemical reaction
• Electroplating process


  The object to coat (Y) is connected to the positive terminal of the battery, the object to be coated (X) is connected to the negative terminal of the battery, the solution must contain Y particles, as the electric current flows through the solution from Y to X, the Y particles will coat the surface of X layer by layer
• Electroplating


  • Producing a silver-coated spoon
• Oxidation


  A chemical change that can take place with or without the presence of oxygen
• Acids


  Substances that have a sour taste, turn blue litmus paper red, have a pH value less than 7, are corrosive, and can conduct electricity, they react with metals, carbonates and bases/alkalis
• Common acids used in the laboratory

  • Sulfuric acid
  • Hydrochloric acid
  • Nitric acid
• Bases


  Substances that react with acids to produce salt and water, when a base dissolves in water, it forms an alkali
• Alkalis


  Have a bitter taste, have a slippery or soapy feel, turn red litmus paper blue, are corrosive, and can conduct electricity, they react with acids to produce salts and water
• Neutralisation reaction


  Acids react with bases/alkalis to form salt and water
• In all neutralisation reactions, water is always formed as a product</b>
• Salts are neutral substances containing an element that is a metal
• Acid-base/alkali-salt + water


  The general word equation for a neutralisation reaction
• Neutralisation reactions


  • Nitric acid + calcium oxide -> calcium nitrate + water
  • Sulfuric acid + sodium hydroxide -> sodium sulfate + water
  • Hydrochloric acid + lithium hydroxide -> lithium chloride + water
• Acid-metal reaction


  Acids react with certain metals to produce salt and hydrogen
• Reaction between acids and bases/alkalis

  1. Acids react with bases/alkalis to form salt and water

  2. This reaction is called the neutralisation reaction

  3. In all neutralisation reactions, water is always formed as a product
• Salts


  Neutral substances containing an element that is a metal
• When the salts produced from the neutralisation reactions dissolve in water, they form a neutral salt solution
• Different acids give different names to the salt

  • Nitric acid - nitrate
  • Sulfuric acid - sulfate
  • Hydrochloric acid - chloride