Changes

• Substances at the start of a reaction are called reactants
• Substances left at the end of a reaction are called products
• A reaction that gives out heat is known as an exothermic reaction
• A reaction that takes in heat is known as an endothermic reaction
• In a chemical reaction, three things happen:

  • A new substance is made which usually looks different from the substances you started with
  • Energy is taken in or given out (e.g., heat energy)
  • The change may be impossible to reverse
• Physical changes:

  • Physical change is a temporary change and is reversible
  • Physical change affects only the physical properties (e.g., shape, size)
  • A physical change involves very little to no absorption of energy
  • Physical changes do not involve the production of heat

  Examples of physical changes are freezing of water, melting of wax, boiling of water
• Chemical changes:

  • A chemical change is a permanent change to the substance and is irreversible
  • In a chemical change, both physical and chemical properties of a substance change
  • During a chemical change, absorption and evolution of energy take place

  Examples of chemical changes are digestion of food, burning of coal, rusting, cooking an egg

  Chemical changes involve the production of heat (e.g., Fe + S heat FeS)
• When an egg is cooked, physical and chemical changes occur. The white changes quickly to become more solid and opaque white, which is a physical change. The heat of the pan causes proteins in the egg to denature and coagulate, which is a chemical change and cannot be reversed
• One thing that chemical and physical changes have in common is that the total mass of a substance remains the same during a chemical or physical change. This is because mass is neither created nor destroyed during a change (Law of conservation of mass)