Chemical and Physical Changes - Notes

4B Chemical and Physical Change

Introduction

• Whakawhanaungatanga: Using specific learning strategies to enhance rangatiratanga.
• Topics covered:
  • Introduction to chemical and physical change
  • Physical changes
  • Chemical changes
  • Chemical equations

Chemical and Physical Change

• Discuss the meaning of these words:
  • Melting
  • Burning
  • Condensation

Chemical Change

• A chemical change occurs when the atoms of one or more molecules are rearranged.
• This rearrangement results in the formation of new molecule(s).
• The chemical properties of the new molecules are different from the original molecules.

Physical Change

• In a physical change, the molecules remain the same.
• The molecules gain or lose energy.
• This energy change causes a change of state (e.g., liquid to a gas, solid to a liquid).

Physical Change - Definition

• A physical change is any change that does not alter the substance's chemical identity.
• Examples of physical changes include:
  • Changes of state (e.g., from solid to liquid or gas to a liquid)
  • Making and/or separating mixtures
• Physical changes are easy to reverse since the molecules have NOT been rearranged.

Physical Change: Example - Solid Ice Melting to Liquid Water

• $H2O(s) \rightarrow H2O(l)$
• Both ice and water are made of the same molecules, $H2O$.
• Solid $H2O$ is still $H2O$, just in a different form.

Equation to Describe a Physical Change

• $H2O(s) \rightarrow H2O(l)$
• This equation shows that solid ice melts to liquid water

Physical Change: Example - Solid Carbon Dioxide Subliming to a Gas

• $CO2(s) \rightarrow CO2(g)$
• Solid carbon dioxide (dry ice) subliming to a gas.

Chemical Change

• A chemical change occurs when a substance is changed into one or more new substances.
• The atoms in the original substance are rearranged into new combinations to form different substances.
• Chemical changes are NOT easy to reverse.

Chemical Change: Examples

• Burning: Once burnt, the wood cannot be retrieved.
• Cooking an egg.

Chemical Change: Example - Gummy Bear + Potassium Chlorate

• This chemical change releases a lot of energy in the form of heat and light!

Chemical Equations

• Chemical equations show how the relationships between the atoms change after a chemical reaction.
• Example: Burning magnesium ribbon in air (which contains 20% oxygen)

Information Given by Chemical Equations

• Solid sodium reacts with chlorine gas to form solid sodium chloride:
  $2Na(s) + Cl2(g) \rightarrow 2NaCl(s)$
• The atoms of $Na$ and $Cl$ have combined to form the compound sodium chloride, $NaCl$, a new substance.
• $NaCl$ has different properties to both sodium and chlorine.
• A lot of energy is released during the reaction (heat and light).

Comparing Chemical and Physical Change