Chemical Changes and Compounds

Indicators of Chemical Change

• Evidence of chemical change:
  • Color change
  • Odor change
  • Gas evolution
  • Heat evolution or absorption
  • Formation of precipitate
  • Change in state (e.g., solid to liquid)

Compounds, Molecules, and Chemical Formula

• Compound: a substance formed by the chemical combination of two or more different elements
• Molecule: a group of atoms chemically bonded together
• Chemical Formula: a representation of the composition of a molecule using symbols and numbers
  • Example: H2O (water) - 2 hydrogen atoms and 1 oxygen atom
  • Subscript numbers indicate the number of atoms of each element in the molecule

Combustion Reactions

• Combustion reaction: a chemical reaction between a substance and oxygen, resulting in heat and light
• General equation: Fuel + Oxygen → Products (e.g., CO2, H2O)
• Examples:
  • 2CH4 (methane) + 3O2 → 2CO2 + 3H2O
  • 2C8H18 (octane) + 25O2 → 16CO2 + 18H2O

Corrosion Reactions

• Corrosion reaction: a chemical reaction between a substance (usually a metal) and its environment, resulting in deterioration
• Examples:
  • 4Fe (iron) + 3O2 → 2Fe2O3 (rust)
  • 2Al (aluminum) + Fe2O3 → 2Fe + Al2O3

Word Equations

• A way to represent chemical reactions using words instead of chemical formulas
• Example: Methane + Oxygen → Carbon Dioxide + Water
• Can be converted to chemical formulas using the symbols and formulas of the reactants and products

Indicators of Chemical Change

• Color change is an indicator of chemical change
• Odor change can also indicate chemical change
• Evolution of gas is a sign of chemical change
• Heat evolution or absorption is a characteristic of chemical change
• Formation of precipitate is evidence of chemical change
• Change in state, such as solid to liquid, is an indicator of chemical change

Compounds, Molecules, and Chemical Formula

• A compound is a substance formed by combining two or more different elements
• A molecule is a group of atoms chemically bonded together
• Chemical formula is a representation of a molecule's composition using symbols and numbers
• Chemical formula example: H2O (water) consists of 2 hydrogen atoms and 1 oxygen atom
• Subscript numbers in chemical formulas indicate the number of atoms of each element in the molecule

Combustion Reactions

• Combustion reaction is a chemical reaction between a substance and oxygen, resulting in heat and light
• General equation for combustion reaction: Fuel + Oxygen → Products (e.g., CO2, H2O)
• Combustion reaction example: 2CH4 (methane) + 3O2 → 2CO2 + 3H2O
• Combustion reaction example: 2C8H18 (octane) + 25O2 → 16CO2 + 18H2O

Corrosion Reactions

• Corrosion reaction is a chemical reaction between a substance (usually a metal) and its environment, resulting in deterioration
• Corrosion reaction example: 4Fe (iron) + 3O2 → 2Fe2O3 (rust)
• Corrosion reaction example: 2Al (aluminum) + Fe2O3 → 2Fe + Al2O3

Word Equations

• Word equation is a way to represent chemical reactions using words instead of chemical formulas
• Word equation example: Methane + Oxygen → Carbon Dioxide + Water
• Word equations can be converted to chemical formulas using the symbols and formulas of the reactants and products