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

This quiz covers the indicators of chemical change, including evidence such as color change, odor change, and gas evolution. It also explores the concepts of compounds, molecules, and chemical formulas.