Overview
This lecture explains the difference between physical and chemical changes, providing examples and clues to identify each type, and discusses the basics of chemical reactions and balanced equations.
Physical Changes
- Physical changes alter appearance but not the substance’s chemical identity.
- Examples include melting, boiling, breaking, bending, or dissolving a substance.
- Changing states of water (ice, liquid, vapor) is a physical change because it remains H₂O.
- Boiling water produces water vapor, not hydrogen or oxygen gases.
- Cutting or breaking objects (like bricks or pens) are physical changes if the material stays the same.
- Dissolving sugar in tea is a physical change as the sugar is surrounded by water but not chemically altered.
Chemical Changes
- Chemical changes produce new substances with different properties from the reactants.
- Breaking down water into hydrogen and oxygen gases is a chemical change.
- Evidence of chemical change includes gas production, formation of a precipitate (cloudiness), temperature change, color change, light production, or a new smell.
- Mixing baking soda and vinegar creates bubbles of a new gas, indicating a chemical change.
- Combustion, such as burning methane with oxygen, releases energy and forms new products.
- Color changes in burning salts (like fireworks) show a chemical change.
- Bioluminescence (light production in jellyfish) results from chemical reactions.
- Smell produced while baking bread is an example of a chemical change.
Identifying Change Types
- Boiling water and sublimating mothballs are both physical changes.
- Burning a sparkler or baking bread are chemical changes.
- Tasting unknown substances in a lab is unsafe and should be avoided.
Balancing Chemical Equations
- Reactants are on the left, products on the right, and the arrow means "yields."
- Subscripts indicate the number of atoms in a molecule; coefficients show the number of molecules.
- In a balanced equation, the number of each type of atom is equal on both sides.
- Only coefficients can be changed when balancing equations, not subscripts.
Key Terms & Definitions
- Physical Change — a change in appearance or state without altering chemical identity.
- Chemical Change — a change that produces new substances with different chemical properties.
- Reactants — substances present at the start of a chemical reaction.
- Products — substances formed as a result of a chemical reaction.
- Balanced Equation — a chemical equation where the number of each type of atom matches on both sides.
- Coefficient — number in front of a molecule indicating amount.
- Subscript — small number in a formula showing the number of atoms in a molecule.
Action Items / Next Steps
- Review examples of physical and chemical changes.
- Practice identifying changes in everyday situations.
- Study how to balance chemical equations.