Overview
This lecture explains the differences between chemical and physical changes, distinguishes their properties, and introduces fundamental concepts about energy in chemical systems.
Physical vs. Chemical Changes
- Physical changes do not alter the chemical composition of a substance.
- Examples of physical changes include melting, boiling, tearing, or reshaping a material.
- In physical changes, molecules retain their identity; only their state or form changes.
- Chemical changes (chemical reactions) result in the formation of new substances with different properties.
- During chemical changes, atoms rearrange to create substances with different chemical identities.
- Rusting of iron (formation of iron oxide) and burning gasoline (producing CO₂ and H₂O) are examples of chemical changes.
Physical and Chemical Properties
- Physical properties can be observed without changing a substance's identity (e.g., color, odor, taste, melting point, density).
- Smell and taste involve interaction with receptors but do not alter chemical composition.
- Chemical properties are observed only during chemical reactions (e.g., flammability, corrosiveness, toxicity).
- Flammability is a chemical property describing how easily a substance participates in combustion.
Energy: Forms and Conservation
- Energy is the capacity to do work; work is the action of a force through a distance.
- Two main types of energy: kinetic (energy of motion) and potential (energy of position).
- Objects at height possess potential energy, which converts to kinetic energy as they fall.
- In chemistry, energy is often stored in chemical bonds and is released or absorbed during reactions.
- Kinetic energy at the molecular level is the motion (vibration) of atoms and molecules; temperature measures this kinetic energy.
- Energy is conserved; it can only change form (e.g., chemical energy to heat and mechanical energy in a car engine).
Key Terms & Definitions
- Physical Change — A change affecting the form or state of a substance without altering its chemical identity.
- Chemical Change — A transformation that alters a substance's chemical composition, producing new substances.
- Physical Property — A characteristic observable without changing the substance's identity (color, boiling point).
- Chemical Property — A trait observed only during a chemical reaction (flammability, reactivity).
- Energy — The capacity to do work.
- Kinetic Energy — Energy due to motion.
- Potential Energy — Stored energy due to position.
- Work — The application of a force over a distance.
- Law of Conservation of Energy — Energy cannot be created or destroyed, only changed in form.
Action Items / Next Steps
- Review examples of physical and chemical changes.
- Memorize key terms and definitions related to energy and properties.
- Prepare for further discussion of energy forms in upcoming lectures.