Physical and Chemical Changes

• Physical and chemical changes are fundamental concepts in chemistry and physics that describe how matter can be altered.

Physical Change

• A physical change alters the form or appearance of a substance, but not its chemical composition.
• The chemical composition of the substance remains the same.
• Physical changes are often reversible, allowing the substance to return to its original form.
• Examples include changes in state (solid to liquid to gas), changes in size or shape (cutting paper, crushing a can), and dissolving.
• Change of state: Substance changes from one state to another (ice melting into water, water boiling into steam), chemical identity remains the same (H2O).
• Dissolving: A substance dissolves in another (sugar dissolving in water), it forms a mixture, but the individual components retain their original properties; the sugar is still sugar, and the water is still water.
• Deformation: Changes like bending a metal rod or crushing a rock are physical because the chemical makeup of the metal or rock doesn't change.

Chemical Change

• A chemical change, known as a chemical reaction, involves the rearrangement of atoms and molecules to form new substances.
• The chemical composition of the substance changes, resulting in the formation of a new substance with different properties.
• Chemical changes are generally irreversible, making it difficult to recover the original substance.
• Examples include burning, rusting, cooking, and reactions that produce a gas, precipitate, or color change.
• Burning: Wood reacts with oxygen to produce carbon dioxide, water vapor, ash, and heat, converting the original wood into entirely new substances.
• Rusting: Iron reacts with oxygen and water to form iron oxide (rust), which has different properties than the original iron.
• Cooking: Heating an egg causes its proteins to denature and form new bonds, resulting in a solid structure different from the original liquid egg.

Indicators of Chemical Change

• Indicators suggest when a chemical change has occurred.
• Change in Color: Color change can indicate a chemical reaction.
• Formation of a Precipitate: The formation of a solid when two solutions are mixed suggests a chemical reaction; the solid is called a precipitate.
• Production of a Gas: The release of gas bubbles when two substances are mixed is often a sign of a chemical reaction
• Change in Temperature: Chemical reactions can either release heat (exothermic) or absorb heat (endothermic), leading to a change in temperature.
• Emission of Light: Some chemical reactions produce light.

Examples

• Physical:
  • Ice melting: Solid water (ice) changes to liquid water, but it is still H2O.
  • Boiling water: Liquid water changes to gaseous water (steam), but it is still H2O.
  • Cutting paper: The size and shape of the paper change, but it is still paper (cellulose).
  • Dissolving salt in water: Salt (NaCl) disperses in water, but it remains NaCl, and the water remains H2O.
• Chemical:
  • Burning wood: Wood reacts with oxygen to produce carbon dioxide, water, and ash.
  • Iron rusting: Iron reacts with oxygen and water to form iron oxide (rust).
  • Baking a cake: Ingredients (flour, sugar, eggs) undergo chemical reactions to form a new substance (cake).
  • Neutralization of an acid with a base: An acid (e.g., hydrochloric acid) reacts with a base (e.g., sodium hydroxide) to form a salt and water.

Conservation of Mass

• In both physical and chemical changes, mass is conserved.
• The law of conservation of mass states that matter cannot be created or destroyed in a closed system.
• During a physical change, the mass of the substance remains the same even though its form may change.
• During a chemical change, the total mass of the reactants (starting materials) equals the total mass of the products (substances formed) within a closed system.

Energy Changes

• Energy is often involved in both physical and chemical changes.
• Physical changes may require energy (melting ice requires heat) or release energy (water freezing releases heat).
• Chemical changes also involve energy changes.
• Exothermic reactions release energy, usually as heat or light.
• Endothermic reactions require energy to proceed.
• The energy change in a chemical reaction is known as the enthalpy change (ΔH).
• Exothermic reactions have a negative ΔH, while endothermic reactions have a positive ΔH.

Reversibility

• Physical changes are generally reversible.
• The substance can be returned to its original form by reversing the conditions.
• Chemical changes are generally irreversible.
• Reversing a chemical change typically requires another chemical reaction.

Mixtures vs. Compounds

• Physical changes often involve the separation or formation of mixtures.
• A mixture is a combination of two or more substances that are physically combined but not chemically bonded.
• Chemical changes involve the formation of compounds.
• Compounds are substances formed when two or more elements are chemically bonded together.
• Separating a mixture can be achieved through physical means.
• Separating a compound requires chemical reactions.

Examples of Physical Changes

• Crushing a can
• Melting an ice cube
• Boiling water
• Dissolving sugar in water
• Breaking a glass
• Sublimation of dry ice
• Magnetizing a nail
• Bending a paper clip
• Chopping wood
• Grinding coffee beans

Examples of Chemical Changes

• Burning wood
• Rusting of iron
• Cooking an egg
• Baking a cake
• Fermentation of grapes into wine
• Souring of milk
• Exploding fireworks
• Photosynthesis in plants
• Digestion of food
• Neutralizing an acid with a base