Physical & Chemical Changes
Section: 6. Chemical Reactions | Syllabus: Cambridge AS Level Physics 9702
What is a Physical Change? Physical changes are changes in which no new substances are formed. The substance remains the same chemically, but its physical appearance or state may change. Key characteristics of physical changes: No new substances are formed Changes are usually easily reversed Only the physical properties change (state, shape, size) The chemical composition stays the same Examples of Physical Changes Type of Change Examples What Happens Changes of state Melting ice, boiling water, freezing water, condensing steam State changes but H₂O remains H₂O Dissolving Sugar in water, salt in water Substance disperses but doesn't change chemically Changing shape/size Cutting paper, breaking glass, crushing rocks Physical form changes but material stays the same Mixing substances Mixing sand and salt No new substance - can be separated again What is a Chemical Change?
Chemical changes (also called chemical reactions) are changes in which one or more new substances are formed with different properties from the original substances. Key characteristics of chemical changes: New substances are formed Usually difficult or impossible to reverse Chemical composition changes Often involves energy changes (heat, light) Examples of Chemical Changes Type of Change Examples What Happens Combustion (burning) Burning wood, burning candle New substances: CO₂, H₂O, ash Rusting Iron + oxygen + water Iron(III) oxide (rust) forms Cooking Baking bread, frying eggs Proteins and starches change structure Reactions Acid + metal, neutralization New products formed (salts, gases, water) Decomposition Thermal decomposition of limestone CaCO₃ → CaO + CO₂ Observable Signs of Chemical Changes You can often tell a chemical change has occurred by observing: Color change - e.g., copper turning green when it oxidizes Gas produced - bubbles/fizzing (e.g., acid + carbonate → CO₂) Precipitate forms - solid appears in solution Temperature change - energy released (exothermic) or absorbed (endothermic) Light produced - e.g., combustion reactions Smell produced - new substance with odor forms Irreversible - difficult to get back original substances Comparing Physical and Chemical Changes Aspect Physical Change Chemical Change New substance formed?
NO - same substance YES - new substance(s) Reversible? Usually YES - easily reversed Usually NO - difficult to reverse Chemical composition Stays the SAME CHANGES Energy change Usually small Often significant (heat/light) Bonds No bonds broken or made Bonds broken and new bonds formed Examples Melting, dissolving, cutting Burning, rusting, cooking Tricky Examples - Is it Physical or Chemical?
Dissolving salt in water = PHYSICAL Reason: Salt (NaCl) stays as Na⁺ and Cl⁻ ions - no new substance. Can be recovered by evaporation. Burning sugar = CHEMICAL Reason: New substances formed (carbon and water vapor).
Cannot be reversed. Heating sugar until it melts = PHYSICAL (if it doesn't burn) Reason: Sugar changes state but is still sugar. Will re-solidify when cooled. Magnetizing iron = PHYSICAL Reason: Iron stays as iron - only magnetic properties change.
Conservation of Mass In both physical and chemical changes, mass is conserved - the total mass before = total mass after. In physical changes: Obviously conserved - same substance just in different form Example: 100g ice → 100g water In chemical changes: Total mass of reactants = total mass of products Example: 12g carbon + 32g oxygen → 44g carbon dioxide Important: Must count ALL products (including gases that escape) Why Mass Seems to Change Sometimes mass appears to change in chemical reactions: Mass appears to DECREASE: This happens when a gas is produced and escapes.
Example: Heating calcium carbonate in an open container CaCO₃(s) → CaO(s) + CO₂(g) The CO₂ gas escapes, so the solid left behind weighs less. Mass appears to INCREASE: This happens when a gas from the air reacts with the substance.
Example: Burning magnesium in air 2Mg(s) + O₂(g) → 2MgO(s) Oxygen from air adds to the magnesium, so product weighs more. Remember: If you measure ALL reactants and ALL products (including gases), mass is ALWAYS conserved!
Key Points Summary Physical changes: No new substance, usually reversible, same chemical composition Chemical changes: New substance(s) formed, usually irreversible, different composition Observable signs of chemical change: color change, gas, precipitate, temperature change, light Mass is conserved in ALL changes (physical and chemical) Apparent mass changes occur when gases escape or are absorbed from air State changes (melting, boiling) are physical; reactions are chemical
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