Explanation: Physical changes alter form/state (melting, cutting) while the chemical composition remains unchanged.

Explanation: Rusting produces a new substance (iron oxide) and changes properties irreversibly under normal conditions.

Explanation: A state change is a physical change and doesn't necessarily indicate a chemical reaction.

Explanation: Salt can be recovered by evaporating water — a physical reversal; other options are chemical/irreversible.

Explanation: Vinegar and baking soda react to produce CO₂ gas (effervescence), indicating a chemical change.

Explanation: Distillation separates liquids by boiling point — a physical method; others are chemical processes.

Explanation: New chemical properties suggest new substances formed during chemical change.

Explanation: Dissolving ammonium nitrate absorbs heat (temperature drops) — an endothermic process.

Explanation: CO₂ reacts with limewater (Ca(OH)₂) forming CaCO₃ precipitate, turning it milky.

Explanation: Hydrogen burns with a characteristic 'pop' when ignited — simple test for H₂.

Explanation: A precipitate (BaSO₄) forms when two solutions react — classic precipitation reaction.

Explanation: Burning a candle releases heat (exothermic); others absorb heat or are physical.

Explanation: Formation of insoluble solid makes the solution cloudy; on settling, solid deposits as precipitate.

Explanation: Combustion converts paper into new substances (ash, CO₂) with heat and light — chemical and irreversible.

Explanation: Reactive metals like Zn, Mg displace hydrogen from acids forming H₂ gas (pop test confirms).

Explanation: Oxygen supports combustion; a glowing splint relights in oxygen-rich environment — diagnostic test.

Explanation: Browning is due to oxidation of compounds in apple — new coloured compounds form (chemical change).

Explanation: Acids turn blue litmus paper red; this simple test indicates acidity.

Explanation: Evaporation or distillation recovers salt by removing water — physical separation based on boiling points.

Explanation: Catalysts provide an alternative pathway with lower activation energy and remain unchanged post-reaction (e.g., MnO₂ in H₂O₂ decomposition).

Explanation: Burning mixes wood chemically with oxygen producing ash and gases — cannot be reversed simply.

Explanation: Painting and oiling form physical barriers preventing contact with moisture and oxygen, reducing rust formation.

Explanation: Fermentation uses microbes to convert sugars into CO₂ and alcohol — chemical changes mediated biologically.

Explanation: Zinc corrodes preferentially (sacrificial protection) protecting iron even when coating is scratched.

Explanation: Melting chocolate is a physical state change; others involve chemical reactions producing new substances.

Explanation: Baking involves fermentation and Maillard reactions producing new flavours and textures — chemical changes.

Explanation: Salt water acts as electrolyte accelerating corrosion by facilitating electron flow between anodic and cathodic sites.

Explanation: Gold is a noble metal, very unreactive and resists corrosion/tarnish in normal environments.

Explanation: Catalysts lower activation energy and increase rate; they are unchanged at end of reaction.

Explanation: Filtration separates insoluble solids from liquids — a physical process based on particle size.

Explanation: Chlorination uses chlorine to chemically inactivate pathogens; boiling is physical and also effective but chlorine provides residual protection.

Explanation: Soaps and detergents have hydrophobic and hydrophilic ends that emulsify oils enabling removal with water (physical/mechanistic action).

Explanation: Bleach contains sodium hypochlorite, an oxidiser that destroys microbes and decolorises stains chemically.

Explanation: Goggles protect eyes; maintaining safe distance and using tongs prevents burns and accidents.

Explanation: Fermentation by yeast converts sugars into ethanol and CO₂ — chemical change used in baking and brewing.

Explanation: Incomplete combustion makes CO (toxic) and soot (particulates) harmful to health and environment.

Explanation: Copper conducts electricity well and is ductile, making it ideal for wires; aluminium is also used where lighter weight is needed.

Explanation: Cold temperatures slow microbial growth and sealing prevents contamination, reducing chemical/biological spoilage.

Explanation: Fermentation is harnessed to produce alcohol and food products under controlled conditions for desired outcomes.

Explanation: Bleach oxidises coloured molecules causing decolourisation — a chemical process used in cleaning.

Explanation: Catalysts lower activation energy making reactions faster without being consumed; they do not alter equilibrium position.

Explanation: Dissolved oxygen often needs chemical/analytical tests (e.g., Winkler method) to quantify; others are visually evident.

Explanation: Photosynthesis builds glucose using light (endothermic), while combustion oxidises fuels releasing energy (exothermic).

Explanation: Dissolving disperses existing molecules (recoverable); chemical reactions change composition forming new substances with new properties.

Explanation: Souring is due to bacterial fermentation producing lactic acid — a biological chemical change.

Explanation: Salt is soluble; dissolve in water, filter sand out, then evaporate water to recover salt — physical separation steps.

Explanation: Spectrophotometers measure absorbance allowing quantitative tracking of concentration and colour changes during reactions.

Explanation: Cleaner fuels and efficient combustion reduce CO, particulates and greenhouse gas emissions, mitigating environmental impacts.

Explanation: Dissolving ammonium nitrate absorbs heat causing temperature drop — safe classroom demonstration of endothermic behaviour.

Explanation: Using multiple indicators strengthens conclusion and avoids mistakes from single misleading signs.