Which of the following is a chemical change?

Chemical change: new substances are formed with different chemical properties; chemical bonds break and form. Wood burning: cellulose + oxygen → carbon dioxide + water vapor + ash; new substances produced; cannot easily reverse. Signs of chemical change: (1) Color change; (2) Temperature change (exothermic — releases heat, or endothermic — absorbs heat); (3) Gas formation/bubbles (not from boiling); (4) Precipitate formation (solid in solution); (5) Light emission; (6) Odor change; (7) Difficulty reversing. Physical change: substance changes appearance or state but composition remains the same. Examples: melting, freezing, boiling, dissolving, breaking, cutting, mixing. Ice → water → vapor are all the same H₂O molecules, just different arrangements. Distinguishing: chemical change creates NEW substances; physical change just rearranges what's already there. Some changes mix both — dissolving sugar in water is mostly physical (sugar molecules still exist), but dissolving NaCl involves ionic dissociation (slight chemical change). Burning, rusting, fermenting, photosynthesis, digestion are chemical. Melting wax, dissolving salt, boiling water, freezing, evaporation are physical.