The technician observed the iron's melting process.
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Three close words: 熔解 (melting of solids - physics/metallurgy), 溶解 (dissolving in liquid - chemistry), 融化 (melting/thawing - everyday, ice cream, snow). The left radicals distinguish them: 火 (fire) in 熔 versus 氵 (water) in 溶.
