Hi there, the correct answer to this would be reactivity because color, melting point, and density are all examples of physical properties.
Regard the principle of utilization of two gas.
Make a consistent control of hardware containing gas.
Make a consistent control of weight diminishing valves giving gas.
No smoking zone.
Cooper is the substance that can't be decomposed by a chemical chAnge. So 4)
Probably Sulfur, low mp and hardness compared to the other metals
