Physics
A radioactive substance does not undergo any change (in mass or charge) during

Alpha-emission
Beta-emission
Gamma-emission
Oxidation

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Physics
On a cold day when the room temperature is 15°C, the metallic cap of a pen becomes much colder than its plastic body, though both are at the same temperature of 15°C, because :

Plastics have a higher thermal conductivity than metals
Plastics have a lower density than metals
Metals are good conductors of heat
Metals have higher thermal capacity than plastics

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