Which substance is most likely to heat up the fastest?
water (specific heat of 4.19 /g • c°)
copper (specific heat of .39 /g • c°)
aluminum (specific heat of .90 /g • c°)
dry air (specific heat of 1.01 /g • c°)

Specific heat is measuring how much heat is needed to raise one gram of a substance by one degree Celsius. Therefore, the substance with the lowest specific heat requires the least amount of energy to raise its temperature. This means if all of the substances got the same amount of heat, the substance with the lowest specific heat's temperature would raise the most. Or, if each substance was exposed to constant heat, the substance with the lowest specific heat would heat up the fastest. Since copper has the lowest specific heat, the answer is copper.

They are all decomposition reactions.