Sulphur has a gravity of 2. That means if you were to weigh a 2k of wet Sulphur , it would have an apparent weight of only 1kg. All things weigh more when they burn (oxygen gets added). Sulphur has an atomic mass of 16. If it combines with oxygen on a one-to-one basis,and thus it will weigh 1 1/2 times as much, That means 2kg will weigh 3kg.
therefore, your answer is sulphur!!!