Addition of water to an alkyne gives a keto-enol tautomer product. draw an enol that is in equilibrium with the given ketone.

If water were added to an alkyne and the product was 2-pentanone, then the alkyne must have been 1-pentyne. 2-pentyne would have given a mixture of 2- and 3-pentanone. 

The enol that would give 2-pentanone would have been pent-1-en-2-ol. Because an equilibrium favors the ketone so greatly, equilibrium is not a good description. If the ketone were treated with bromine, little reaction would be seen as the enol content would be too low. If a catalyst were added, NaOH for example, then formation of the enolate of pent-1-en-2-ol would form and react with bromine. This would eventually give a bromoform product. Under acidic conditions, the enol would favor formation of the more substituted enol consistent with alkene stability.

Exothermic,  physical change after it is already baked

The density would be 1.622