Whak-A-Pack Valentines

Around this time of the year a local dollar store sells four Valentines Day-themed Wack-A-Packs for a dollar.  These are fun chemistry demos!  Inside each pack (bottom right in picture) is a deflated polyester balloon.  In the balloon is powdered citric acid and powdered sodium hydrogen carbonate (upper left in picture - much like an Alka-Seltzer tablet) and a pouch filled with water (lower left).  When the pack is struck hard, the water pouch breaks, enabling the water to dissolve the acid and salt.  These react to produce carbon dioxide gas which causes the balloon to inflate (upper right) and pop out of the pouch. I recommend passing the balloons around the classroom as soon as they open, because for a little while, the reaction can be heard fizzing in the balloons as carbon dioxide continues to be produced, and the balloon feels cold (I attribute this to the enthalpy of solvation of the sodium hydrogen carbonate).