Weighing reaction mixture At school we do an experiment with baking soda and vinegar (in a cup) both sealed in a ziplock bag. The bag is masses on a triple beam balance (example 32.7 g). Bag is tilted to allow the vinegar and baking soda to mix.

The students note the signs of chemical reaction and mass the bag again. There is always a 0.3 g – 0.5 g loss of mass. How is this explained? Cheap ziplock bags? Is some of the mass from the endothermic reaction converted?
Think about the reaction:
$$\ce{NaCHO3(s) + CH3COOH(l) \to CO2(g) + H2O(g) + Na+(aq) + CH3COO- (aq)}$$
The resulting $\ce{CO2}$ will make the bag expand, therefore increasing the volume and the subsequent buoyant force. This force opposes the force of gravity, making it seem like the mass of the matter inside the bag has decreased.