Final answer:
The reason the Erlenmeyer flask experiment shows a mass decrease is because it's not a closed system, allowing gas to escape, thus failing to demonstrate the law of conservation of mass. To correctly demonstrate this law, you must conduct the experiment in a closed system.
Step-by-step explanation:
The reason the scale shows less mass after the reaction between baking soda and vinegar in an Erlenmeyer flask is due to the release of gases, such as carbon dioxide, which are not accounted for by the scale. This does not mean the law of conservation of mass is violated; it simply indicates that the setup of the experiment does not capture all products of the reaction. To demonstrate the law of conservation of mass, the experiment should be conducted in a closed system where no mass (gas or otherwise) can escape. For example, by using a sealed container or a balloon over the flask to trap the gases, you could then measure the total mass before and after the reaction to show it remains constant.
In classroom or laboratory demonstrations, such as reacting an effervescent tablet in water contained within a balloon attached to a test tube, the mass is seen to remain the same before and after the reaction when captured in a closed system. Similarly, in other scenarios like cooking bread dough or burning magnesium, mass can appear to change due to the release or absorption of gases, but in a sealed environment, the mass would remain unchanged, showcasing the conservation of mass. Antoine Lavoisier's experiment with tin and air in a sealed flask exemplifies this principle perfectly, as the mass of the sealed flask and contents remained the same after the heating and reaction.