The Law of Conservation of Mass states that mass in a closed system is neither created nor destroyed by chemical reactions or physical transformations; thus, the mass of the reactants must equal the mass of the products.

Demonstrating Mass Conservation

To best support this law, a reaction must be conducted in a way that ensures all reactants and products are accounted for. This usually means preventing the escape or addition of substances from the surroundings.

Analysis of Options:

• Heating iron in open air: Iron reacts with oxygen. In an open system, gaseous reactants (oxygen) or products might not be fully measured, making mass conservation difficult to verify precisely.
• Evaporation of alcohol in open air: Evaporation is a physical change. In an open container, the alcohol vapor dissipates into the air, making it impossible to measure the total mass accurately.
• Burning of paper in open air: Burning is a chemical reaction producing gases (like CO₂, water vapor) and ash. These gaseous products escape into the open air, leading to a significant loss of measurable mass, seemingly violating the law.
• Mixing of vinegar and baking soda in a sealed container: This classic reaction produces carbon dioxide gas:

  CH₃COOH (aq) + NaHCO₃ (s) → CH₃COONa (aq) + H₂O (l) + CO₂ (g)

  In a sealed container, all reactants and products, including the gaseous CO₂, are contained. This allows for accurate measurement of the total mass before and after the reaction, clearly demonstrating that the mass remains constant, thus best supporting the Law of Conservation of Mass.

Conclusion

Conducting the reaction in a sealed container is crucial for observing the Law of Conservation of Mass, as it prevents the loss or gain of matter. Therefore, mixing vinegar and baking soda in a sealed container is the best option.