Assertion (A): In an open container, a reaction between baking soda and vinegar seems to lose mass after some time.
Reason (R): Some gas is released during the reaction and escapes into the air.

In this reaction between baking soda (sodium bicarbonate) and vinegar (acetic acid), a chemical reaction occurs that produces carbon dioxide gas, water, and salt. In an open container, the carbon dioxide gas can escape into the air.

As per the law of conservation of mass, the total mass should remain the same. However, if the reaction happens in an open container, it might seem like mass is lost because the gas, which is part of the mass, escapes and isn’t counted in the final mass of the container contents.

This makes the assertion (A) true—there appears to be a loss of mass when, in reality, it’s just the gas escaping. The reason (R) correctly explains why this apparent loss occurs: the carbon dioxide gas produced in the reaction escapes into the air, giving the impression that mass has decreased.