Question

A group of students added baking soda to a cup containing vinegar. As soon as the baking soda touched the vinegar, bubbles and a gas formed. After the bubbling stopped, the students measured the mass of the cup and its contents. The total mass was 2 grams less than the combined mass of the baking soda, vinegar, and cup before the reaction.



Which statement BEST explains why the students’ measured mass was less after the reaction?



A.
The mass was less because the baking soda dissolved in the vinegar.

B.
The mass was less because a gas formed and escaped into the air.

C.
The mass was less because a chemical reaction occurred.

D.
The mass was less because gases do not have mass.

Answer 1

B

When baking soda (sodium bicarbonate) reacted with vinegar (acetic acid) it produced carbon dioxide gas that escaped from the cup. Mass was lost because some of the product (the gas) left the system. (A is wrong because dissolving doesn't reduce mass, C is incomplete—reactions don't by themselves change mass of a closed system, and D is false—gases do have mass.)