Question

nitrogen dioxide is a gas that can be generated by emissions from vehicles and factories. it can also be generated by natural sources, such as forest fires, lightning, and microbial activity in soil. the equation for producing nitrogen dioxide is shown below. 2no + o₂ → 2no₂ colorless gas colorless gas brown gas which of these provides evidence that a chemical change occurs? a. a gas is present. b. the total number of atoms remains unchanged. c. the state of matter remains the same. d. a brown gas is produced.

Explanation:

Step1: Recall chemical - change criteria

A chemical change results in the formation of new substances with different chemical properties. In the given reaction $2NO + O_2
ightarrow2NO_2$, color - less gases ($NO$ and $O_2$) react to form a brown gas ($NO_2$), which is a new substance.

Step2: Analyze each option

• Option A: Just the presence of a gas does not necessarily mean a chemical change. For example, a gas can be in a physical state change like evaporation.
• Option B: The conservation of the total number of atoms is a characteristic of both physical and chemical changes (due to the law of conservation of mass), so it is not conclusive evidence of a chemical change.
• Option C: A change in the state of matter (e.g., solid - liquid - gas) is a physical change, not a chemical change indicator.
• Option D: The formation of a new substance (a brown gas $NO_2$ from color - less gases $NO$ and $O_2$) is evidence of a chemical change.

Answer:

D. A brown gas is produced.