Chemical changes sometimes produce a gas which you can detect by observing

Observations of a chemical change is when chemical changes sometimes produce a gas, which you can detect by observing bubbles or by a change in odor. A chemical reaction is usually accompanied by easily observed physical effects, such as the emission of heat and light, the formation of a precipitate, the evolution of gas, or a color change.

The answer is 4 i don't know if its right let me see.

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