GED Science TestPhysical ScienceMedium
A scientist is observing a chemical reaction in which a solid reactant is heated and decomposes into two gaseous products. Which of the following observations would most clearly indicate that a chemical change has occurred?
- AThe solid becomes warmer to the touch.
- BThe solid melts into a liquid.
- CThe solid changes shape.
- DBubbles are produced and a new odor is detected.
Show answer & explanationAnswer & explanation
Correct answer: D. Bubbles are produced and a new odor is detected.
The production of bubbles (indicating gas formation) and the detection of a new odor are strong indicators of a chemical change, as new substances with different properties (gases and odor) are being formed. Melting, changing shape, or becoming warmer are typically physical changes or energy transfers.
Why the other options are wrong
- A. Becoming warmer indicates an energy transfer (exothermic reaction), which can accompany both physical and chemical changes but isn't a definitive sign of a new substance.
- B. Melting is a physical change of state, not necessarily a chemical change.
- C. Changing shape is a physical change, not indicative of new substances being formed.
Indicators of Chemical Change
Observable signs that suggest a chemical reaction has occurred, resulting in the formation of new substances with different properties.
- Formation of a gas (bubbles)
- Formation of a precipitate (solid)
- Change in color
- Change in temperature (release or absorption of heat)
- Production of light or sound
- Change in odor
Memory trick: New Stuff, New Signs: That's Chemistry's Design.