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Many students who study using our platform have reported that a large number of the practice questions here are very similar to those that appear in real exams. View Courses
Many students who study using our platform have reported that a large number of the practice questions here are very similar to those that appear in real exams. View Courses
Which observation most strongly indicates that a new substance has been produced during a reaction?
Correct Answer: A. Gas bubbles forming after mixing two clear liquids. The appearance of gas bubbles indicates that a chemical reaction has occurred, producing a new substance in the form of a gas that was not present before. This is a strong indicator of chemical change because it suggests that bonds in the original substances were broken and new bonds formed to create different compounds. For example, mixing vinegar and baking soda produces carbon dioxide gas, which is visible as bubbles. Other signs of chemical change include color changes, energy release (heat or light), or the formation of a precipitate.
Why the Other Options Are Incorrect:
B. Salt dispersing evenly when stirred into water
Dissolving salt in water is a physical process known as dissolution. The salt ions separate and disperse in the water but do not form new chemical substances. The original compounds (NaCl and H₂O) can be recovered through evaporation, showing no chemical reaction took place.
C. Ice changing to liquid water after heat exposure
This is a phase change involving the addition of heat energy, but the water molecules remain identical. The process affects only the physical state (solid to liquid) and not the chemical makeup.
D. Metal expanding slightly after temperature increase
Thermal expansion occurs because heat causes the metal’s atoms to vibrate more vigorously, increasing the distance between them. This is purely a physical effect related to temperature, not a chemical transformation.
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