Which observation most likely indicates that only a chemical change has taken place?
1. State of matter changes
2. The change cannot be reversed
3. The shape changes
4. A reaction occurs
Option 4. A reaction occurs most likely indicates that only a chemical change has taken place.
The observation that most likely indicates that only a chemical change has taken place is option 4: "A reaction occurs."
To understand why this is the correct answer, let's first define what a chemical change is. A chemical change, also known as a chemical reaction, involves the formation or breaking of chemical bonds, resulting in the creation of new substances with different chemical properties.
Now, let's analyze the other options:
1. "State of matter changes": Changes in the state of matter, such as from a solid to a liquid or a gas, can indicate a physical change rather than a chemical change. Physical changes do not involve the formation or breaking of chemical bonds.
2. "The change cannot be reversed": This observation alone does not provide conclusive evidence of a chemical change. Both physical and chemical changes can be irreversible.
3. "The shape changes": Changes in shape typically suggest a physical change resulting from processes like bending or breaking, which do not involve the formation of new substances.
Therefore, when option 4 states that "a reaction occurs," it implies that a chemical reaction has taken place. Chemical reactions often involve the release or absorption of energy, the production of gas or precipitate, changes in color, or other indicators of a chemical change.