To determine which of the reactions represents a physical change, we need to understand the differences between physical and chemical changes:
- Physical changes: These changes affect the form of a chemical substance, but not its chemical composition. Examples include changes in state (solid to liquid, liquid to gas, etc.), dissolving, and melting.
- Chemical changes: These lead to the formation of new chemical substances with different compositions and properties. This often involves breaking and forming chemical bonds.
Now, let's analyze each option:
### Option A
[tex]\[ 2 CH_3OH (\ell) + 3 O_2 (g) \rightarrow 2 CO_2 (g) + 4 H_2O (g) \][/tex]
This reaction describes the combustion of methanol, resulting in the formation of carbon dioxide and water. Since new substances (carbon dioxide and water) are formed, this is a chemical change.
### Option B
[tex]\[ NaCl (s) \rightarrow NaCl (aq) \][/tex]
This reaction shows solid sodium chloride dissolving in water to form an aqueous solution. The chemical composition of sodium chloride remains the same; it is only changing its state from solid to aqueous. Thus, this is a physical change.
### Option C
[tex]\[ 2 AgNO_3 (aq) + CaBr_2 (aq) \rightarrow 2 AgBr (s) + Ca(NO_3)_2 (aq) \][/tex]
This reaction involves the mixing of aqueous solutions of silver nitrate and calcium bromide to form solid silver bromide and aqueous calcium nitrate. New substances are formed, indicating a chemical change.
### Option D
[tex]\[ H_2 (g) + O_2 (g) \rightarrow H_2O (g) \][/tex]
Here, hydrogen gas reacts with oxygen gas to produce water vapor. The formation of water from hydrogen and oxygen indicates the creation of a new substance, making it a chemical change.
### Conclusion
Based on the analysis, the reaction that describes a physical change is:
### Option B:
[tex]\[ NaCl (s) \rightarrow NaCl (aq) \][/tex]
This concludes that the correct answer is:
[tex]\[ 2 \][/tex]
