Which of the following reactions shows a physical change?
A. [tex]$H _2(g)+ O _2(g) \rightarrow H _2 O ( l )$[/tex]
B. [tex]$2 AgNO _3(a q)+ CaBr _2(a q) \rightarrow 2 AgBr (s)+ Ca \left( NO _3\right)_2(a q)$[/tex]
C. [tex]$2 CH _3 OH ( l )+3 O _2(g) \rightarrow 2 CO _2(g)+4 H _2 O ( l )$[/tex]
D. [tex]$NaCl (s) \rightarrow NaCl (a q)$[/tex]
Answer (2)
The question asks to identify a physical change among the given chemical reactions. A physical change is a change in the form or appearance of a substance, but not in its chemical composition. Option D, N a Cl ( s ) → N a Cl ( a q ) , represents the dissolution of sodium chloride (salt) in water, where the salt dissociates into ions but does not change its chemical composition, indicating a physical change. The final answer is D.
Explanation
Identifying Physical Change We are asked to identify which of the given reactions represents a physical change. A physical change alters the form or appearance of a substance but does not change its chemical composition. Let's analyze each option:
Option A: H 2 ( g ) + O 2 ( g ) → H 2 O ( l ) involves the formation of water from hydrogen and oxygen. This is a chemical reaction because new chemical bonds are formed.
Option B: 2 A g N O 3 ( a q ) + C a B r 2 ( a q ) → 2 A g B r ( s ) + C a ( N O 3 ) 2 ( a q ) involves the formation of silver bromide and calcium nitrate from silver nitrate and calcium bromide. This is a chemical reaction because new compounds are formed.
Option C: 2 C H 3 O H ( l ) + 3 O 2 ( g ) → 2 C O 2 ( g ) + 4 H 2 O ( l ) is a combustion reaction where methanol reacts with oxygen to form carbon dioxide and water. This is a chemical reaction because new chemical bonds are formed.
Option D: N a Cl ( s ) → N a Cl ( a q ) represents the dissolution of sodium chloride (salt) in water. The salt dissociates into ions ( N a + and C l − ) but does not change its chemical composition. This is a physical change.
Determining the Answer Based on the analysis, the reaction that represents a physical change is the dissolution of sodium chloride in water.
Final Answer Therefore, the correct answer is D.
Examples
Consider making a cup of coffee. When you add sugar to your coffee and stir, the sugar dissolves. This is a physical change because the sugar is still sugar, just dispersed within the coffee. Similarly, when salt dissolves in water, it's still salt, just in ionic form. Understanding physical changes helps us see how substances can change form without changing their fundamental nature, which is crucial in cooking, chemistry, and everyday life.
The reaction that represents a physical change is option D: N a Cl ( s ) → N a Cl ( a q ) , as it shows sodium chloride dissolving in water without changing its chemical composition. This involves a change in state from solid to aqueous. Understanding physical changes is crucial as they do not alter the chemical identity of the substance.
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