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Difficulty: MediumChemical Reactions and Compounds

During a laboratory experiment, a student mixes an aqueous solution of sodium carbonate (Na2CO3\text{Na}_2\text{CO}_3) with an aqueous solution of calcium chloride (CaCl2\text{CaCl}_2). A solid white precipitate of calcium carbonate (CaCO3\text{CaCO}_3) immediately forms and settles at the bottom, while sodium chloride (NaCl\text{NaCl}) remains dissolved. Which of the following statements best explains why this process is classified as a chemical reaction rather than a physical change?

  1. A
    The liquid solution underwent a physical phase change where dissolved particles froze into solid crystals due to temperature loss.
  2. B
    The substances formed a simple mechanical mixture in which both original compounds retain their identical chemical bonds.
  3. Chemical bonds were broken and reformed to produce a new substance with different chemical properties from the original reactants.Answer
  4. D
    New atoms of calcium and carbon were generated during the reaction to create the additional mass observed in the solid precipitate.

Answer

Chemical bonds were broken and reformed to produce a new substance with different chemical properties from the original reactants.
A chemical reaction occurs when bonds within reactant molecules break and new bonds form to create one or more chemically distinct substances. The formation of solid calcium carbonate precipitate from aqueous sodium carbonate and calcium chloride demonstrates the synthesis of a new compound with unique solubility and chemical properties.

Step-by-Step Solution

1
Analyze the starting reactants and resulting products in the reaction.
Reactants are soluble Na2CO3\text{Na}_2\text{CO}_3 and CaCl2\text{CaCl}_2; products are solid CaCO3\text{CaCO}_3 precipitate and dissolved NaCl\text{NaCl}.
Identifying the initial and final chemical species confirms whether new substances were produced.
2
Distinguish between chemical transformations and physical changes.
The formation of a solid precipitate from two clear liquids via double displacement indicates bonds broke and reformed to yield a new insoluble compound.
A physical change alters form or state without changing chemical identity, whereas this reaction alters chemical identity and molecular structure.
3
Apply the Law of Conservation of Mass to verify atom balance.
The balanced equation is Na2CO3(aq)+CaCl2(aq)→CaCO3(s)+2NaCl(aq)\text{Na}_2\text{CO}_3\text{(aq)} + \text{CaCl}_2\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} + 2\text{NaCl}\text{(aq)}, showing atom conservation rather than atom creation.
All atoms present in the products originate from the reactants; no atoms are generated or destroyed.

Key Concept

Chemical change involves breaking and forming chemical bonds to produce new substances, distinguished from physical phase changes and governed by the Law of Conservation of Mass.
Estimated Time:1m 0s
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Topics

Chemical Reactions and Compounds

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