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Difficulty: MediumChemical Reactions and Compounds

An electric current is passed through liquid water containing a small amount of an electrolyte, producing hydrogen gas and oxygen gas according to the equation:

2H2O(l)→2H2(g)+O2(g)2\text{H}_2\text{O}\text{(l)} \rightarrow 2\text{H}_2\text{(g)} + \text{O}_2\text{(g)}

Which statement best explains why this process is classified as a chemical change rather than a physical change?

  1. A
    The liquid water absorbs energy and undergoes a physical phase change into water vapor.
  2. Covalent bonds within water molecules are broken and new substances with distinct chemical properties are formed.Answer
  3. C
    The total mass of the resulting gases is greater than the mass of the liquid water consumed.
  4. D
    Water molecules physically dissolve and separate into individual ions without forming new neutral molecules.

Answer

Covalent bonds within water molecules are broken and new substances with distinct chemical properties are formed.
The process is a chemical change because intramolecular covalent bonds in the reactant compound are broken, rearranging the atoms to form new elemental substances with entirely different chemical and physical properties.

Step-by-Step Solution

1
Analyze the chemical formulas of the reactants and products in the reaction equation (2H2O→2H2+O22\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2).
Liquid water (a chemical compound) is converted into two distinct elemental gases: hydrogen and oxygen.
Determines whether the chemical identity of the starting substance is preserved or transformed.
2
Differentiate between a physical change and a chemical change at the molecular level.
The existing H−O\text{H}-\text{O} covalent bonds in H2O\text{H}_2\text{O} are broken, and new H−H\text{H}-\text{H} and O=O\text{O}=\text{O} bonds are created.
A chemical change is fundamentally defined by the rearrangement of atoms through the breaking and forming of chemical bonds to create new substances.

Key Concept

A chemical change involves breaking and forming chemical bonds to produce new substances with distinct chemical properties, in accordance with the law of conservation of mass.
Estimated Time:1m 0s
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Topics

Chemical Reactions and Compounds

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