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Difficulty: EasyProperties of Matter

A cook dissolves table salt into a pot of warm water until the crystals are no longer visible. When the water is later boiled away completely, solid salt crystals remain at the bottom of the pot. Which of the following best classifies the dissolving of the salt?

  1. A
    A chemical change, because the solid crystals initially disappeared from view.
  2. B
    A decrease in total mass, because the dissolved salt no longer occupies physical space.
  3. A physical change, because the chemical identity of the salt was preserved.Answer
  4. D
    A chemical change, because thermal energy was required to separate the salt from the water.

Answer

Dissolving salt in water is a physical change because the chemical identity of the substance does not change and the process is reversible.
A physical change occurs when the form, phase, or appearance of a substance changes without altering its chemical identity. Because salt can be completely recovered in its original form simply by evaporating the water, no chemical reaction took place.

Step-by-Step Solution

1
Analyze whether new substances were formed during the dissolving process.
The salt (NaClNaCl) disperses into sodium and chloride ions in water, but its chemical identity remains unchanged.
Chemical changes require the formation of entirely new substances with distinct chemical formulas.
2
Evaluate the reversibility of the separation.
Evaporating or boiling off the water leaves the original salt crystals intact.
Physical mixtures can be separated by physical means such as evaporation, confirming a physical change.

Key Concept

Physical changes alter the form or state of matter without changing its chemical composition, allowing original substances to be recovered by physical methods.
Estimated Time:45s
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Topics

Properties of Matter

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