Is dissolving table salt (sodium chloride or NaCl) a chemical change or a physical change? This is a common general chemistry question. The problem is that the answer is disputed. Here are valid arguments for both answers.
Many chemists consider dissolving salt (or any ionic solid) to be a chemical change. Dissolving sugar (or any covalent solid) is a physical change.
Why Dissolving Salt Is a Chemical Change
A chemical change involves a chemical reaction and the formation of new products. Dissolving salt in water may be written as a chemical reaction, where sodium chloride dissociates into Na+ ions and Cl– ions in water.
NaCl(s) → Na+(aq) + Cl–(aq)
When salt dissolves, the ionic bonds between the atoms break. The reactant (sodium chloride or NaCl) differs from the products (sodium and chloride ions), so a chemical change occurs. The same reaction occurs when other ionic compounds dissolve in water. To generalize: Dissolving an ionic compound is a chemical change. In contrast, dissolving sugar or another covalent compound is a physical change because chemical bonds are not broken and new products are not formed. If you dissolve sugar in water, you get sugar molecules in water.
Why Dissolving Salt in Water Is a Physical Change
A physical change involves a change in a physical property, but not a change in chemical composition. Examples include changes in states of matter or alterations to crystal structure.
Dissolving salt in water may be considered a physical change because no change occurs in the electron shells of the sodium and chlorine atoms and no chemical reaction occurs between sodium chloride and its solvent (water). In contrast, if you dissolve salt in acetic acid (CH₃COOH), you get sodium ethanoate (CH3COONa) and hydrochloric acid (HCl).
Sometimes the reason given for saying dissolving salt is a physical change is that the process is reversible. If you remove the water, you recover salt. The problem is that many chemical changes are reversible. For example, weak acids and weak bases typically participate in reversible reactions that eventually reach equilibrium. Mixing carbonic acid in water is an example:
H2CO3 (l) + H2O(l) ⇌ HCO3−(aq) + H3O+(aq).
Meanwhile, many physical changes are not reversible. For example, you can’t put a sheet of paper back together again after you shred it.
Teaching Considerations
On the one hand, discussing whether dissolving sugar and salt are chemical or physical changes is a good way to get students thinking about changes in matter. It’s an opportunity to talk about how you know whether a chemical reaction has occurred. On the other hand, all of the signs of a chemical change (temperature change, color change, odor, bubbles, precipitate formation) occur with some physical changes.
If a student is asked whether dissolving salt is a chemical change or a physical change, some instructors consider it unfair to mark either answer incorrect, providing the student can explain the answer. Other teachers feel strongly about the answer. In this case, it’s important to convey expectations to the class prior to homework or an exam.
But, what do you think? Feel free to post a comment.
References
- Hill, John W., et al. (2004) General Chemistry (4th ed.). Prentice Hall. ISBN: 978-0131402836.
- Zumdahl, Steven S.; Zumdahl, Susan A. (2000). Chemistry (5th ed.). Houghton Mifflin. ISBN: 0-395-98583-8.