An example of a physical change is

Analysis

To solve this, we must distinguish between a physical change and a chemical change:

• Physical Change: A change where no new substance is formed. The chemical composition remains the same, and the change is often reversible (e.g., melting, boiling, dissolving).
• Chemical Change: A change that results in the formation of one or more new substances with different chemical properties. These are usually difficult to reverse.

Concept Explanation

• Addition of water to calcium oxide: This is a chemical reaction (slaking of lime). Calcium oxide reacts with water to form calcium hydroxide ($CaO + H_2O \rightarrow Ca(OH)_2$). This releases heat, indicating a chemical change.
• Corrosion of iron: This is a chemical process (oxidation). Iron reacts with oxygen and moisture to form hydrated iron(III) oxide (rust). A new substance is formed.
• Dissolving sodium chloride in water: When salt dissolves in water, it dissociates into its constituent ions ($Na^+$ and $Cl^-$). However, the chemical identity of the salt remains. If you evaporate the water, you recover the original sodium chloride crystals. No new chemical substance is created.
• Burning of charcoal: This is a combustion reaction. Carbon reacts with oxygen to form carbon dioxide ($C + O_2 \rightarrow CO_2$). This is a classic chemical change.

Step-by-Step Solution

1. Evaluate A: Calcium oxide + water $\rightarrow$ Calcium hydroxide. New substance formed = Chemical change.
2. Evaluate B: Iron + Oxygen $\rightarrow$ Rust. New substance formed = Chemical change.
3. Evaluate C: Sodium chloride + water $\rightarrow$ Aqueous solution. The salt can be recovered by evaporation. No new substance = Physical change.
4. Evaluate D: Carbon + Oxygen $\rightarrow$ Carbon dioxide. New substance formed = Chemical change.

Comparing these, only option C represents a process where the original substances can be recovered without a chemical reaction occurring.

Conclusion

The correct option is C.

Exam Tip

A simple way to remember the difference: If you can easily "undo" the process to get your original materials back (like evaporating water from salt), it is likely a physical change. If you see heat, light, gas bubbles, or a permanent color change, it is almost certainly a chemical change.