Identifying whether a process is a chemical reaction or a physical change is fundamental in chemistry. A chemical reaction involves the rearrangement of atoms and molecules to form new substances with different properties. A physical change, on the other hand, only alters the form or appearance of a substance, but not its chemical composition.

Understanding Chemical vs. Physical Changes

Let's look at the key differences:

• Chemical Change:
  • New substances are formed.
  • Often irreversible or difficult to reverse.
  • Changes in chemical properties occur (e.g., flammability, reactivity).
  • Energy is typically absorbed or released (heat, light, sound).
  • Examples: burning, rusting, cooking an egg, souring milk.
• Physical Change:
  • No new substances are formed; the chemical composition remains the same.
  • Often reversible.
  • Only physical properties change (e.g., state, shape, size, color, density).
  • Examples: melting, freezing, boiling, cutting paper, dissolving sugar in water.

Analyzing the Given Processes

Let's examine each option to determine if it represents a chemical reaction:

1. Drying of wet clothes: This process involves the evaporation of water from the clothes. Water changes from a liquid state to a gaseous state (water vapor). The chemical composition of water ($\text{H}_2\text{O}$) remains the same. This is a physical change.
2. Melting of ice: Ice is solid water ($\text{H}_2\text{O}$). Melting is the change of state from solid to liquid water. The chemical composition ($\text{H}_2\text{O}$) does not change. This is a physical change.
3. Sublimation of Iodine crystals: Sublimation is the process where a substance transitions directly from the solid state to the gaseous state without passing through the liquid state. Iodine ($\text{I}_2$) crystals turning into iodine gas is a change of state. The substance is still iodine ($\text{I}_2$). This is a physical change.
4. Butter turning rancid: When butter turns rancid, the fats within it undergo chemical reactions, primarily oxidation and hydrolysis. These reactions break down the fat molecules into smaller compounds like aldehydes and ketones, which have distinct and unpleasant smells and tastes. New substances are formed with different chemical properties. This is a chemical reaction.

Conclusion on Chemical Reaction

Based on the analysis, butter turning rancid is the only process among the options that involves the formation of new substances due to chemical transformation. Therefore, it is a chemical reaction.

Revision Table: Identifying Chemical Reactions

To quickly identify chemical reactions, look for signs that new substances are being formed, such as:

• Color change (sometimes)
• Gas production (bubbles)
• Formation of a precipitate (a solid forms in a liquid)
• Energy change (heat absorbed or released, light produced)
• Change in odor
• Difficulty in reversing the process

However, none of these signs individually guarantee a chemical reaction; they are indicators that suggest one might be happening. The definitive characteristic is the formation of new substances.

Additional Information: Types of Physical Changes

Physical changes include various processes that alter appearance but not composition:

• Changes of State: Melting, freezing, boiling, condensation, sublimation, deposition.
• Changes in Form: Cutting, tearing, crushing, bending, stretching.
• Dissolving: Mixing a solute into a solvent (e.g., salt in water). While the substance is dispersed, its chemical identity remains unchanged. Evaporating the solvent can often recover the original substance.