The correct option is Hydrogenation.
Explanation
Vegetable oils typically contain long chains of unsaturated fatty acids, characterized by the presence of carbon-carbon double bonds ($C=C$). To convert these liquid oils into semi-solid or solid fats (saturated fats), the double bonds must be converted into single bonds through a chemical addition reaction.
Analysis of Options:
- Hydrogenation is Correct: Hydrogenation is the chemical process where dihydrogen ($H_2$) is added to unsaturated vegetable oils in the presence of a catalyst, typically finely divided Nickel (Ni), Palladium (Pd), or Platinum (Pt). This addition reaction saturates the double bonds, resulting in the formation of saturated fats (e.g., the production of Vanaspati ghee from vegetable oils).
- Saponification is Incorrect: Saponification is the process used in soap manufacturing. It involves the alkaline hydrolysis of esters (fats or oils) to produce glycerol and the salt of a fatty acid (soap). It does not convert unsaturated oils to saturated fats.
- Substitution is Incorrect: Substitution reactions involve replacing an atom or group of atoms in a molecule with another atom or group. This type of reaction is characteristic of saturated hydrocarbons (alkanes), whereas converting oils to fats requires an addition reaction.
- Cracking is Incorrect: Cracking (or pyrolysis) involves breaking down large, complex hydrocarbon molecules into smaller, simpler molecules using heat and pressure. This process is primarily utilized in the petroleum industry to produce fuels like petrol, not to harden oils.
Key Takeaway: The "hardening of oils" is achieved through hydrogenation, an addition reaction that converts unsaturated fatty acids into saturated fats using a Nickel catalyst.