Many lyophilic sols and few lyophobic sols when coagulated under some special conditions changes into semi rigid mass, enclosing whole amount of liquid within of liquid within itself, it is called gel and the process is called gelation. Gelatin, agar-agar, gum-Arabic can be converted into gels by cooling them under moderate concentration conditions. Hydrophobic sols like silicic acid. `Al(OH)_(3)` are prepared by double decomposition and exchanged of solvent method.
Types of gel :
1. Syneresis/weeping of gel : The spontaneous liberation of liquid from a gel is called syneresis or weeping of gels. It is reverse of swelling.
e.g., geletin, agar-agar show syneresis at low concentration while silicic shows it at high concentration.
2. Imbibition or swelling of gel : When gel is kept in a suitable liquid (water) it absorb large volume of liquid. The phenomenon is called imbibition or sweeling of gel.
3. Thixotropic : Some gels when shaken to form a sol, on keeping changes into gel are termed as thixotropic gel and phenomenon is called thixotropy.
e.g., gelatin and silica liquify on shaking changing into corresponding sol and the sol on keeping changes back into gel.
Some types of gels like gelatin and silica liquify on shaking thereby changing into sols. The sols on standing change back into gels. This process is know as :

Options

A. synersis
B. thixotropy
C. double decompostion
D. peptization

Explanation

To solve the question regarding the process in which certain gels like gelatin and silica liquefy upon shaking and revert back to gels upon standing, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Gels**:
- Recognize that gels are semi-rigid structures that can hold liquids within them. Examples include gelatin and silica.

2. **Identifying the Process**:
- The question describes a phenomenon where shaking the gel causes it to liquefy, transitioning from a gel to a sol (a colloidal solution).

3. **Reversibility of the Process**:
- After shaking, when the stress is removed (i.e., when the gel is allowed to stand), it reverts back to its original gel state. This indicates that the process is reversible.

4. **Defining the Term**:
- The specific term for this behavior, where a gel becomes less viscous upon agitation and regains its viscosity upon standing, is known as "thixotropy".

5. **Conclusion**:
- Therefore, the process described in the question is called thixotropic process.

### Final Answer:
The process is known as **thixotropic process**.

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