Which of the following process is responsible for the formation of delta at a place where rivers meet the sea?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Coagulation
River water carries clay as a colloidal solution; the clay particles carry a charge and stay suspended. Sea water contains many electrolytes (\( \ce{Na^{+}} \), \( \ce{Mg^{2+}} \), \( \ce{Cl^{-}} \), \( \ce{SO4^{2-}} \) …).
Where the river meets the sea, these ions neutralise the charge on the clay particles. The particles come together and settle: this is coagulation. The deposited clay builds up over years into a delta.
Why the other options are incorrect
Option (1) Emulsification — Incorrect. Dispersing one liquid in another with an emulsifier; no liquid-in-liquid system is involved here.
Option (2) Colloid formation — Incorrect. The clay is already colloidal in the river; at the sea the colloid is destroyed, not formed.
Option (4) Peptization — Incorrect. Converting a precipitate into a sol with an electrolyte, the reverse of what happens at a delta.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Colloids Around Us – “… (vi) Formation of delta: River water is a colloidal solution of clay. Sea water contains a number of electrolytes. When river water meets the sea water, the electrolytes present in sea water coagulate the colloidal solution of clay resulting in its deposition with the formation of delta.”