Discussion
Assertion (A): Calcium carbonate contributes almost no opacity in a solvent based paint.
Reason (R): Its refractive index is close to that of the binder surrounding it.
Answer: C
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason is the same principle applied in reverse. Calcium carbonate has a refractive index near 1.6 and the binder about 1.5, so there is almost no contrast, almost no scattering and effectively no hiding power. It is used as an extender for bulk, rheology and cost, not for opacity - and in a dry film above the critical pigment volume concentration it does contribute, because the surrounding medium is then air.
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