Question

The amorphous form of silica is:
A. Quartz
B. Kieselguhr
C. Cristobalite
D. Tridymite

Answer 1

We know that silica exists in four allotropic forms, such as, quartz, tridymite, cristobalite and fused quartz or Kieselguhr.

Complete step-by-step answer:
An amorphous substance does not possess long range molecular order and well defined arrangement.

A.) A chemical compound composed of two part oxygen and one part silicon is termed as quartz. Its chemical formula is $SiO_2$ . On Earth's surface, it is the most available mineral. As quartz has a well defined arrangement, so it is not the amorphous form of silica. Hence, option A is not correct.
B.) The composition of Kieselguhr is silicon containing shells of unicellular aquatic plants. It does not possess a well defined arrangement. So, it is the amorphous form of silica. Hence, option B is correct.
C.) Cristobalite is one of the allotropic forms of silica which has the same molecular formula as quartz but distinct crystalline structure. It is formed at a very high temperature and used in medicine in dentistry to make models of teeth. As it has a well defined structure, it is not the amorphous form of silica. So, option C is not correct.
D.) Tridymite is a form of quartz found in the form of crystals having thin hexagonal shapes in rocks and meteorites. It has a well defined arrangement. So, it is not the amorphous form of silica. Hence, option D is not correct.

Therefore, the correct answer is (B).

Note: Students might get confused with different forms of solid. There are two forms, amorphous and crystalline. In crystalline forms, molecules are in an ordered manner but in amorphous form, molecules are in a random manner.