Which of the following is a non – crystalline solid?
(A)- ${\text{CsCl}}$
(B)- ${\text{NaCl}}$
(C)- ${\text{Ca}}{{\text{F}}_{\text{2}}}$
(D)- Glass

Answer
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Hint:. Non – crystalline solids are also known as amorphous solids, and among the above given options that solid is amorphous solid which is relatively weaker than other and gets break easily.

Complete step by step answer:
Non – crystalline solids are those solids in which arrangements of atoms or molecules or their constituents are uneven.
-And in crystalline solids arrangements of atoms or molecules or their constituents is even.
-In option (A) ${\text{CsCl}}$ is given, which is a crystalline solid because in it regular cubic arrangement of Cesium (${\text{Cs}}$) and chlorine (${\text{Cl}}$) is observed.
-In option (B) ${\text{NaCl}}$is given, which is a crystalline solid because in it regular face centered cubic arrangement of Sodium (${\text{Na}}$) and chlorine (${\text{Cl}}$) is observed.
-In option (C) ${\text{Ca}}{{\text{F}}_{\text{2}}}$ is given, which is a crystalline solid because in it regular face centered cubic arrangement of Calcium (${\text{Ca}}$) and Fluorine (${\text{F}}$) is observed.
-In option (D) Glass is given, which is a non - crystalline solid because no regular arrangement of its constituent is observed.
Hence, Glass is a non - crystalline solid among all given options in the question.
So, the correct answer is “Option D”.

Note: Here you may think that glass is a crystalline solid by considering its appearance only, but that consideration will be wrong because glass is prepared by the random cooling of liquid sand at low temperature, so that there is not proper arrangement of constituents.