Question

Which of the following materials fall in the category of a pure substance?
 A.Ice
B.Milk
C.Iron
D.Hydrochloric acid
E.Calcium oxide
F.Mercury
G.Brick
H.Wood
I.Air

Answer 1

Hint: A pure substance is a substance whose constituent particles have the same chemical nature.
Pure substances can be of two types: elements and compounds.

Complete step by step answer:
Ice is a pure substance because it is a compound composed of two elements, hydrogen and oxygen combined chemically in a fixed proportion, i.e., because it has a definite composition. Thus, (A) is correct.
Iron is a pure substance since it is a metal and metals are a class of elements which cannot be broken down into simpler substances. In other words, iron has a homogeneous chemical composition. Thus, (C) is correct.
Hydrochloric acid is a pure substance because it is a compound composed of two different elements, hydrogen and chlorine combined chemically in a fixed proportion. i.e., it cannot be separated by using physical methods. Thus, (D) is correct.
Calcium oxide is a pure substance because it is a compound which consists of two elements, calcium and oxygen in a constant proportion, i.e., it has a constant composition. Thus, (E) is correct.
Mercury is a pure substance because it is a metal. Since metals are a class of elements, mercury has a uniform composition throughout and cannot be broken down into simpler substances. Thus, (F) is correct.
Milk is not a pure substance. This is because milk is a liquid in liquid emulsion and colloids are heterogeneous mixtures. So, milk does not have a uniform composition. Thus, (B) is incorrect.
Brick is not a pure substance since it is not composed of only one type of atom. It is made up of different types of substances like mud and water. Thus, (G) is incorrect.
Wood is also not a pure substance because it consists of different kinds of components of a tree. Thus, (H) is incorrect.
Air is also not a pure substance because it is a homogeneous mixture of different kinds of gases. Thus, (I) is incorrect.

Hence A,C,D,E and F are correct options.


Note:
-Pure substances have fixed melting and boiling points and usually take part in chemical reactions to form predictable products.
-On the other hand, impure substances do not have any specific properties, they have variable composition and their melting and boiling points are not fixed.