Question

Identify the compounds of the following.
(a)Milk (b)Sodium (c)Calcium carbonate (d)Methane
A. Milk, calcium carbonate
B. Sodium
C. Calcium carbonate, methane
D. None of the above

Answer 1

Compounds are those chemical substances which are made up of two or more atoms of the same or different elements. In other words, when two or more elements (same or different) combine chemically in a fixed ratio by mass, the product obtained is called a compound.

Complete step by step answer:
A pure substance is a substance whose constituent particles are the same in their chemical nature. Pure substances are of two types: elements and compounds. Those chemical substances in which all the atoms are exactly the same are known as elements. Thus, elements can be defined as species of atoms which have the same number of protons in their atomic nuclei. They cannot be broken down further into simpler fragments. On the other hand, compounds can be broken down further into simpler fragments.
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. Hence, it is not a compound.
Sodium is an element since it is a metal and metals cannot be broken down into simpler substances. In other words, sodium has a homogeneous chemical composition. Hence, sodium is not a compound.
Calcium carbonate has the chemical formula ${\text{CaC}}{{\text{O}}_{\text{3}}}$ . Hence, it is a compound because it is a pure substance composed of atoms of three different elements, calcium, carbon and oxygen, combined chemically in a fixed proportion.
Methane has the chemical formula ${\text{C}}{{\text{H}}_{\text{4}}}$ . Hence, it is a compound as it is composed of atoms of two different elements, carbon and hydrogen, combined chemically in a fixed proportion.
Hence, both calcium carbonate and methane are compounds.

So option C is correct.

Note:
Some other examples of compounds are ice (which is composed of hydrogen and oxygen combined chemically in a fixed proportion), hydrochloric acid (which is composed of hydrogen and chlorine combined chemically in a fixed proportion) and calcium oxide (which is composed of calcium and oxygen combined chemically in a fixed proportion). Some other examples of elements are iron and mercury both of which are both metals.