Question

A compound is a substance composed of two or more different types of _________ chemically combined in a fixed proportion.
A. Element
B. Compound
C. Molecule
D. Ion

Answer 1

Hint: We know that the law of constant proportions was proposed by Lavoisier. This law states that in a chemical compound, elements are present definite proportions by mass irrespective of who prepared it or from where the compound came from.

Complete step by step answer:
Let’s discuss compounds in detail. When atoms of different elements join together in definite proportions, a compound forms. Let’s take the example of water. Water is a compound made of two elements namely hydrogen and oxygen. The compound water always forms in the ratio by mass 1:8. Similarly ammonia is a compound and ammonia also forms by the ratio of mass of 14:3.
So, a compound is formed due to the chemical combination of elements in fixed proportion.

So, the correct answer is Option A.

Additional Information:
We know that the smallest portion of matter is an atom and the atoms of the same or different elements combine to form molecules. The chemical reactions involve atoms and molecules. Since the seventeenth century, scientists were trying to establish the basis of these chemical combinations. Ultimately they came out with five laws namely law of conservation of mass, law of definite proportions, law of multiple proportions, law of reciprocal proportions and Lay-Lussac’s law.
Always remember that law of constant proportions deals with the composition of various elements present in a compound. This law is not applicable to the impure compound. The compound must be pure.

Note: It is to be noted that sometimes the elements present in the same ratio may give rise to different compounds. For example, C, H and O combining in the ratio of 12:3:8 by mass may give two compounds ethanol $\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}\right)$
and methoxymethane $\left(\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{C}{\mathrm{H}}_3\right)$ .These two compounds are termed as isomers.