Question

Two substances, $A$ and $B$ were made to react to form a third substance, ${\mathrm{A}}_2\mathrm{B}$ according to the following reaction:
$2\mathrm{A}+\mathrm{B}\to {\mathrm{A}}_2\mathrm{B}$
Which of the following statements concerning this reaction are incorrect?
(i) The product ${\mathrm{A}}_2\mathrm{B}$ shows the properties of substances $A$ and $B$
(ii) The product will always have a fixed composition
(iii) The product so formed cannot be classified as a compound
(iv) The product so formed is an element

• A. (i), (ii) and (iii)
• B. (ii), (iii) and (iv)
• C. (i), (iii) and (iv)
• D. (ii), (iii) and (iv)

Answer 1

The correct option is C

(i), (iii) and (iv)

Pure substances:

1. It is made up of only one kind of atom.
2. A pure material cannot be divided into different types of matter by physical processes.
3. Elements: It is made up of a single type of atom which can't be broken further. Example: Argon $\left(\mathrm{Ar}\right)$.
4. Compound: It is a pure substance made up of two or more elements chemically combines with one another in a fixed proportion by mass. Example: Sodium hydroxide$\left(\mathrm{NaOH}\right)$.

The explanation for correct options:

The product ${\mathrm{A}}_2\mathrm{B}$ shows the properties of substances $A$ and $B$:

The properties of a compound (e.g., ${\mathrm{A}}_2\mathrm{B}$) are entirely different from those of its constituent elements (i.e., $\mathrm{A}$ and $\mathrm{B}$).

The product so formed cannot be classified as a compound:

${\mathrm{A}}_2\mathrm{B}$ is a compound made up of two elements $\mathrm{A}$ and $\mathrm{B}$ in a fixed ratio.

2A + B → A₂B

The product so formed is an element:

The product formed i.e. is a compound.

Since all the above statements are not true about the product formed thus option (c) is correct.

The explanation for incorrect options:

The product will always have a fixed composition:

1. ${\mathrm{A}}_2\mathrm{B}$ is a compound made up of two elements $\mathrm{A}$ and $\mathrm{B}$ in a fixed ratio.
2. The composition of a compound is fixed.

Since the above statement is true about the compound ${\mathrm{A}}_2\mathrm{B}$ thus option (a), (b) and (d) are incorrect.

Hence option (c) is the correct option.