Question

Which of the following statement is false regarding the ionic solids $CaO$ and $KI$?

• A. $CaO$ has high melting point
• B. The lattice energy of $CaO$ is much larger than that of $KI$
• C. $KI$ has low melting point
• D. $KI$ is soluble in benzene

Answer 1

The correct option is D $KI$ is soluble in benzene

$\left(a\right)$ $CaO$ has high melting point.

This statement is true because $CaO$ has high lattice energy in comparison to $KI$ due to more $\frac{\text{charge}}{\text{size}}$ ratio. Hence, $CaO$ has more stable arrangement of ions than $KI$ and so it is difficult to break and thus requires high temperature to melt which leads to the high melting point.
Overall we can say that the order of melting point is directly proportional to the order of lattice energy.

$\left(b\right)$ The lattice energy of $CaO$ is much higher than that of $KI$.

This statement is also true as the lattice energy depends upon the ratio of charge/size of ion. Calcium ion $\left(C{a}^{2+}\right)$ is not only smaller than Potassium ion $\left(K^{+}\right)$ but also has twice the charge as that of Potassium ion. Similarly, Oxide ion $\left(O^{2-}\right)$ also has a smaller size as compared to the Iodide ion $\left(I^{-}\right)$ and even higher charge (magnitude wise). Hence, the lattice energy of $CaO$ is higher than $KI$.

$\left(c\right)$ $KI$ has a low melting point.

This statement is true because, low lattice enthalpy leads to low melting point as explained in the first statement.

$\left(d\right)$ $KI$ is soluble in benzene.

$KI$ is an ionic compound therefore it will be soluble in polar solvents like water as the famous saying 'Like dissolves Like'.
Benzene is a non-polar solvent and so $KI$ is insoluble in it. Therefore, this statement is false.