Question

Which of the following has the lowest melting point?
(a)- $SrC{{l}_{2}}$
(b)- $CaC{{l}_{2}}$
(c)- $MgC{{l}_{2}}$
(d)- $BaC{{l}_{2}}$

Answer 1

First, we have to check the change in the trend in the options and we can see that all the compounds are salts in which all the anions are chloride but the cations are changing. By checking the size of the cation we can find the trend of the melting and boiling point.

Complete answer:
The chemical compounds have many properties like solubility, melting point, boiling point, color, a force of interactions, etc. When there is a similarity in the compounds we can find the trend in the properties.
As we know that the melting point is the temperature at which the solid form of the compounds starts to change in the liquid form. All the compounds given in the options are salts because they are made up of cations and anions.
$SrC{{l}_{2}}$ is strontium chloride, $CaC{{l}_{2}}$ is calcium chloride, $MgC{{l}_{2}}$ is magnesium chloride, and $BaC{{l}_{2}}$ is barium chloride. As we can see that the anion in all the compounds is the same but the cations are changing. All the cations are from group 2, i.e., alkaline earth metals.
So, as the size of the cation increases the surface area also increases due to which the strength of the bond also increases. So, from the given compounds magnesium has the smallest size so, its surface will be the lowest. Therefore, the size of magnesium chloride ($MgC{{l}_{2}}$) is the smallest, and the melting point is the lowest.

Hence, the correct answer is an option (c).

Note:
If the cation was the same and the anions were different then the trend will be different, as the size of the anion increases then the melting point decreases because of the lattice energy.