Solid $CO_2$ is an example of which type of crystal?

Which of the following processes is exothermic?

$A$ solid melts at a temperature above $100\,^oC$ and is a non-conductor of electricity in both solid and molten states. It is soluble in hydrocarbon solvents. What is the most likely structure of the solid?

Carbon dioxide is a gas,whereas silica is a solid because ...........

In silicon dioxide $(SiO_2)$:

An aqueous solution of potash alum gives .......