Question

Which of the following is most likely a covalent compound?

Substance X, which has low melting and boiling points, and does not conduct electricity in any phase.

Substance Y, which has a high melting point, is malleable and a conductor of electricity when solid.

Substance Z, which has high melting and boiling points, is not malleable and is a conductor of electricity when liquid but not solid.

Answer 1

`"X` is most likely a covalent compound.

Explanation:

All of the properties of `"X"` are properties of covalent compounds.

`"Y"` is malleable and a conductor when solid, so it is most likely a metal.

`"Z"` is not malleable and is a conductor of electricity when liquid, but not when solid, so it is most likely an ionic compound.

Answer 2

Substance X

Explanation:

Covalent compounds have strong bonds between atoms, but weak interactions between molecules.

This is because covalent bonds are usually formed between non-polar atoms, forming non-polar molecules. Non-polar molecules are only held together by London dispersion forces, which is a very weak interaction.

Covalent compounds have a low melting point and boiling point
Since interactions between covalent molecules is weak, it does not take much energy to pull apart molecules in a covalent compound. Thus, melting point and boiling point are low.

Covalent compounds are usually not conductive
For a compound to be conductive, there must be free ions or electrons to carry charge through the material. Since covalent compounds are usually non-polar, they do not have free electrons or ions that can carry charge, and thus are non-conductive. There are exceptions to this e.g. graphene, a covalent network of carbon, which is a superconductor.

Covalent compounds are usually soft and malleable
Since the interactions between covalent molecules are weak, the molecules can easily slide over each other, and move around. This means that covalent compounds are ususally soft and easily malleable. There are exceptions to this e.g. diamond, a covalent network of carbon, which is very hard.